Tektronix ADA400A CMRR fails miserably

[FlexibleMammoth]

Dear all,

I have recently treated myself to a Tektronix ADA400A differential probe. It is basically a TekProbe version of some older series 5000/7000 plugins.

I have started doing a performance verification, and found that while gain, zero offset and LF CMRR are excellent, the HF CMRR is way off. Like, "10x higher than it should be" off.

The manual [1] has a procedure for alignment, so I went through that. On page 47, HF CMRR is adjusted to be <1mV when applying a 100kHz, 1Vpp, 50Ohm terminated signal to both inputs. For my probe, it's more like 12-13mV (see attached screenshot) and I am unable to get it down further using CV4 and R18.

Sanity checks: I have verified +/- 15V and +5V on the board. It appears the last wire (Brown) does not carry -5V, but rather indicates the attenuation factor to the scope. For giggles, I have played around with different lead lengths on the inputs, but even a 2.5m length difference does not really change the picture other than introducing additional spikes from reflections. There is no visual damage to any component that I could see and no leakage from the capacitors.

EDIT: I am using the Siglent SAG1021I frequency generator for my experiments. While it does not have great rise times, it's the best I have on hand and it should not really matter at the frequencies we are interested in here, especially when the same signal is fed into both inputs.

What am I missing here?

[1] https://download.tek.com/manual/070916402.pdf

[alm]

Are you feeding it a sine wave as the manual specifies? The screen shot looks like the higher frequency component of a square wave to me, which would be much higher frequency than the fundamental frequency. So you might actually be measuring the CMRR at 1 MHz instead of 100 kHz.

Tektronix does specify a length-matched cable: https://w140.com/tekwiki/wiki/067-0525-02.

[FlexibleMammoth]

Oh. My. God. Thanks for noticing! I was feeding a square wave, which was still configured from the last measurement. That one naturally has frequency components to the moon. I will retry and report back asap.

Also, thanks for the hint about the special Tek cable - unfortunately eBay was not kind on that one, so I had to come up with *ahem* custom solutions which definitely is not just the shortest wire I could fit between the inputs.

EDIT: As expected, this magically cured the CMRR illness and i was even able to adjust to <100uV common mode voltage! I am leaving this post up for the next person that is scratching their head and as a humorous reminder to RTFM carefully. Thanks again to alm for your quick help!