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ADC7762 on board differentail amplifier

I am seeking performance specification for the on-chip differential amplifier on the ADC7762, in particular the maximum and recommended voltages for VinA+ (Pin 19) and VinA- (pin 20).  The data sheet appears to be silent in this regard.

In previous designs using AD7678 I have used an AD8139 to level shift and generate the differential input signal, but presumably the on-chip diff amp will provide a more precise level shift.

I am also curious about the recommendation (at fig 27 of the data sheet) to use an AD8021 to derive a differential input for the on chip amplifier A1 from a single ended signal.  Since A1 is going to level shift the input signal anyway, why not just apply the signal  to one input and ground reference to the other?  Does A1 perform better if the differential inputs are balanced?

My signal input is a single ended bipolar output from a current to voltage conversion circuit.  The gain can be adjusted as required, currently the measurement range is +/- 3.275V.   However I would like to allow the voltage to exceed this range in the overload condition.  Currently the signal can range up to +/- 15V which I presume would damage the input?  Would the input be safe over voltage ranges of say +/- 5V? 

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  • Hi,

    Apologies for the delay.

    The diff-amp does not perform well in Single-ended to differential conversion due to common mode variation.

    With regards to the amplitude of the input, the AD7760's differential amplifier can take a +/-5V input (common mode of 0V on input) and attenuate to the required 80% for the modulator input. You  must ensure that the input to the modulator doesn't go outside the 80% of Full scale input. This would over range the modulator input and mean that the output of the digital filter will clip, thus, the linearity would obviously be affected.

    Thanks and Best Regards,

    Chris

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  • Hi,

    Apologies for the delay.

    The diff-amp does not perform well in Single-ended to differential conversion due to common mode variation.

    With regards to the amplitude of the input, the AD7760's differential amplifier can take a +/-5V input (common mode of 0V on input) and attenuate to the required 80% for the modulator input. You  must ensure that the input to the modulator doesn't go outside the 80% of Full scale input. This would over range the modulator input and mean that the output of the digital filter will clip, thus, the linearity would obviously be affected.

    Thanks and Best Regards,

    Chris

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