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active low-pass filter from Valhalla 2701C DC standard
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fenugrec:
Hi,
I'm looking at the analog portion of the Valhalla 2701C "precision DC voltage / current standard" and am intrigued by this one low-pass filter that I don't recognize.

For a bit of context , this unit works by generating a PWM waveform with a period of 40ms, filtering aggressively then driving the voltage/current amplifier (chopper amp, relay-switched feedback networks for different ranges; nothing very special).



Their low-pass filter is very interesting - instead of feeding the signal *through* the opamp like for example a Sallen-Key (which would hurt offset and drift specs), it's acting as a kind of "shunt" filter [EDIT - perhaps more akin to a capacitance multiplier ?] The only DC effect I can imagine is whatever leakage through the caps C6 and C7, but even that would be minimal since all nodes of the opamp should be within a few mV of 0 V.

I'm wondering if this is a common type of filter, how is it called, and how is it designed ?
tggzzz:
The Tek 4x5 DM44 contains a similar filter. To function correctly it needs a constant source impedance, unsurprisingly.
Kleinstein:
This type of question came up before, but AFAIK no name for this type of filter was found.
If nothing other helps one can do the design / choice of values with the support of a simulation. If needed one could so the circuit analysis by pen and paper the old way.

This type of filter and the higher order variants are used quite a bit in other instruments too, e.g. Fluke 5700, Datron 4910, Fluke 8840, Datron 1281. The analog filter in the 3456 is similar, though not exactly the same.

A change in the source impedance only shifts the cross over frequency. So some DMMs like the Datron 1061 have such a filter also at the input, where the impedance can change.
Wimberleytech:
Never seen this before.  Excluding the effects downstream of the right side of the 99.5k resistor, we have:
G1 = 1/95k
G = 1/45k
C = 1u

H(s)= -G1*G2 / s3c3 + s2*2c2G + SCG2 + G1G2

...assuming I did make some mistake

<edit>
lol

...assuming I did NOT make some mistake
RandallMcRee:
This type of low pass filter is generally known as a "zero offset" or "no dc offset" filter.

E.g.
https://ir.library.louisville.edu/cgi/viewcontent.cgi?article=2077&context=etd
https://www.edn.com/zero-offset-active-lowpass-filter-part-1/

I think with better opamps now available it has seen its day in the sun.
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