Transient behavior on first samples of AD7980

I am using the AD7980 where I sample 512 values, delay for 10ms then start again. To test the ADC I just input a fixed voltage. But what I am finding is that about the first 30 samples exhibit some form of transient behavior (see image). After that the data seems OK. The ADC is being driven using a buffer amp. I did also have a low pass filter before the ADC but temporarily removed it as it didn't seem to alter the result. Does anyone have any suggestions on what could be causing this? Thanks

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  • Hi Percy,

    From the AD7980 data sheet:

    When REF is driven by a very low impedance source, for example, a reference buffer using the AD8031 or the ADA4805-1, a ceramic chip capacitor is appropriate for optimum performance. If an unbuffered reference voltage is used, the decoupling value depends on the reference used. For instance, a 22 μF (X5R, 1206 size) ceramic chip capacitor is appropriate for optimum performance using a low temperature drift ADR435 reference.

    The AD7980 evaluation board uses the ADR435 with a 22uF decoupling cap. They also use the COMP pin on this product to reduce ringing. See the user guide for more information. This same circuit can be used with the ADR431.

    The ADR4525 does not have as strong an output stage as the ADR431 and also does not have a COMP pin to help with the dynamic switched cap load that the ADC presents to the reference output stage, and so you will have problems using the part without a buffer such as the ADA4805-1. If you require the better temperature coefficient performance of the ADR4525 then I would recommend using a buffer or else perhaps evaluating the LT6657.

    The LT6657 has improved TC compared to the ADR4525 and has a stronger output stage. I have not tested this product combination specifically however.

    Regards,

    Rob

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  • Hi Percy,

    From the AD7980 data sheet:

    When REF is driven by a very low impedance source, for example, a reference buffer using the AD8031 or the ADA4805-1, a ceramic chip capacitor is appropriate for optimum performance. If an unbuffered reference voltage is used, the decoupling value depends on the reference used. For instance, a 22 μF (X5R, 1206 size) ceramic chip capacitor is appropriate for optimum performance using a low temperature drift ADR435 reference.

    The AD7980 evaluation board uses the ADR435 with a 22uF decoupling cap. They also use the COMP pin on this product to reduce ringing. See the user guide for more information. This same circuit can be used with the ADR431.

    The ADR4525 does not have as strong an output stage as the ADR431 and also does not have a COMP pin to help with the dynamic switched cap load that the ADC presents to the reference output stage, and so you will have problems using the part without a buffer such as the ADA4805-1. If you require the better temperature coefficient performance of the ADR4525 then I would recommend using a buffer or else perhaps evaluating the LT6657.

    The LT6657 has improved TC compared to the ADR4525 and has a stronger output stage. I have not tested this product combination specifically however.

    Regards,

    Rob

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