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LMP90100 Input Impedance

Rui Silva
Prodigy 40 points

Hello, I have a working prototipe with the LPM90100 in the following configuration.

ODR = 13.42 SPS

VA=VIO=5V

Buffer ON

Gain=16

Background Calibration Correction ON

I am trying to determine the input impedance of ADC, but the datasheet only make a note about a ZIN  = 15.4M (ODR=13.42).

Wen the buffers are enable the input impedance are higher or are the same? 

Is there a way so dermine the ADC impedance experimentaly?

Thank You

  • 0
    TI__Genius 16890 points
    Hello,

    When you have a gain of 16 the input impedance is from the FGA. This has the high impedance of an op amp so the spec that you mentioned of 15.4M applies. The Buffer also has this impedance but you would only see the buffer impedance if your gain was <16x so that you are bypassing the FGA.

    Mike
  • 0 in reply to Michael Stout
    Prodigy 40 points

    Thank you Michael.

    So you are saying that the FGA and the BUFFER have the same input impedance of 15.4M, I understand that, but if I have a gain <16 and buffer OFF, the only impedance in the path are the ADC internal impedance, can you tell me what is the internal impedance in that configuration?

    Thank you

  • 0 in reply to Rui Silva
    TI__Genius 16890 points

    Hi Rui,

    Looking at the datasheet, page 8, there is some information but not exactly what you are looking for.  The 15.4M ohm 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.

    Mike

  • 0 in reply to Michael Stout
    Prodigy 40 points
    Thank you Michael for your explication.
    Rui