What is Common Mode Rejection Ratio (CMRR) and how to measure it on a high voltage differential probe or differential amplifier.
“10 things your doctor won’t tell you about CMRR”
At least for the lower frequencies one can get a bit more amplitude (an extra 6dB) if one skips on the 50 ohm termination. It gets a bit dubious later on.
An amplifier with only a gain of 4 makes some sense. 4 V may be still just OK for the internal 50 Ohm terminations, though the program calls for 3.5 V max.
Part of the common mode gain could depend on the exact twisting / position of the cables. So ideally one should repeat the measurement a few times with different cable positions.
Our situation has not improved, so why is it 8dB out of spec there at 20 kHz?
Printed circuit board "hook" will cause that in high impedance circuits. It means that they did not use the right type of printed circuit board material, and its dielectric constant versus frequency at low frequencies changed after testing or design. One of my older posts includes an article about "hook".
This is why before "hook" was understood, Tektronix built their high impedance circuits on special board materials like polysulfone. FR4 type board materials must be qualified for high impedance low frequency applications.