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Ohmmeter Circuit

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Burning Circuits:
Hi,
the attached picture is the ohmmeter stage of a bench multimeter.

We have two current sources (IC26 and IC9-B), and an analogue multiplexer (IC8).

I don't understand the purpose of IC9-A ... maybe limit the output voltage when there isn't a resistor to probe?

Kleinstein:
IC9 is limiting the voltage with little leakage current. It is still odd to used a zener diode at near zero voltage for low leakage.

David Hess:
I agree with Kleinstein; the two series zener diodes clamp the output voltage and IC9 bootstraps zener diode D20 for minimum leakage.

I think the designer incorrectly assumed that D20 would have low leakage at low voltages without being a low leakage diode; bootstrapping it does lower the leakage but not to the level of a true low leakage diode.  I would have used a low leakage diode, or two, in place of D20.

Kleinstein:
The choice of IC9A and IC26 is also a little odd:  IC9A still has some avoidable input bias and IC26 has quite some noise and drift, even though attenuated by a factor of 5. I would more like swap the two OP.

Burning Circuits:

--- Quote from: Kleinstein on July 07, 2020, 08:19:20 pm ---The choice of IC9A and IC26 is also a little odd:  IC9A still has some avoidable input bias and IC26 has quite some noise and drift, even though attenuated by a factor of 5. I would more like swap the two OP.

--- End quote ---
LT1112 don't have a very low input bias current? 250 pA from the datasheet  :-//

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