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Minimizing Noise Impacts When Using Oscilloscope Probes

Blog Post created by BoonCampbell Employee on Jul 3, 2018

Have you ever been fighting noise on your measurements and can’t tell where it’s coming from? There are four possible contributors:

  1. Your device under test
  2. Your probe
  3. Your oscilloscope
  4. Or a combination of all three.

 

Learn about the different ways you can minimize noise impacts and improve the quality of your measurements.

 

System Noise Consequences
You should look at your probe and oscilloscope together as one measurement system that can add noise to your measurement. Figure 1 below shows two possible noise sources from your system: one source from your probe and the other from your oscilloscope. Some amount of noise will come from the probe’s amplifier system and ride on your DUT’s signals, which is sent to the attenuator of the oscilloscope. All scopes use an attenuator to vary the vertical scale on your oscilloscope screen. Most oscilloscopes can detect your probe's attenuation ratio and will automatically adjust its vertical scale accordingly. For example, for a 10:1 probe, the oscilloscope will simply amplify both the signal and noise by a factor of ten. Be sure to keep this in mind as you minimize the noise in your signal.

 

 

Signal to noise diagram

Figure 1: Signal-to-noise diagram of the oscilloscope and probe

 

Probe Noise Impacts
Due to added inductance, the probe ground/signal loop formed by the probe and tip contributes to the noise on your signal. This noise can be reduced by:
1) Selecting just enough scope/probe bandwidth to measure your DUTs signals. Excessive bandwidth will contribute to the system’s overall noise.
2) Setting your oscilloscope’s vertical range to the most sensitive voltage range possible while still seeing your DUT’s complete signal on the oscilloscope. This will reduce the amount of gain the oscilloscope needs.

 

Oscilloscope noise floor with no probe connected

Figure 2: Oscilloscope noise floor with no probe connected

 

Oscilloscope noise floor probe connected

Figure 3: Oscilloscope noise floor probe connected

 

In Figure 2, you can see the noise floor of an oscilloscope with no probe attached is 295 µV root mean square (rms).
In Figure 3, with the probe attached to the oscilloscope, the noise increases from 295 to 485 µV rms. So, the probe itself is adding around 200 µV rms (or 67% more noise)! This noise level will reduce when your probe is well grounded, but it is worth noting the increased noise level just by adding a probe. Keep your ground and tip lengths as short as possible to reduce this effect.

 

Probe Attenuation Impacts
The probe attenuation you need is going to depend on the Voltage of the signal you are measuring. The attenuation ratio changes how the signals are fed into your oscilloscope. For example, a 10:1 probe connected to a 1V signal will pass 100 mV to the scope’s input.The oscilloscope will either read (or you can manually enter) the probe’s attenuation ratio. Then the oscilloscope will display the correct signal, factoring in the probe’s attenuation ratio. Having a higher attenuation ratio (100:1, 1000:1) will allow you to view higher voltages, but it will also make the scope’s internal amplifier noise more pronounced. The higher the attenuation ratio, the more scope noise you’ll see. For example, a 10:1 probe will show 10x the noise.

 

One easy way to estimate the amount of your probe noise is to check the attenuation ratio and the probe noise level from the probe’s data sheet or manual. Many probe manufacturers characterize the probe’s noise as equivalent input noise (EIN) and will be listed in volts rms.

 

The probe pictures in Figure 4 (from bottom left going clockwise) are examples of a 10:1 passive; 10:1 single-ended active; 50:1 or 500:1 high voltage; and a 1000:1 high voltage probe. These attenuation ratios are needed to reduce the probed signal down to levels that the oscilloscope attenuators can handle and display on the screen without clipping.

 Examples of probe attenuation and voltage levels
Figure 4: Examples of probe attenuation and voltage levels

 

Common attenuation guidelines and limitations are shown in Figure 5 below. Keep this in mind when determining the attenuation ratio you need.

 

1:110:1100:1
Suitable formeasuring low voltage, low frequency signals (<~25MHz)general purpose measurementhigh voltage measurement
Limitationslimited bandwidth, dynamic rangetypically up to 300 Vhigh probe noise

Figure 5: Probe attenuation guidelines

 

Figure 6 below compares the same signal measured by both a 1:1 and a 10:1 passive probe. The screen shot clearly shows how attenuation from a 10:1 probe can cause the oscilloscope to amplify both the signal and noise. The result is an exaggerated level of noise in the signal (green trace).

 

A 50 mVp-p sinewave measured with bot 1:1 and 10:1 probes - overstated 36%

Figure 6: A 50 mVp-p sine wave measured with both a 1:1 and 10:1 probe.

 

Higher attenuation ratios lead to higher levels of noise shown on the oscilloscope. This might lead you to believe that you should always use a 1:1 probe. But that’s not true! Lower attenuation probes typically have much higher loading on your system and may have lower dynamic range. There are tradeoffs, and you will need to pick the probe that fits your measurement best. You want a probe that can effectively measure the level of voltage on your DUT with the least amount of attenuation and lowest loading effects.

 

Conclusion
Your system noise can be exaggerated by probe and oscilloscope noise levels. Selecting the correct probe for your application with the correct attenuation ratio will lower the probe/oscilloscope added noise. As a result, your measured signal is a cleaner representation of what is on your DUT.

 

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