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ADS8684: Max. differences of input impedances and input impedance drift of analog inputs withing same device (tracking) of ADS8684.

Beat Kramer
Prodigy 20 points
Part Number: ADS8684

Dear all,

Input impedance Zi of analog inputs is specified at Ta=25°C between min.=0.85MOhm, typ.=1MOhm, max. 1.15MOhm with a drift of typ. 7ppm/°C, max. 25 ppm/°C (page 7 of SBAS582C; http://www.ti.com/lit/ds/symlink/ads8684.pdf).

Is it possible to specify differences of input impedances and input impedance drifts of the 4 channels within the same device? There is the hope, that e.g. the input impedance and input impedance drift tracking of the 4 analog inputs with in the same device are much better?

I couldn't find any information about CMRR of the differential inputs of ADS8684. Are there any characteristic values available?

Thank you for your support!

Best regards,
Beat 

  • 0
    TI__Mastermind 24970 points

    Hello,

    The spec that you have listed is per channel. Each channel can experience a drift of 7ppm/C and so on. It is not a cumulative spec that takes in all channels.

    Within the datasheet, Figure 99, on page 60 provides a Common Mode rejection ration vs Frequency graph.

    There are two short pieces of collateral what might be helpful within this topic:

    Reducing Effects of External RC Filter on Gain Error and Drift in SAR ADC with Integrated AFE

    and the Circuit Cookbook that goes with it.

    Regards, Cynthia

  • 0 in reply to Cynthia
    Prodigy 20 points

    Hello,

    Thank you for you fast feedback!
    My question was mainly about the matching of different channels within the same physical device. Is it possible that one input channel of the AD converter has an input impedance of 0.85 MOhm and an other one has 1.15 MOhm within the same device. Is it possible to specify the max difference between the channels of the same device (tracking / matching)? Helpfull would be also the matching of the channels regarding temperature drift.

    Best regards
    Beat

  • 0 in reply to Beat Kramer
    TI__Mastermind 24970 points

    Each channel within the device can fall within the specification given, thus one channel can be in the minimum or .85Mohm, and the following can be at 1.15Mohm. This is completely possible. The max difference between the channels is in fact the min and max possible for each channel.

    The same applies for drift, it is completely possible for one channel in a device to drift max positive, while the next channel in the same device to drift in the negative.

    Regards

    Cynthia