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TM4C1294NCPDT: leakage current ADC

Kevin Den Toom
Prodigy 450 points
Part Number: TM4C1294NCPDT

Hi,

the datasheet of the device shows a leakage current of max. 2µA at 1MSPS. Is the leakage current really dependent on the sample frequency?

Chapter 15.3.2.5 , Sample and Hold Window Control describes that Rs,max can be up to 189k5 Ohm.

With 189k5Ohm and up to 2µA leakage current, the resulting error is 0.38V. The bigger sample-hold time doesnt help, unless you calibrate permanently.

Any further information on this matter is appreciated.

Rgds
Kevin

  • 0
    TI__Guru*** 135355 points

    Hello Kevin,

    As the same max leakage current is spec'd for both 1MSPS and 2MSPS, I don't see any dependence there and in general would not expect leakage current (which is due to transistor properties) to be impacted by the sample frequency. That is generally intrinsic to the device.

    The max resistance however, is impacted by the sample frequency. You referenced Rs,Max is up to 189.5k Ohm and the 2 µA max spec, but that isn't for 1MSPS anymore.

    For 1MSPS you'd have 500 ohm resistance which means only 0.001V error.

    I think there is an added layer of detail here I am not quite catching onto, maybe you can help me understand further what the customers concerns are regarding the ADC and leakage current?

  • 0 in reply to Ralph Jacobi
    TI__Prodigy 450 points

    Hi Ralph,

    Using the TM4C129 it is possible to change the conversion rate. If we sample at 60kSp, Rs,max can be up to 189kOhm (datasheet 15.3.2.5, table 15.4). Does it really make sense to show a  Rs,max with a value above say 10kOhm in the datasheet, if the specified max. leakage current is 2uA, resulting in an uncontrollable error that is very large?

  • 0 in reply to Kevin Den Toom
    TI__Guru*** 135355 points

    Hello Kevin,

    That table isn't meant to be a recommendation of the resistance values that should be used, but rather outlining what can be used. Having such a large input impedance would also have issues with impedance matching as well. The point is more that if a customer has such a large impedance, they are limited to lower sample rates to even attempt to get any sort of measurement. Analysis of the leakage current and error would have to be applied as well at that point, but I haven't seen any customers getting to that point.