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That CMRR Video - trying to replicate Rigol MSO5000 and MicSig DP750

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6SN7WGTB:
As it says, trying to replicate the CMRR measurement technique.


Im using 5V pp sine wave at 10MHz from G1, 50Ω termination on CH1, tee with +ve output to DP joint probes on another channel, just as per Dave's video. No BW limits, 100MHz scope.

What I'm confused about are the readings of Vrms with the Rigol on the DP trace.

The Vrms from the MEASURE > ADD > VERTICAL > VRMS function says 236µV AVG (using 10,000 counts).

The "AC+DC RMS" from the MEASURE > ANALYZE > DVM says 260µV, but the "AC RMS" says 210µV, and the "DC" says -170µV.

From my maths, the DVM AC and DC readings would produce a combined RMS of indeed about 230µV.

I appreciate the DVM values are 'instantaneous' and the others obviously smoothed, but the DVM ones I am quoting are reasonably stable.

What am I missing?

I am now also, therefore, confused about the std. deviation measurement Dave refererd to...

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