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INA186: INA186 reference voltage input changes as input changes when used for bidirectional current measurement

Vasu Poojary
Intellectual 380 points
Part Number: INA186

Hello Experts .

We have used INA186 for measuring the bidirectional current and have given reference of 0.6V  offset the output while measuring the bidirectional current

Below is the schematic

But we see that when input sense current is changing , Reference voltage also shifting .   i.e when diffeential input is changes form +/-100mV .   the reference voltage (which is 0.6V )  also shifts by +/-50mV.

If we see internal architecture of INA186 , reference input is fed to non inverting terminal of 2nd stage amplifier as reference which may change and impact of input swing   (internal series resistance to reference is not known )

But change is 50mV is  effecting our ADC reading against the theoretical calculations ,   How to correct this ?

Are we missing anything to understand ?

Please update

thanks

regards

Vasu

  • 0
    TI__Genius 12720 points

    Hello Vasu, 

    Can you please provide a scope shot of your 3.3V supply being used to create the reference? I want to make sure this is a stable 3.3V supply.

    I also would like you to try removing C30 and C33. Either try this or increasing the size of C31 by 10x. The idea behind C31 is to reduce any dynamic offset voltage between the inputs created by inherent mismatches between the input traces, resistances, parasitics, etc. 

    Please reference this Driving Voltage Reference Pins of Current-Sensing Amplifiers app note for error calculations when driving the reference pins.

    From the example in this app note, the error for a 5V supply, 2.5V Vref, 50kOhm load on the reference is -11mV. I would suggest reducing the size of the resistor divider values as much as possible. Of course, as much as your power constraints permit.

    I would also suggest that you buffer the reference voltage for further accuracy improvements. 

    What kind of ADC is the output of the INA186 driving?

    Please also take a look at Current Shunt Amplifier Considerations for Driving SAR ADCs if the output of the INA186 is driving a SAR ADC.

    Thank you,

    Joe