AC RMS is Stdev ... 
Please see attached red-circled measurements from my KS - the "RMS without DC" is called "AC RMS" and "Std Dev" is a statistical standard deviation of measurement values. This "RMS without DC" measurement on a Siglent (and some others) are called "Stdev". Threw me for a loop a few times until I got it straight.
Never thought about the AC RMS (DMM) and Std Dev relationship, thanks for pointing that out 2N3055.
In the KS 34465A (TrueVolt) the RMS value is achieved in a different manner than with the older 34401A which used a "Analog RMS (AD636) circuit". Think this is a sampled data type RMS computation, rather than an Analog type RMS computation.
Best,
This got my curiosity going after I did a partial calibration of an HP34401A and a Agilent 34401A using some of the custom developed Reference Circuits & PCBs. These circuits are based upon a couple of Precision Buried Zeners (LM399 & LTZ1000) and a Analog Devices IC reference, and one circuit uses a CMOS precision chopper circuit to produce a very accurate AC waveform derived from a precision voltage reference and a crystal oscillator at 4.096MHz divided down with a precise 50% duty cycle (guaranteed by design). A new Keysight 34465A was used as the standard measurement, and the HP and Agilent 34401A also used.
First off DC measurements of the SDS2102X Plus input using statistics of C1
Input KS34465A of 5.00021VDC, DSO reads Mean C1 5.000398VDC (mean of mean)
Input of 10.00001VDC, DSO reads 10.0031VDC
Input of 7.04184VDC, DSO reads 7.04192VDC
Input of 0.99985VDC, DSO reads 999.39mvDC
Input of 100.0073mvDC, DSO reads 99.825mvDC
Now AC measurement with 50% duty cycle square wave of 5.00021V peak value, so Mean & StdDev (RMS) should be 5.00021/2 or 2.500011V
Input at 250Hz KS34465A 2.49993VAC, DSO reads 2.4916VDC (mean) and 2.5085VAC (stddev)
500Hz 2.49993VAC, DSO reads 2.4912VDC & 2.5094VAC
1KHz 2.49886VAC, DSO reads 2.4910VDC & 2.5095VAC
4KHz 2.49452VAC, DSO reads 2.4913VDC & 2.5092VAC
8KHz 2.48868VAC, DSO reads 2.4921VDC & 2.5085VAC
Note: The peak to peak readings of ~6Vpp include waveform ringing.
This is better performance than I would expect from this DSO
Don't know if I got lucky or this is typical performance from the SDS2102X Plus, maybe others could make some measurements
Edit: In the 2 last PCB images the LTZ1000 Buried Zener Reference isn't installed (LM399 is), I forgot to order the proper Op-Amp that works with this reference
Best,
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