# How to make simultaneous measurements with a digital multimeter

Blog Post created by Seok Jin Wong on Jul 2, 2018

Traditionally, digital multimeters (DMMs) have been single-measurement instruments. When engineers want to measure more than one parameter on their signal, they need to have two multimeters to measure two different measurements at the same time. This is not time and cost efficient.

With the right architecture and design, DMMs can make multiple measurements. This advanced feature can help you save cost and analysis time and finish your analysis faster!

### Two measurements in a single screen

Secondary measurements are defined as auxiliary measurements that augment information provided by a main primary measurement function. Depending on the function, you can measure complementary data that traditionally would have taken two different operations to acquire. The table below illustrates all secondary measurement capabilities of the Keysight’s Truevolt DMMs.

 Primary measurement function 34460A secondary measurement function 34465A/70A secondary measurement function DCV ACV ACV, peak, pre-math ACV Frequency DCV, frequency, pre-math 2-wire, 4-wire resistance - Pre-math DCI ACI ACI, peak, pre-math ACI Frequency DCI, frequency, pre-math Frequency Period Period, ACV, pre-math Period Frequency Frequency, ACV, pre-math Temperature Sensor Sensor, pre-math Ratio Input/Ref Input/ref, pre-math Capacitance - Pre-math Continuity - None Diode - None

An example of a common secondary measurement would be the ability to measure the frequency of an AC signal, as shown in Figure 1. Figure 1. AC voltage with frequency.

The secondary measurement provides more information than is possible with other digital multimeters because of the advanced secondary features in Keysight’s 34465A and 34470A DMMs. As an example, Figure 2 shows the primary measurement of DC voltage (DCV) with a secondary measurement of AC voltage (ACV). This is an especially important measurement if your signal has both an AC and DC component. Figure 2. DC voltage (primary) and AC voltage (secondary) measurements.

In DCV mode, there are two additional secondary measurements that can be made to provide insight into your signal: Peak and Pre-Math. The Peak measurement, as shown in Figure 3, keeps track of the minimum and maximum DCV readings read by the DMM. Figure 3. Peak measurement of DCV.

The Pre-Math is a very valuable measurement because it allows you to see modified readings and raw readings in one screen (Figure 4). You can also modify your primary display by applying useful math functions to your data (e.g., a null value or scaling) or filtering your data (Figure 5). See Adding Math Enables Faster Analysis section for more information on applying a math function.

Once you have applied the desired math function, the secondary display will display the raw reading without the math. This is useful for determining if the applied math is correct and if the readings are within the expected range. Figure 4. A DCV signal with dB scaling with the Pre-Math measurement Figure 5. A with the null value applied with the raw measurement on the secondary display.

You have seen the benefits of the secondary measurement capability. Below you will see how to do this without changing the instrument’s configuration.

### Example 1

A test engineer wants to monitor the temperature inside of an environmental chamber and needs a high level of confidence that the measurements are accurate. A 34465A DMM is selected due to its ability to log data and provide simple trend charts. A 5-KΩ NTC thermistor is used to spot check for accuracy. The engineer notices that the thermistor has a temperature error of a few degrees. To understand the error, the secondary display on the Truevolt DMM is turned on and temperature and resistor readings are read at the same time. According to the datasheet for the thermistor, it should read 25 ºC at 5 KΩ. The engineer’s probe is put inside of a calibrated chamber set to 27 ºC, but the probe reads 25 ºC with a 5-KΩ resistance reading, a two-degree error. After a bit of characterization, the engineer decides that he can simply add an offset value to adjust for the offset of his thermistor.

### Example 2

A system designed to apply a linear force to a small structure can provide an oscillating force with an AC signal and a constant force with a DC signal. The system designer wants to keep track of both signals concurrently to characterize how much force is being applied. Using a Truevolt DMM with its ability to make secondary measurements, he can read both the DC and AC components of his control signal at the same time.