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Digital Ammeter with shunt help please

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_gmd_

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digital ammeter circuit

Following problem:
I have some digital ammeters with ICs and they require the shunt to be mounted in the ground line. Wont work in plus line. When I have the + voltage on the Analogue Input the ammeter will not work and blow something up.

How can I convert the +12V signal against ground to the 0V signal required with the shunt in the + line. An analogue meter will work no problem .. the voltage between the shunt points is 0-80mV. With the shunt in ground line it works no problem.

Anybody has a little circuit to convert the input signal ??

thanks
gmd
 

AdrianN

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digital ammeter

Your ammeter cannot take high common-mode voltage levels and that is why it blows.

You could use a sense resistor in the positive rail to measure current, and use a differential amplifier to bring the voltage drop across that resistor to some useful levels. Or use an instrumentation amplifier. Use a voltmeter at the amplifier output.

I have an article that explains this. You can find it here:

http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-2/

The article explains the common-mode voltage error for precision applications. For your case, read the example with the power supply output current measurement.
 

    _gmd_

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_gmd_

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ammeter shunt

Spot on mate .. you get all the points I have :))
Ive done some research and came to that conclusion and tried to built a little circuit with a LM358 but so far have no decent result. The output is stuck to 0.7
I will read your article and hopefully I will get something working.
Thanks so much for answer
gmd
 

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