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Lars Gunnarsson
Participant

Join Date: Sep 2011
Posts: 4

Zero setting on Fluke 887AB diff.voltmeters

09/29/2011 12:27 PM

Hi,

I have two Fluke 887 diff.voltmeters.
One with electromechanical chopper, and one (modified by Fluke) with OP-amp, (OPA111 Burr-Brown).
I can't adjust the zero setting, on the AC range , to zero.
DC range works fine
Both instruments have the same behavior.

The problem seems to be that the AC amplifier generates a small voltage, typical
20-40 microvolt which is enough to prevent the balancing.
Input post is shorted to common post.
The chopper i replaced by a NOS.

I have spent hours (nights) to solve the problem but........

I would be most grateful if anybody can give me a hint.
Regards
/Lars Gunnarsson
Stockholm/Sweden
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SimpleMind
Guru

Join Date: Sep 2009
Posts: 1056
Good Answers: 88
#1

Re: Zero setting on Fluke 887AB diff.voltmeters

09/29/2011 4:13 PM

Sounds like a ripple on the the AC amplifier power rail(s) and/or bad CMR. Test with a scope. And more details on AC stage would help. S.M.

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Lars Gunnarsson
Participant

Join Date: Sep 2011
Posts: 4
#2
In reply to #1

Re: Zero setting on Fluke 887AB diff.voltmeters

09/29/2011 5:20 PM

Hello SimpleMind

Thanks for your prompt answer and interesting suggestions!

I will measure and respond to morrow.

Regards
/Lars G
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Lars Gunnarsson
Participant

Join Date: Sep 2011
Posts: 4
#3
In reply to #1

Re: Zero setting on Fluke 887AB diff.voltmeters

10/02/2011 4:22 PM

SimpleMind,

I have measured the ripple (and noise) on the power rails +/- 18Volt with a R&S URE voltmeter.

Fluke req. 0,2mV BW > 120Hz.

Result;
a) Approx. 0,4 mV (BW 20MHz) Same on both voltages.
b) Approx. 0,08 mV (BW 100kHz). Same on both rails.
I guess ripple is ok

CMRR
Fluke req. 70dB at 400Hz

Result:
10V 400Hz put on the 877 input.
Balance at 0,15mV which is > 70dB.
I guess CMRR is ok.

There are five transistors and one n-channel FET (front end) in the AC amplifier.
The transistors are mounted in holders and seems to be selected (color coded).
I may have a noisy amplifier, a ground loop problem, bad galvanic contact somewhere or...

You asked for more details on the AC amplifier. See link below.

http://bama.edebris.com/manuals/fluke/887a/ for a circuit diagram.

Regards
/Lars G
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Lars Gunnarsson
Participant

Join Date: Sep 2011
Posts: 4
#4
In reply to #3

Re: Zero setting on Fluke 887AB diff.voltmeters

10/03/2011 11:05 AM

S.M.

I forgot to tell that I also have powered the Fluke 877 with batteries.
Result, same as with AC power i.e. no progress.
Regards
/Lars G
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