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LMP90080: Input impedance equivalent schematic

M.A.M
Expert 8555 points
Part Number: LMP90080

Hi,

My customer is troubleshooting a deployed system right now and wish to know the equivalent impedance of the inputs, capacitance etc, dependence on sample frequency e.t.c, that could affect the measurements. Datasheet only express Cin to be 4pF for 214 SPS, so the info is not complete for other frequencies for instance..

Thanks for enlightening us.

B.r  M.A.M

  • 0
    TI__Mastermind 40331 points

    Hi MAM,

    Looking at the datasheet, page 8, there is some information about this topic. The 15.4Mohm is the differential input impedance. The datasheet does not give any information on the state of the buffer or FGA for this spec. There is another spec, Input Leakage Current that is used to indirectly specify the input impedance of the FGA and the buffer. The part has 500pA input leakage when the buffer is used. It has 100pA leakage when the FGA is used.  (Gain is >= 16 so the FGA is used). There is no specification listed if the FGA is not used and the buffer is bypassed (ADC leakage). Because of the internal sampling cap of the ADC the leakage is most likely higher than the buffer specification of 500pA.

    -Bryan

  • 0 in reply to Bryan Lizon86
    TI__Expert 8555 points

    Bryan,

    Could you please dive a bit deeper for me and find data, i.e ADC input impedance/leakage changes when the Buffer and FGA are in all possible states?
    My customer clarifies:

    "In our application we use a Gain = 1 and the buffer is disabled. As mentioned, we have identified a relative huge difference by enable and disable the buffer in our application. We use 13.42 SPS in our application." 

    Thanks again.

    B.r M.A.M

  • 0 in reply to M.A.M
    TI__Mastermind 40331 points

    Hi MAM,

    Can you clarify what is the "huge difference" that the customer mentions? Does the buffer make things better or worse?

    If the customer has a high impedance load directly connected to the ADC, then the buffer is likely necessary. If the buffer is causing issues, please let me know what those issues are so we can try to resolve them. A schematic would help in the latter case

    Please feel free to reach out to me offline for additional information on input impedance

    -Bryan