EEVblog 1521 - Common Mode Rejection Ratio (CMRR) Explained & Measured

What is Common Mode Rejection Ratio (CMRR) and how to measure it on a high voltage differential probe or differential amplifier. Using the new Rohde & Schwarz MXO4 oscilloscope.

Use coupon code ProbeyMcprobeFace for 15% of the HVP70 differential probe:

00:00 - What is CMRR?
01:53 - Why twist the wires?
03:26 - Typical CMRR figures
04:51 - How to measure CMRR - The Setup
08:54 - Remote R&S Oscilloscope desktop view & settings
11:22 - Output to input voltage ratio
11:58 - Beware of DC offset
13:36 - Does the 20MHz CMRR measaurement match?
14:02 - Input referred measurement
15:02 - Compensating for the gain
16:29 - Automated Frequency Response Analysis

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[TheDefpom]

Part of the common mode amplification of the probe when connected together @8:00 could also be due to the connection point used, if the noise/source frequency is high enough the connections will act like a transmission line and there could be a phase difference over the length between the connections being side by side and not stacked at the same point, that phase difference could produce an effective shift for the differential probe.

[KeritechElectronics]

1kV RMS max, nice thing to measure the signal across the vacuum tube amp's output transformer primary winding.
Cool way of demonstrating the common mode interference.

[cowshittt]

I wouldn't call it "phase reversal", that kind of implies change of impedance response (capacitive to inductive). It is just a phase wrap, since the screen is limited to +- 180 degree.

[electronics.unmessed]

👍Wow, pretty advanced features that you are explaining here! Many thanks.

[IanScottJohnston]

Very nice explanation Dave, enjoyed this one very much.

[AppleTheBandit]

Dave, you should make a note that you’re measuring CMRR from input to output (the smaller, the better), while the formulas you show and what you describe display CMRR as the ratio of differential gain to common mode gain (the larger, the better). I can see an astute viewer being confused. Otherwise, great video.

[dardosordi]

This is really great stuff Dave! I loved every bit of it, it's well polished.

[qzorn]

I love these kind of videos. They make learn and relearn things I have dealt with for some time. 😎 Thanks and happy 2023 new year. 🥳

[erin]

One-of my first projects in electronics as a design tech was to build a common mode amplifier. For the life of I had no idea what there was to achieve expect build an amplifier that had a gain of 1 or less. The cathode follower ! I used an RCA low noise high frequency 6DS4.

[djpenton]

Beginner here. I am stumbling around attempting to build me own constant current supply. So CMRR in this context is mostly about (sort of) DC voltages across an op amp or current sense IC. By "sort of" DC I mean that of course there is some variation across the shunt resistor. But here's my question: WHy, in this context is the common mode voltage defined as the average of the two inputs? The actual voltage across the inputs (shunt voltage drop) is clearly different, not the same. So why do we actually calculate a common mode voltage as the average? This seems to be different that the notion of say AC hum noise common to both inputs.

[SeanBZA]

Wanted to buy one, but country is not available off the online

[optroncordian]

For me, a real test is to solder both inputs together on the board and then measure. Now the output can result from the difference in length and position in the twisted pair of the probe wires. However, it is not worth the effort considering the very high rejection ratio even without that.

[NeverTalkToCops]

Today, Daiyve demonstrates a set up where the probe costs LESS than the oscilloscope!
Just grab that R & S scope for $23, 632.

[GoBO]

I know this doesn't make sense, but I actually prefer looking at the scope from the cameras perspective rather than the screen capture. Wouldn't be the same for a smaller scope, but for this one...

[DK-elektronik]

Thank you Dave!
I've a question. I don't saw you set up HVP-70 with MXO4. With 1/10 attenuation on Probe, MXO4 will get 5mV/div minimum; but the screen showed 2mV/div. is this right?

[ZergZfTw]

Wouldn't a VNA be a better choice to test CMRR over a frequency sweep? Something like the Bode 100 or similar.

[MrSnoots]

I never got a satisfying answer to what "common mode" noise was. I always saw CM chokes and wondered why they were needed.

My understanding is common mode is simply a signal that it applies to all input lines (without prejudice). Further, since lengths of wire are effectively antennas, they can pick up high frequency interference which a choke will filter out due to the magnetic flux, which differential signals will pass through due to the topology of the choke and its windings.

This video seems to confirm my understanding. Is there any additional nuance that I'm missing?

[guillepk]

Why Ch1 changed from 1.75V to 1.80V when changing frequencies? The attenuation in the circuit should be linear, right?

[denk]

Too late to ask Santa this gorgeous oscilloscope

[joopterwijn]

Guess you need to put the whole setup in a faraday cage to reject as much noise as possible