How to Test the CMRR of Differential Probes

Figure 1: CMRR plots for two attenuation
settings of an HVD3106A differential probe.

While recently we told you not to connect two probes to the same place at the same time, there is a case where connecting two tips of a differential probe to the same place at the same time is useful, and that is when testing the probe’s common mode rejection ratio (CMRR). CMRR is frequency dependent, so part of developing “situational awareness” of your test environment is to know how your probe behaves with different signals at different frequencies. 

Although CMRR as a function of frequency is a principal specification for differential probes, manufacturer's CMRR plots are the result of testing with a narrowband source under strictly controlled laboratory conditions. In real-world applications of probes to broadband sources, you can expect a different result. This quick test will inform you how different.

CMRR is measured by applying a common mode signal, usually a sine wave, to both inputs of a differential probe and measuring the attenuation of the probe output compared to the amplitude of the input signal. The input signal frequency is varied over the bandwidth of the probe being tested, and the CMRR is plotted as a function of frequency, as shown in Figure 1. This ratio is generally expressed in decibels. 

An in-situ test for evaluating the CMRR of your differential probe is to:

1. Acquire a signal within the probe's bandwidth in the conventional manner.
2. Short the differential inputs of the probe under test and connect both probe tips to the same point in the circuit.
3. Divide the amplitude of the test probe output by the amplitude of the input signal.

Figure 2: Testing the CMRR of HVD3106
using an upper gate drive (VG-E) signal
as the common mode signal source.

Ideally, repeat the experiment at a few different frequencies on the CMRR plot.

Figure 2 shows such a test using the upper side gate drive (VG-E) of an IGBT. This is a 15 V signal riding on a 500 V common mode signal.

The top trace in yellow shows the gate drive signal acquired in the usual manner using a High Voltage Fiber Optically Isolated Probe (HVFO). The signal shows a fast, step transition with an amplitude of about 15 V.

The center trace, shown in blue, is the output of the HVD3106A High Voltage Differential Probe being tested. This is the probe that has its two inputs shorted together, so both differential inputs see the same signal. Probe attenuation is set to the 500x factor, and the oscilloscope vertical sensitivity is set to 200 mV per division. The peak amplitude of the measured impulse is about 1 V. This is the common mode signal leakage due to the finite CMRR of the probe.

Relative to the 15 V input signal, the attenuation is 0.066, or about -24 dB. The impulse has a rise time of about 40 ns. The signal’s bandwidth is about 9 MHz. If we use that as an estimate of the signal frequency and consult the CMRR plot in Figure 1, we see that the probe specification for the 500x attenuation setting used for this test shows an expected CMRR of about -32 dB. Keep in mind that the real-world test uses a broadband source and a less-than-ideal lead configuration, but it shows us what to expect from this probe. 

If we do the same test on an HVFO probe with its inputs shorted together, then the bottom trace in magenta is the result. The sensitivity is increased to 100 mV per division to show that, basically, there is no evidence of an impulse. That is because the CMRR of the HVFO probe is greater than 140 dB down from the input signal amplitude. 

The outlined procedure is a good test of differential probe performance in a working circuit environment. It serves to confirm how accurate your measurement is and if your differential probe is matched to the measurement requirements.

Watch Ken Johnson explain more about probe CMRR in the on-demand webinar, Probing in Power Electronics, What to Use and Why, Part 1 and Part 2.

Also see:

Back to Basics: Differential Probing