 Difference amplifier acting as a voltage integrator
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TheMerovingian
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Mon Dec 08 2008, 11:28PM
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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
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For my project (a Wind generator monitor station with LCD display and radio transmissions) i use a 1mOhm shunt for 0-20A of current passed in a precision difference opamp with 249x gain to obtain 5V full scale fed to a analog port of a PIC16f876a microcontroller. The difference amplifier (not rail to rail, using a -12V supply also) is a classic design with a 2 precision 1Kohm resistor and 2 precision 249Kohm resistor with 1uF 50V lytic capacitors in parallel acting as a low pass filter, at least i thought that they would work in that way. Anyway i apply a 2.5A current to the shunt and the display quickly jumps to that value, and than drifts positive at constant rate. Removing the two capacitors of course solved the problem but will screw up adc conversion with a current with some ripple on it(the adc conversion is much faster than the frequency of the alternator). I don't understant why the amplifier acts as a integrator, the 249K parallel resistor should limit the gain to 249x for DC and less for higher frequency components. Maybe capacitor inequalities or leakage currents play a role in this strange behauviour. If i cannot find a solution i will stick to an output filter with an RC network
Edit: this behauvior starts when the wire from the shunt to the battery gets hot, rising its resistance, so it is probably a common-mode gain problem, keeping the resistance of the wires low (preventing them to change resistance, causing rapid change in common mode voltage) eliminates the drift.
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