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LMH5401: Min input resistance of CM pin

Bernd Stelling
Prodigy 20 points

Part Number: LMH5401

Tool/software:

Hi,

the datasheet does not give any numbers for the input resistance of the CM pin.

It only states that it is "very high".

Do you know the minimum resistance for this pin?

I have to make sure that it can be driven by the ADC output buffer.

Best regards,

Bernd

  • 0
    TI__Intellectual 2780 points

    Bernd,

    The CM pin is designed to be directly driven from ADC buffers that do not typically have high current drive.

    Expect the current to be in the order of micro amperes as the VCM pin is looking into a basic high impedance amplifier input.

    If you need to know the exact values and don't have an EVM, we can make quick measurements under your supply conditions and the voltage level you intend to drive the VCM pin to.

    Keyur

  • 0 in reply to Keyur Tejani
    Prodigy 20 points

    Hi Keyur,

    currents in the microampere range sound fine.

    I just need to make sure that the impedance is higher than 10 kOhm.

    The measurements on the eval boards were mucher higher than that, but I don't know if this is true for all devices.

    I am biasing the amp with +/- 2.5 V. The CM voltage is 0.7 V.

  • 0 in reply to Bernd Stelling
    TI__Intellectual 2780 points

    My micro ampere comment was more if Vcm was at midsupply. The current will be a bit higher as Vcm goes away from midsupply.

    We'll check in the lab and report.

    Keyur

  • +1 in reply to Keyur Tejani
    TI__Intellectual 2780 points

    Bernd,

    Below are the Vcm pin currents we measured in the lab...

    Vcm bias current varies with VOCM pin drive voltage. It is minimum for mid supply and Ibias< 0.5uA. For min and max VOCM values Ibias   < +/- 15uA.

    Keyur