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CMRR measurement, am I doing it right?
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ballsystemlord:
Hello,
I followed Dave's video here: https://youtu.be/vDe_BHvRpks
And arrived at the following formula:
(20 * log10( RMS_diff_probe / input )) + dB
dB == 20 for 10x probe and dB == 40 for 100x probe.

I then applied it to a differential probe I got and I achieved a much higher than expected CMRR number, so I thought I'd ask here.

Attached is a screen shot of my test setup and the HT8100's (100MHz by Hantek) results. I also am including a screen shot of my PVP2350 passive probes's CMRR for comparison. I'm using an MSO5354. I'm feeding a 1MHz signal into the scope through a 50ohm termination. The differential probe is a 50X probe. Just out of curiosity, I turned on the FFT to see where the noise was coming from.

My calculation is as follows:
(20*log(0.018349/2.4508))+30 == -67.8919dB
The CMRR on the datasheet says >= 50dB at 1MHz.

NOTE: The scope is set to "high res" mode, meaning, it's doing averaging or over sampling to get a more accurate result. Therefore, it's BW is limited to 55MHz in the screen shots. If you turn off "high res" mode the RMS noise increases by about 10mV.

EDIT: The FFT is set to 70dB offset and 5dB/div. It's span is 500Hz to 1.2Mhz. My thinking was to try and see if the 1Mhz signal was leaking through (and it's not).

Am I doing the CMRR measurement right?

Thanks!

Found out what I was doing wrong. See here: https://www.eevblog.com/forum/testgear/cmrr-measurement-am-i-doing-it-right/msg5641429/#msg5641429
CosteC:
Can you describe more setup you used? CMRR for passive, single ended probe puzzles me.

Did you used FFT to measure CMRR?
tszaboo:
Can you please draw a quick sketch about your test setup, including where you are inserting the common mode signal?
This needs special investigations of the ground signals, it's very easy to make a loop.
ballsystemlord:
Here's the drawing of how I connected up the probe. It's pretty much identical to Dave's setup. I have coax to a tee which goes into port 1 through a 50ohm feed-through resistor. The other side of the tee goes into a BNC->DMM adapter. Both leads of the differential probe are connected to one terminal of the adapter. I just unplug the diff probe and connect up the passive one identically.

Thanks!
ballsystemlord:
Here's a picture of the feed-through terminator.
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