Author Topic: Ohmmeter Circuit  (Read 884 times)

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Offline Burning CircuitsTopic starter

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Ohmmeter Circuit
« on: July 06, 2020, 05:58:07 pm »
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?
 

Online Kleinstein

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Re: Ohmmeter Circuit
« Reply #1 on: July 06, 2020, 07:18:09 pm »
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.
 

Offline David Hess

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Re: Ohmmeter Circuit
« Reply #2 on: July 07, 2020, 07:56:04 pm »
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.

 

Online Kleinstein

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Re: Ohmmeter Circuit
« Reply #3 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.
 

Offline Burning CircuitsTopic starter

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Re: Ohmmeter Circuit
« Reply #4 on: July 07, 2020, 08:42:39 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.
LT1112 don't have a very low input bias current? 250 pA from the datasheet  :-//
 


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