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Operational amplifier gain stage and PIC ADC input

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flore

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hi everyone,
I am trying to make one DC current meter like in the schematic attached.
My problem is when the load is the DC motor, the A/D result is random.

Any idea how to avoid this?

 

The load current from a DC motor probably is random ! It depends on the mechanical motor load, friction, inductance of the motor windings and commutator position.

The only way to get an AVERAGE reading is to use an averaging filter. You can do this in software by taking many readings and mathematically finding their mean or you can use an RC filter to average the voltage itself.

You are using very high gain in the amplifier which is not helping the situation.

Be careful with your circuit, if you are using positive and negative supplies to the op-amp you should protect the PIC from possible negative voltages at its ADC pin.

Brian.
 

thank you for your response.

I try both solutions:
- averaging in software with 100 samples per second, did not help;
- RC filter solve the problem but with high values (R=10k C=10uF)

The problem was the DC motor used for testing.
Anyway the gain of the amplifier is not so high, R3 is 10K.


florentin.
 

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