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2017

In this Part 2 of the blog, I will explain to you another way of creating an arb and transferring into an AWG using BenchVue software. If you have not used the BenchVue software before, let me tell you, it’s even easier than the two examples shared in Part 1 of this blog.

 

First, let me give you an introduction of what BenchVue software is all about. It is a software platform for the PC that allows users to easily connect, record and achieve results across multiple test and measurement instruments with no programming. Plug and play functionality enables you to connect your instrument to your PC and immediately begin controlling it in BenchVue. Test Flow is a new application inside of BenchVue that provides an easy method to create custom test sequences using a drag-and-drop interface.

 

For the sake of new potential BenchVue users, please go to www.keysight.com to download a trial version or purchase a licensed version of the BenchVue Function Generator App software for your PC. Hook up your favorite instrument interface whether it is a USB, GPIB or LAN to your PC and launch your BenchVue software. You will quickly get full control of your Keysight Waveform generator in a very short moment. You will see a graphical instrument control window of your waveform generator as shown below. You can easily setup normal sine, square, ramp, pulse, triangle, noise, PRBS and DC waveforms with desired parameters in no time.

 

benchvue control panel

If you select the Arb button on the figure above, you can either open and load current existing Arb waveform pre-loaded in the instrument or an existing Arb waveform available in your PC. You can also choose to create a new Arb waveform by clicking on the button pointed by the arrow as shown on the figure above.

A new window will pop up when you click on the “create Arb” button as shown on the figure below. You can easily create basic as well as advanced waveforms. You can even perform a free hand drawn waveforms using this tool.

waveform builder

The arrow on the figure above points to an “equation editor” button. When you click this button, an equation editor window will appear as shown below.

equation editor

One more important feature when creating your new Arb, this software can help you sequence multiple different waveforms in the order and number of repeated times per waveform as you like. Transferring of the created arb waveform is really easy too with this BenchVue software. You do not have to create a CSV file and manually transfer to your AWG. With only several clicks in BenchVue, your newly created arb waveform will automatically be transferred to your instrument.

I hope this blog helps. If you have not seen Part 1 of this blog, please click here.

Click here to learn more about the 33500A / 33600B series of function / arbitrary waveform generators

 

 

In this blog post we will look at how easy it is to create arbitrary waveforms on a modern function / arbitrary waveform generator (AWG). I am always running into engineers, young and old alike, that try at all costs to avoid creating an arbitrary waveform (arb) for a test. When they hear the word arb they picture the tedious process of learning how to use some type of waveform software or, worst yet, having to write a program to generate your waveform and then remotely connecting to your AWG to upload the arb. With modern AWGs arb creation no longer has to be looked upon with doom and gloom.

 

Let's look at two easy examples for creating an arb and transferring it to an AWG. In the first example, we will look how we can create an arb from scratch and transfer it to an AWG. In the second example, we will capture a waveform from a scope and then transfer and recreate it with an AWG. In both examples two common elements will be used, a USB memory stick and the Comma Separated Value (CSV) file format. I will follow up with a Part 2 of this blog to showcase our BenchVue software in creating Arbs.

 

Using Excel to Build Arbs To build an arb from scratch most engineers turn to either an engineering programming environment, like Matlab, LabVIEW, or VEE, or a custom arb waveform software package that may or may not be free. These are great arb creation solutions, but they can be costly and time consuming if you do not use them on a regular basis. Another option that most engineers do not consider is Excel. Excel is a great tool for building custom arbs since it provides advanced mathematical functions built-in, it can handle large amounts of data (waveform points), and it is already on just about everybody's computer. The next question then is how do you get the waveform from Excel to the AWG? Excel and modern AWGs have something in common; the CSV file format. Excel can read CSV files and Excel spreadsheets can be saved as CSV files. Modern AWGs can read and create arbs from CSV files. To transfer the CSV to the AWG a USB memory stick with the CSV file can be plug into the AWG's front panel and then uploaded into waveform memory.

 

Let's look at an example. Using Excel an arb waveform was created that consisted of a sine wave summed with third harmonic noise and random noise. A screen shot of the Excel spreadsheet can be seen below. Notice the resulting arb is plotted and the built-in Excel functions used to create the waveform are circled in red.

Excel Arb

 

The Excel spreadsheet was then saved as a CSV file. Using a USB memory stick it was then uploaded to Keysight's 33500B or 33600A series function / arbitrary waveform generators. The resulting arb was captured in the below scope screen shot.

Scope Excel Arb

 

As you can see Excel provides an easy no cost way to create an arb and the CSV file format provides a means to easily transfer the arb to an AWG. If you prefer to use a software environment to generate your arb or your arb needs are more advanced then what Excel can do, you can still skip the remote connection / instrument programming part. Most programming environments, like Matlab and LabView, have APIs for writing / reading CSV files. Simply have your program write the arb to a CSV file and "sneaker network" it to the AWG.

 

For the second example we will capture a digitized signal from a scope then transfer it to an AWG and turn it into an arb. In the past, this was typically done using some type of arb waveform software package that would remotely connect to first the scope to grab the digitized signal and then to the AWG to create the arb. With today's scopes and AWGs the process has been streamlined. For our example an Keysight MSO-X 3054A scope was used to capture a Data Word from a Mil-Std-1553 signal. The captured waveform is shown in the below figure.

Mil Std 1553 Data Word

 

Shown below the Mil-Std-1553 signal in blue is 5F67, which is the hexadecimal decoded value of the Data Word. The AWG used in this example was again the 33500B or 33600A series function / arbitrary waveform generator. Here is how it works:

  1. Plug the USB memory stick into the front panel of the scope.
  2. Save the digitized waveform to the USB memory stick as a CSV file.
  3. "Sneaker network" the USB memory from the scope to the front panel of the AWG.
  4. Import the CSV to the AWG's memory. 

It is really that easy! To make this example a little more exciting the Mil-Std-1553 arb on the 33500B / 33600A series was modulated with a lower frequency pulse to simulate coupled transient noise into the signal channel. The modulated arb was captured in the screen shot below.

1553 Data Word Error

 

You can see the simulated transient noise in the figure at the beginning and middle of our arb. Notice at the bottom of the scope in red and blue that because of the transient noise the scope was unable to decode the Data word.

In this post we looked at how easy it is today with modern AWGs to create an arbitrary waveform. We looked at two cases, creating an arb from scratch with Excel and capturing a waveform with a scope. Both methods used the USB memory stick and the CSV file format to transfer the waveform to the AWG with no remote connections or programming. If you have any questions, feel free to email me and if you have any personal incites to add use the "Comments" section below.

You can also click this link -  Creating Arbs Today is Easy - Part 2 of this blog that showcases our BenchVue software in creating Arbs.

Click here to learn more about the 33500A / 33600B series of function / arbitrary waveform generators

Using an electrocardiogram (ECG sometimes called an EKG) is an invaluable way to identify various physical ailments. Today there is a wide array of cardiac equipment that displays and interprets ECG signal patterns. Medical equipment designers need a flexible way to seamlessly generate accurate ECG signal patterns to verify and test their designs. In this post, I will discuss how to generate complex ECG signal patterns with an arbitrary waveform generator (AWG). Below in the figure is a 12-lead ECG waveform.

ECG_waveform_description

There are three methods to create and store an ECG on an AWG:

  1. You can use a device such as a digitizer or oscilloscope to capture an actual ECG signal from a patient. Then you upload the digitized points to the AWG. With modern AWGs, there are many ways to accomplish this, including using a .csv file and a memory stick.
  2. You can use mathematical software to create an ECG signal. There may be custom software for the AWG that can do this, or you could use a standard software package, such as MATLAB ®.
  3. If your instrument has this capability, you can use your AWG’s built-in "typical" ECG waveform. The Keysight 33500B & 33600A series has a built-in ECG waveform.

 

Using an AWG’s arb sequencing capability to simulate complex ECG patterns

AWGs that have arb sequencing ability, like the 33500B/33600A function/arb waveform generators, can seamlessly transition from one arb waveform stored in memory to another without any discontinuities in the output. The figure below shows an example using the 33500B/33600A’s arb sequencing feature to combine three different ECG waveforms stored in different places in memory into one waveform.

ECG_Seq

The first ECG waveform cycle is meant to be an "ideal" ECG waveform. The other two were based on the first one but were changed in a systematic way using MATLAB software. Notice the second ECG waveform has a flattened T wave. In the third ECG waveform, the T wave is inverted.

The 33500B/33600A’s sequencing capability provides flexibility for controlling when it sequences from one waveform to another. One way to control sequencing is to specify how many cycles each waveform is run before sequencing to the next. Sequences can also return to a waveform that was used previously in that sequence.

Combining the 33500B/33600A’s arb sequencing feature with its large arb memory, 1 million points per channel standard with 16 million optional, gives you the ability to simulate complex ECG patterns for thorough testing of cardiac monitoring equipment designs. For example, each ECG waveform shown in the above figure were created with about 500 points. You could store up to 2,000 different ECG waveforms of this size in the 33500B/33600A’s standard arb memory. The 33500B/33600A allows arb sequences to contain up to 512 steps, allowing you to create complex ECG patterns for thorough testing. You can control arb sequences on the 33500B/33600A asynchronously by using triggers to control waveform transitions instead of cycle counts. This provides you with the ability to continuously cycle a waveform for some undetermined time period until it receives a software trigger or external trigger or front-panel trigger. Once it receives the trigger, the 33500B/33600A transitions to the next waveform in the sequence. You can also mix the two ways of transitioning through a sequence, specifying a count and using triggers.

 

Free Matlab ECG simulation program

You can download and use an ECG simulator program created in MATLAB®. You can find the ECG simulator download and instructions at http://www.mathworks.com/matlabcentral/fileexchange/10858-ecg simulation-using-matlab or type “ECG MATLAB” into a search engine and it should be at the top of the results. The program creates ECG waveforms using multiple Fourier series summed together. A Fourier series is used for each distinct wave shape in the ECG waveform, such as the P wave, T wave, etc. The program allows you to adjust various ECG waveform parameters to simulate various cardiac conditions. You can then transfer the ECG waveform you created to a 33500B/33600A either by storing it in a .csv file and using a memory stick or remotely via Matlab's instrument toolbox feature or BenchVue FG application software.

 

Click here for more info on the 33500B and 33600A series function / arbitrary waveform generator

 

 

Single-ended signals are referenced to a common level, such as ground, that they can share with other signals, so a single-ended signal requires only a single path or wire. Differential signals are made up of a pair of paths that are both dedicated to a single signal at any given time. One path is used at a higher potential than the other. Differential signals do add complexity, since they require two wires instead of just one, but they provide a number of performance advantages over single-ended signals.

Differential signal advantages include better signal-to-noise ratio, fewer timing errors, and less crosstalk. These advantages make differential signals common in applications such as ADC inputs, instrumentation amplifiers, measurement sensors (like accelerometers), and communication signals. When engineers design and test devices that use differential signals, simulating the differential signals for testing can be challenging. These challenges are caused by the fact that most function/arbitrary waveform generators (FAWGs) have single-ended outputs; instruments that can generate differential signals tend to be fairly costly. In this post I will explain two ways to create a low cost differential signal: using a FAWG with some custom hardware and using a 2-channel FAWG.

One way to use custom hardware at the output of a single-ended source to create a differential signal is to use a differential amplifier circuit design as shown in this figure.

Differential amp circuit

The resistors in the differential circuit were chosen to achieve a gain value of 1. I set the DC offset to 0 V. When building the circuit, be careful to keep signal paths and wiring as short as possible to keep parasitic reactive affects low for better signal integrity.

FAWG’s with two single-ended channels (isolated from ground) can have their channels combined into a single differential signal channel. To do this, you need to tie together the two "low" or "common" connections of each channel. The "high" of one channel must be used as the high signal path of the differential channel and the "high" of the other channel must be used as the inverse return wire or low signal path, as shown in the figure.

two channel differential

In addition to the two channels, it is also a lot easier to do this on a two-channel FAWG that has channel tracking capability, like Keysight’s 33522B/33612A/33622A two-channel function/arb waveform generator. This feature gives you the ability to create an inverted mirror image of the output signal from channel one onto channel two, which is exactly what is needed to create a differential signal. Also, this capability means you only have to set up the arb or built-in waveform on one channel and the inverted version of the waveform automatically tracks to the other channel. Without this feature you would have to setup an arb or built-in waveform on both channels and try to output them in sync using triggering.

As an example I measured and captured three signals with a differential input high-resolution digitizer. The example signal we used was a squarewave at 500 KHz. The figure below shows a digitizer screen shot of the signals. The three signals:

  1. Signal in yellow is a differential signal from the output of the differential amplifier connected to the single-ended FAWG
  2. Signal in green is the differential signal output created by the two channels from the FAWG
  3. Signal in purple is the output of the single-ended FAWG before the differential amplifier input

 

diff signal 500K

As you can see from the figure there is quite a bit of ringing on the differential signal created with the custom hardware. When I built my diff amp circuit I was careful to keep wiring as short as possible and I provided a large ground plane. Now with further time and engineer effort I could probably further improve the signal integrity of the circuit. But the point of this example is to show you can save test time and achieve better signal quality by using a 2-chan FAWG with tracking to create a differential signal. Also the cost of a 2-chan FAWG is still typically much cheaper than a differential output waveform generator.

 

Click here for 33500B and 33600A series product page

 

True RMS responding DMMs measure the "heating" potential of an applied voltage. Power dissipated in a resistor is proportional to the square of an applied voltage, independent of the wave shape of the signal. Today’s general purpose DMMs can accurately measures true RMS voltage or current, as long as the wave shape contains negligible energy above the meter’s effective bandwidth (more on this in a bit). Most DMM's ACV and ACI functions measure the AC–coupled true RMS value (DC is rejected). For symmetrical waveforms like sinewaves, triangle waves, and square waves, the AC–coupled and AC+DC values are equal, since these waveforms do not contain a DC offset. However, for non–symmetrical waveforms (such as pulse trains) there is a DC voltage content, which is rejected by AC–coupled true RMS measurements. DC rejection is desirable in certain applications such as when you want to measure the AC ripple present on DC power supplies. For situations where you want to know the AC+DC true RMS value, you can determine it by combining results from DC and AC measurements, as shown below:

True RMS AC DC

A common misconception is that "since an AC multimeter is true RMS, its sine wave accuracy specifications apply to all waveforms." Actually, the shape of the input signal can dramatically affect measurement accuracy for any multimeter, especially when that input signal contains high–frequency components which exceed the instrument’s bandwidth. As an example, consider a pulse train, one of the most challenging waveforms for a multimeter. The pulse–width of that waveform largely determines its high–frequency content. The frequency spectrum of an individual pulse is determined by its Fourier Integral. The frequency spectrum of the pulse train is the Fourier Series that samples along the Fourier Integral at multiples of the input pulse repetition frequency (PRF).

 

The below figure shows the Fourier Integral of two different pulses: one of broad width (200 μs); the other narrow (6.7 μs). Keysight's 34460A/61A series and 34465A/70A series of DMMs have an effective AC measurement bandwidth of 300 kHz.  If we used one of these DMMs to measure the RMS ACV value of both pulses in the figure the measured value of the broader pulse will be more accurate than the measured value of the narrow pulse since its frequency components outside of the DMM's bandwidth are larger in amplitude.

Pulse freq spectrum

When making true RMS measurements on non-symmetrical waveforms, accuracy drops as the crest factor and/or the frequency of the waveform increases (for more info on crest factor click here). Here is a list of other tips when making true RMS AC measurements:

  1. For maximum accuracy, measure as close to full scale as you can. You might need to override auto scaling in some cases. Be careful with high-crest-factor signals not to overload and saturate the meter’s input circuitry.
  2. Be sure to select your DMMs appropriate low-frequency filter to allow for the fundamental to be captured. The lower the filter the longer the measurement will take.
  3. You may not want to use the first measured value because many DMMs have a large-value DC-blocking capacitor in the input path. You need to allow this capacitor to charge, especially when you are measuring low-frequency signals or when you are switching between measurement points that have a large DC offset.
  4. When you measure AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will serve as an antenna and the DMM will measure these unwanted signals as well. Reduce the area of the “antenna,” use good shielding techniques, and make sure the AC source and the DMM are connected to the same electrical outlet to minimize ground loops.
  5. AC loading errors: The input impedance of a DMM is often in the region of 10 MΩ in parallel with 100 pF. The cabling you use to connect signals to the multimeter adds additional capacitance and loading. As frequency increases, loading will change. For example, at 1 kHz, the input resistance will now be closer to 850 kΩ, and at 100 kHz it will be closer to 16 kΩ.

 

Click here to check out Keysight's DMMs

 

 

The digital multimeter or DMM offers two methods for measuring resistance: 2–wire and 4–wire ohms. For both methods, the test current flows from the input HI terminal and then through the resistor being measured. For 2–wire ohms, the voltage drop across the resistor being measured is sensed internal to the multimeter. Therefore, test lead resistance is also measured. For 4–wire ohms, separate "sense" connections are required. Since no current flows in the sense leads, the resistance in these leads does not give a measurement error. In this blog post I will discuss some general considerations and tips when making DMM resistance measurements. 4–Wire Ohms Measurements

4-wire ohm measurement use the HI and LO DMM leads as well as the HI-Sense leads (that is why they are called "4-wire"), the setup for a 4-wire ohm measurement is shown below.

Four wire ohm

The sense leads essentially extend the DMM measurement to the DUT junctions instead of the HI and LO terminals. This eliminates the voltage drop across the HI and LO leads caused by the test current. Since the sense leads are high impedance there is essentially no current flow into the sense inputs. The 4–wire ohms method provides the most accurate way to measure small resistances. Test lead resistances and contact resistances are automatically reduced using this method. Four–wire ohm is often used in automated test applications where resistive and/or long cable lengths, numerous connections, or switches exist between the DMM and the DUT. Removing Test Lead Resistance Errors Modern DMMs offer a built-in function, often labeled as "Null" or "Math", for eliminating test lead error. To use the Math function on a DMM you short the test leads to together. The Math function will then make a resistance measurement of the test leads and store it. The DMM will then mathematically subtract the measured lead resistance for subsequent resistance measurements to cancel out the lead resistance error.

Minimizing Power Dissipation Effects When measuring resistors designed for temperature measurements (or other resistive devices with large temperature coefficients), be aware that the DMM will dissipate some power in the device–under–test. If power dissipation is a problem, you should select the DMM's next higher measurement range to reduce the errors to acceptable levels. The following table shows examples of Keysight's 34460A and 34461A DMMs source current for various measurement ranges.

Resistance Ranges

Errors in High Resistance Measurements
When you are measuring large resistances, significant errors can occur due to insulation resistance and surface cleanliness. You should take the necessary precautions to maintain a "clean" high–resistance system. Test leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and "dirty" surface films. Nylon and PVC are relatively poor insulators (10^9 Ω) when compared to PTFE (Teflon) insulators (10^13 Ω). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 MΩ resistance in humid conditions.

Click here to check out the DMMs that Keysight offers

Today I want to talk about what causes an overvoltage condition. An overvoltage condition is a condition that causes the power supply output voltage to exceed its setting. Let’s take a look at some of the things that can cause this to happen.

 

Causes of power supply output voltage exceeding its setting:
User-caused miswires
These miswires should be found and corrected during test setup verification before a device under test (DUT) is connected to the power supply. Possible miswires and their effect on the power supply output voltage are:

  • Shorted sense leads – the output voltage will rapidly rise above the setting. Keysight power supplies will prevent the output from rising above the overvoltage protection (OVP) setting.
  • Reversed sense leads – on most power supplies, the output voltage will rapidly rise above the setting and on Keysight supplies, it will be stopped by the OVP circuit. On our N6900/N7900 Advanced Power System (APS) power supplies, this condition is caught sooner: OV- is triggered when the output reaches about 10% of the rated voltage, so the output does not have to rise to the setting and above.
  • Open sense leads – If your power supply does not have protection for open sense leads, it is possible for your output to rapidly rise above the setting if one or both sense leads are open. Keysight power supplies have built-in sense protect resistors which limit the output voltage rise to about 1% above the setting. The voltage will continue to be regulated there. In addition to limiting the output to about 1% above the setting with an open sense lead, Keysight N6900/N7900 APS power supplies have a feature called open sense lead detection. When enabled, open sense lead detection will cause a sense fault (SF) status about 50 us after open sense leads are detected. This status does not turn off the output, but it can be configured to turn off the output using the advanced signal routing capability.
  • Special note about N7900 power supplies (not N6900): these models have output disconnect relays that open upon a protection fault. These mechanical relays take about 20 ms to open. Before they open, the output downprogrammer circuit is activated for about 2 ms and draws about 10% of rated output current to reduce the output voltage. The N7976A and N7977A (both higher voltage models) also have solid state relays in series with the mechanical relays. Upon a protection fault on these 2 models, the downprogrammer activates for 2 ms followed immediately by the solid state relays opening and then the mechanical relays open about 20 ms later.

 

Download the free "4 Ways to Build Your Power Supply Skill Set" eBook.

 

Inadvertent wiring failure
  • Sense leads inadvertently become shorted – power supply response is the same as mentioned above under shorted sense leads
  • Sense leads inadvertently become open – power supply response is the same as mentioned above under open sense leads
  • Sense leads should never become inadvertently reversed, nevertheless, the power supply response is the same as mentioned above under reversed sense leads

Power supply fault (circuit failure)
Note that Keysight’s overall power supply failure rate is very low. Since the below mentioned failures are a subset of all failures, they are very rare. This means that failures that cause the output to go to a higher-than-desired value are a small percent of a small percent, and while not impossible, they are extremely unlikely events.
  • Power element fails (shorts)
    • Series regulator – when a series regulator power element shorts, the output very quickly rises above the rated voltage of the power supply. The only way to limit this is to trip OVP and either fire an SCR across the output to bring the voltage back down or open output relays. For example, the Keysight N678xA models use a series regulator. When OVP trips on N678xA models, output relays are opened to protect the DUT. Solid state relays very quickly open first followed by mechanical relays about 6 ms later.
    • Switching regulator – when a Keysight switching regulator power element shorts, the output will go toward zero volts instead of rising since Keysight switching regulators use power transformers and no power can be transferred through the transformer without the switching elements turning on and off. For example, all N6700 and N6900/N7900 series models use switching regulators except the N678xA models (series regulators).
    • Note that if a power element fails open using either power regulation scheme, the output voltage will fall, not rise, so this condition is not a concern when looking at excessive output voltage possibilities.
  • Regulation circuit failure (bias supply, DAC, amplifier, digital comparison processor, etc.)
    • There are various circuits that could fail and cause the output voltage to rise in an uncontrolled manner. Keysight power supplies have OVP designed to respond to these failures. In series regulators, an SCR across the output can fire to reduce the voltage or output relays can open. In switching regulators, the pulse width modulator is turned off to prevent power from flowing to the output, downprogrammers are activated to pull any excessive voltage down, and output relays are opened (when present) to disconnect the output from the DUT.
    • Multiple parallel failures – if both a regulating circuit fails that causes the output to rise AND the OVP circuit fails, there would be nothing to prevent the output voltage from rising above the setting. While this is possible, it requires just the right combination of multiple circuit failures and is therefore extremely unlikely.
Output response to load current transients
  • It is possible for the output voltage to temporarily rise above the setting for short transients in response to fast load current changes (especially unloading). If the voltage excursion is high enough and long enough, it is possible that the OVP will activate and respond as outlined above.
External power source
  • It is possible for an external source of power (such as a battery, charged capacitor, inductor with changing current, or another power supply) to cause the voltage to go above the setting. The OVP will respond to this condition as outlined above. If the external power source can provide more current than the rating of the power supply and an SCR circuit is used in the power supply, it is prudent to put a fuse in series with the external source of power to prevent damage to the power supply SCR and/or output circuit from excessive current.
So you can see that there are a number of ways in which the output voltage can rise above the setting. Luckily, Keysight design engineers are aware of these possibilities and have lots of experience adding protection circuits to prevent damage to your DUT!

Occasionally, one of our power supply users contacts us with a question about voltages measured from one of the power supply output terminals to earth ground (same as chassis ground). All of our power supply outputs are floating with respect to earth ground. See my previous post about this here. In that post, I stated that neither output terminal is connected to earth ground. To be more specific, no output terminal is connected directly to earth ground. We do have internal components, mainly resistors and capacitors, connected from each output terminal to earth ground. These components, especially the caps to ground, help mitigate issues with RFI (radio-frequency interference) and ESD (electrostatic discharge). They help prevent our power supplies from being susceptible to externally generated RFI and ESD, and also help to reduce or eliminate any internally generated RFI from being conducted to wires connected to the output terminals thereby reducing RFI emissions.

So even though our outputs are considered floating with respect to earth ground, there frequently is a DC path from at least one of our output terminals to earth ground. It is typically a very high value resistor, such as several megohms, but could be as low as 0.5 MΩ. This resistor acts as a bleed resistor to discharge any RFI or ESD caps to earth ground that could be charged to a high float voltage.

As an example of a power supply with a resistor to earth ground, the Keysight N6743A has 511 kΩ (~0.5 M) from the minus output terminal to earth ground. This resistor was responsible for the voltage measurements to earth ground observed and questioned by one of our power supply users. He was using this power supply in the configuration shown in Figure 1 and measured 9.7 Vdc from his common reference point to earth ground (again, same as chassis ground).



He understandably did not expect to measure any stable voltage between these points given that the output terminals are floating from earth ground. But once we explained the high impedance DC path from the minus output terminal to earth ground inside each power supply (see Figure 2), and the 10 MΩ input impedance of his DMM, the measurement made sense. The input impedance of the voltmeter (DMM) must be considered to accurately calculate the measured voltage. This is especially true when high impedance resistors are in the circuit to be measured.



Figure 3 shows the equivalent circuit which is just a resistor divider accounting for the 9.7 V measurement. (The exact calculation results in 9.751 V.) Notice that the voltage of the 28 V power supply does not impact this particular voltage measurement (but its resistor to ground does). If the user had measured the voltage from the plus output of the 28 V power supply to earth ground, both the 28 V supply and 20 V supply would have contributed to his measurement which calculates out to be 37.05 V (if you check this yourself, don’t forget to move the 10 MΩ resistor accounting for the different placement of the DMM impedance).



So you can see that even with power supply output terminals that are considered floating, there can still be a DC path to earth ground inside the supply that will cause you to measure voltages from the floating terminals to ground. As one of my colleagues always said, “There are no mysteries in electronics!”

Previously I posted about hurricane Irene and inverters. In that post (click here to read), I talked about the power ratings for inverters and just skimmed the surface about the differences between ratings in watts (W) and volt-amperes (VA). In this post, I want to go further into detail about these differences. Both watts and VA are units of measure for power (in this case, electrical). Watts refer to “real power” while VA refer to “apparent power”.

Inverters take DC power in (like from a car battery) and convert it to AC power out (like from your wall sockets) so you can power your electrical devices that run off of AC (like refrigerators, TVs, hair dryers, light bulbs, etc.) from a DC source during a blackout or when away from home (like when you are camping). Note that this power discussion is centered on AC electrical power and is a relatively short discussion about W, VA, and inverters. Look for a future post with more details about the differences between W and VA.

Watts: real power (W)
Watts do work (like run a motor) or generate heat or light. The watt ratings of inverters and of the electronic devices you want to power from your inverter will help you choose a properly sized inverter. Watt ratings are also useful for you to know if you have to get rid of the heat that is generated by your device that is consuming the watts or if you want to know how much you will pay your utility company to use your device when it is plugged in a wall socket since you pay for kilowatt-hours (power used for a period of time).

The circuitry inside all electronic devices (TVs, laptops, cell phones, light bulbs, etc.) consumes real power in watts and typically dissipates it as heat. To properly power these devices from an inverter, you must know the amount of power (number of watts, abbreviated W) each device will consume. Each device should show a power rating in W on it somewhere (390 W in the picture below) and you can just add the W ratings of each device together to get the total expected power that will be consumed. Most inverters are rated to provide a maximum amount of power also shown in watts (W) – they can provide any number of watts less than or equal to the rating. So, choose an inverter that has a W rating that is larger than the total number of watts expected to be consumed by all of your devices that will be powered by the inverter.


Volt-Amperes: apparent power (VA)
VA ratings are useful to get the amount of current that your device will draw. Knowing the current helps you properly size wires and circuit breakers or fuses that supply electricity to your device. A VA rating can also be used to infer information about a W rating if the W rating is not shown on a device, which can help size an inverter. Volt-amperes (abbreviated VA) are calculated simply by multiplying the AC voltage by the AC current (technically, the rms voltage and rms current). Since VA = Vac x Aac, you can divide the VA rating by your AC voltage (usually a known, fixed number, like 120 Vac in the United States, or 230 Vac in Europe) to get the AC current the device will draw. To combine the apparent power (or current) of multiple devices, there is no straightforward way to get an exact total because the currents for each device are not necessarily in phase with each other, so they don’t add linearly. But if you do simply add the individual VA ratings (or currents) together, the total will be a conservative estimate to use since this VA (or current) total will be greater than or equal to the actual total.


What if your device does not show a W rating?
Some electrical devices will show a VA rating and not a W rating. The number of watts (W) that a device will consume is always less than or equal to the number of volt-amperes (VA) it will consume. So if you need to size an inverter based on a VA rating when no W rating is shown, you will always be safe if you assume the W rating is equal to the VA rating. For example, assume 300 W for the 300 VA device shown in the picture above. This assumption may cause you to choose an oversized inverter, but it is better to have an inverter will too much capacity than one with too little capacity. An inverter with too little capacity will make it necessary for you to unplug some of your devices; otherwise, the inverter will simply turn itself off to protect its own circuitry each time you try to start it up, so it won’t work at all if you try to pull too many watts from it.

Some electrical devices will show a current rating (shown in amps, or A) and not a VA rating or W rating. Usually, this current rating is a maximum expected current. Maximum current usually occurs at the lowest input voltage, so calculate the VA by multiplying the current rating (A) times the lowest voltage shown on the device. Then, assume the device consumes an equal number of W as mentioned in the previous paragraph. For example, the picture below shows an input voltage range of 100 to 240 V and 2 A (all are AC). The VA would be the current, 2 A, times the lowest voltage, 100, which yields 200 VA. You could then assume this device consumes 200 W.

Ground loop errorA true ground potential is something that only exists on paper or in simulations. In the real world there is no such thing as a true ground which in test and measurement leads to ground loop errors. Ground loops present problems when measuring low level signals such as thermocouple measurements. When measuring voltages in circuits where the DMM and the device-under-test are both referenced to a common earth ground, a ground loop is formed. As shown in the figure, any voltage difference between the two ground reference points (Vground) causes a current to flow through the LO measurement lead. This causes an error voltage (VL) which leads to inaccuracies in the DMM’s measurement.

When considering ground loops just in terms of DC, as long as Ri is a large value (meaning air between the two potentials) the error will be fairly insignificant when measuring mV and up. Keysight’s Truevolt DMMs such as 34460/61/65/70A have a Ri of 10 Gohm at 80% humidity. 80% humidity is high for a lab environment so in most settings the actual Ri is much greater than 10 Gohms. Error caused by DC ground loops can be further reduced by keeping the ground path of low level signals as short as possible.

The bigger source of noise and error from ground loops is the AC component. The DMMs impedance to ground is lower with AC because of the capacitive component, Ci, in parallel with Ri. The capacitive component results from the windings in the transformer inside the DMM. Referring to the Z calculation at the bottom of the figure, as the frequency increases the Z isolation of the DMM to ground begins to decrease. Now in most low frequency settings the ground loop noise is from the power line so it is 60 or 50 Hz. The effect of AC power line ground loop noise can be reduced by setting the DMM’s measurement integration time to 1 or more power line cycles (for 60 Hz that is 16.67 ms). If your testing environment consists of high frequency signals, high speed digital signals, or noisy components like relays or motor it is best to put any sensitive voltage measurements on a separate ground potential if possible.

 

For the ground loop Wikipedia page click here

This is part 2 of a 2-part post that takes a comprehensive look at all of the factors that can lead to errors in a DC voltage measurement with a DMM and how to eliminate them so you can achieve the highest accuracy possible in your measurement. In part 2 we will cover the following topics: loading errors, power-line noise and injected current noise. If you are a seasoned DMM measurement veteran and you feel I missed something in the following sections, please add it as a comment.

 

Loading Errors Due to Input Resistance — Measurement loading errors occur when the resistance of the DUT is an appreciable percentage of the DMM’s own input resistance. The figure below shows this error source. To reduce the effects of loading errors, and to minimize noise pickup, see if your DMM allows you to set its input resistance to a higher value. For instance, using any of Keysight’s Truevolt DMMs such as 34460/61/65/70A, input resistance can be set from 10 M to > 10 G for the 100 mVdc, 1 Vdc, and 10 Vdc ranges.

Loading errors

Ri should be much larger than Rs or loading error will be a factor in the measurement

 

Power-Line Noise — This type of noise is caused by the powerline voltage signal (50 Hz or 60 Hz) being coupled onto the measurement setup either from the DUT, the DMM, or both. This noise appears as an AC ripple summed on top of the DC level you are measuring. To eliminate this common noise source DMM designers use integrating or averaging measurement time settings that are integer multiples of the powerline noise's period. Remember if you integrate over a sine wave you get zero. This is typically called normal mode rejection or NMR. If you set the integration time to an integer value of the powerline cycles (PLCs) of the spurious input, these errors (and their harmonics) will average out to approximately zero. For instance, the Keysight Truevolt DMM provides three integration times to reject power-line frequency noise (and power-line frequency harmonics). When you apply power to the DMM, it measures the power-line frequency (50 Hz or 60 Hz), and then determines the proper integration time. The table below shows the noise rejection achieved with various configurations. For better resolution and increased noise rejection, select a longer integration time.

Integration time to integer value of the PLC

 

Noise Caused by Injected Current — Residual capacitances in the DMM’s power transformer cause small currents to flow from the LO terminal to earth ground. The frequency of the injected current is the power line frequency or possibly harmonics of the power line frequency. The injected current is dependent upon the power line configuration and frequency. With Connection A (see figure below), the injected current flows from the earth connection provided by the circuit to the LO terminal of the DMM, adding no noise to the measurement. However, with Connection B, the injected current flows through the resistor R, thereby adding noise to the measurement. With Connection B, larger values of R will worsen the problem.

Injected current noise

The measurement noise caused by injected current can be significantly reduced by setting the integration time of the DMM to 1 power line cycle (PLC) or greater.

 

If you think I missed anything or if you have a question, please leave it in a comment

 

Click here to read part 1 of this 2-part post

 

Click here for more info on Keysight DMMs

 

 

In this two-part post we will (or at least attempt to) take a comprehensive look at all of the factors that can lead to errors in a DC voltage measurement with a DMM and how to eliminate them so you can achieve the highest accuracy possible in your measurement. In part one we will cover radio frequency interference, thermal EMF errors, noise caused by magnetic fields, and common mode rejection. If you are a seasoned DMM measurement veteran and you feel I missed something in the following sections please add it as a comment.

Radio Frequency Interference -- Most voltage-measuring instruments can generate false readings in the presence of large, high-frequency signal sources such as nearby radio and television transmitters, computer monitors, and cellular telephones. Especially when the high frequency energy is coupled to the multimeter on the system cabling. This effect can be severe when the cabling is 1/4, 1/2, or any integer multiple of the high frequency wavelength. You probably have experienced this type of effect first hand if you ever placed a mobile phone near speaker wiring and heard bursts of noise from the speaker that were certainly not part of the intended audio experience. To reduce interference, try to minimize the exposure of the system cabling to high-frequency RF sources. You can add shielding to the cabling or use shielded cabling.  If the measurement is extremely sensitive to RFI radiating from the DMM or your DUT, use a common mode choke in the system cabling, as shown in the figure below, to attenuate DMM emissions. Often you can see this same EMI reducing method being used on the data cable for your computer monitor.

Figure 1

Thermal EMF Errors -- Thermoelectric voltages, the most common source of error in low level voltage measurements, are generated when circuit connections are made with dissimilar metals at different temperatures. Each metal-to-metal junction forms a thermocouple, which generates a voltage proportional to the junction temperature. It is a good idea to take the necessary precautions to minimize thermocouple voltages and temperature variations in low level voltage measurements. The best connections are formed using copper-to-copper crimped connections. The figure below shows common thermoelectric voltages for connections between dissimilar metals.

Figure 2. Thermoelectric voltage table of dissimilar metals

 

Keysight benchtop DMMs use copper alloy for their input connectors

Noise Caused by Magnetic Fields -- When you make measurements near magnetic fields, take precautionary steps to avoid inducing voltages in the measurement connections. Voltage can be induced by either movement of the input connection wiring in a fixed magnetic field, or by a varying magnetic field. An unshielded, poorly dressed input wire moving in the earth’s magnetic field can generate several millivolts. The varying magnetic field around the ac power line can also induce voltages up to several hundred millivolts. Be especially careful when working near conductors carrying large currents. Where possible, route cabling away from magnetic fields, which are commonly present around electric motors, generators, televisions and computer monitors. In addition, when you are operating near magnetic fields, be certain that the input wiring has proper strain relief and is tied down securely. Use twisted-pair connections to the multimeter to reduce the noise pickup loop area, or dress the wires as closely together as possible.

 

Stay tuned for part 2!

 

Click here for more info on Keysight DMMs