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active low-pass filter from Valhalla 2701C DC standard
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David Hess:
Tggzzz mentioned that it was used in the Tektronix DM44 for the 465 series of oscilloscopes and I think Tektronix also used it in the 2236 oscilloscope which has a DMM built in.  I spent some time studying it when I first ran across it.

An alternative way to look at that circuit is as an operational amplifier based capacitance multiplier as part of an RC lowpass filter; it allows the use of low leakage film or ceramic capacitors in low frequency low pass filters.  The T-network provides the input bias current to the operational amplifier with the shunt capacitance increasing the effective feedback resistance as frequency increases.
SiliconWizard:
Couldn't that be considered a variant of a gyrator?
SilverSolder:

Same concept in Fluke 8505A:



Kleinstein:
The simple version with just 1 OP has not that many advantages over a simple active filter. One still has the leakage from the 2 capacitors. The bigger advantage comes with the higher order versions. The other advantage is that one can use different OPs for there needs: a fast JFET bases OP for the filter part and a slow AZ OP for the buffer.

The circuit is a little similar to a gyrator, in that a special impedance is simulated with the OP circuit. However this time not an inductor, but more like a extra slow reacting capacitor or "super" capacitor.
SilverSolder:

--- Quote from: Kleinstein on February 26, 2020, 08:11:15 pm ---The simple version with just 1 OP has not that many advantages over a simple active filter. One still has the leakage from the 2 capacitors.

[...]


--- End quote ---

Do film capacitors have leakage issues, generally?
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