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INA333: microvolt bridge measurement

allen dominek
Expert 1515 points
Part Number: INA333

Hi,


Back in 2013, there was a post on measuring microvolt signals.  Michael Mock responded that one should us an ina333 instrumentation amplifier (5852.Shardul_Bridge_Sensing_10.4.13.TSC).  I do not know if the  person tried the suggestion or not.

I have a similar need.  I have to measure the transient pressure variation in a chamber which lasts about 2 ms (measurement bandwidth of a couple kHz).  I was going to attempt a solution as suggested above but the cmrr is in the order of 100 dB.  If the cm voltage on the bridge is 2.5 volts, the smallest signal the I could measure might be about 25 uvolts.  This is not sufficient for me.  I would like my minimum to be 1 uvolt or so.  If I am correct, is there anything else that I could do?  Would I have a better chance to succeed if I used the ina333 in a dual supply mode instead of single ended so the cm voltage from the bridge 0 if properly balanced?  Would this relax my cmrr requirement?

Michael did not address the power supply requirement.  I suppose I would also need a low noise supply for this measurement.

A completely difference approach would be to use a adc such as a ads12701.  This would have the bandwidth I need and I believe a better cmrr too.  This would be a more complex solution since a processor would have to be interfaced between the adc and computer.  The ina333 approach could use an oscilloscope to collect the data.

Thanks.

Al

  • 0
    TI__Guru 67601 points

    I believe you might be incorrectly interpreting CMRR limitation of INA333.  You would have to change the input common mode voltage, Vcm, by 2.5V to see up to 25uV CMRR related error.  However, you are hardly changing Vcm at all; if you use the circuit in a high gain, like G=47,047 shown below, the most you may change the input common-mode voltage is 100uV (peak-to-peak value of the input signal), resulting in the input referred CMRR related error of just 1nV (100uV/100,000).

    Of course, for such high gains, first you would have to calibrate the system, based on 0V initial input signal, and include the initial error in your final calculations.