Replacing AD624 with AD8295

Hi all,

We are using AD624 Instrumentation Amplifier for our signal Conditioning Application for Strain gauge based sensor like Force and Torque Sensors.  Here we get Voltage in mV and we are using AD624 as IA and then a second order Butter worth filter using OP177.

We are planning to replace AD624 with AD8295 IA, due to space constrains of AD624.

We have few doubts regarding how to implement the same circuit using AD8295.

1. We have implemented a offset null circuit in AD624[to null the offset voltage coming from the sensor] using the pins 4 & 5[Input Null].  As there is no such pin as Input Null in AD8295, please suggest us circuit which we could you to null offset either before or after AD8295.

2. Further we are using pins 1 and 8[VosTRIM] of OP177 to fine tune the Zero Voltage of the output.  Please suggest a circuit where we can do the same without using the VosTRIM Pins, as we are planning to use small Op-Amp array IC where VosTRIM pin wont be available.

I have attached the circuit that we are using, for reference.

attachments.zip
  • Hi anatha,

    The easiest answer would be to remove the offset in software, but that comes at the expense of some resolution. The AD8295 IA output can be level-shifted by applying a voltage a the REF pin, however the voltage must be low impedance. So if you are using a resistive divider with a trim potentiometer, you will need a buffer to apply the voltage to the REF pin.

    As for the trim of the later op-amps, it depends on your error budget, but I wouldn't expect this to affect you much. Offset errors from these amplifiers are divided by the in-amp gain when referred to the input. For example, if your in amp is in a gain of 60 and you use a precision op amp like the OP177 with a max Vos of 60uV, that's only 1uV offset referred to input.

    Unlike Vos trim pins, the REF pin method can be used to zero the system offset. Still, you will probably want to make sure your range of adjustment is not much more than your maximum expected offset, that way the potentiometer tolerance and tempco doesn't affect the circuit as much. There are a few more tips and tricks in this tutorial: http://www.analog.com/static/imported-files/tutorials/MT-037.pdf

    I hope this helps.

    Regards,
    Scott

  • Thank you Mr. Scott.

    I will try your solution.  I think it will solve the problem.