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THS4521: What is the input impedance of VOCM pin?

John11513
Expert 5360 points
Part Number: THS4521
Other Parts Discussed in Thread: THS4522

Hi,

Based on Figure 78 in data sheet, VOCM input ckt, the input impedance of pin VOCM should be 275k/2=137.5k;

Based on Equation 9 in data sheet, it should be 50k/2=25k;

Based on Electrical Characteristics, it should be 72k;

Based on TINA Spice Model simulation, it should be 50k.

Which one is correct?

Thank you,

John

  • 0
    TI__Genius 15975 points

    Hi John,

    Sorry for the confusion in the VOCM input circuit. Figure 78 in datasheet should be correct with the 275kohm from each supply to the VOCM pin.

    We will work on updating the VOCM input impedance specification in the datasheet as well as the model. Thanks for bringing this up.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    TI__Expert 5360 points
    Hi Rohit,

    But customer's test result do not support this.
    A customer applies 2.5V through 82k resistor onto VOCM1 pin of THS4522 which is powered from +/-2.5V supply. The tested voltage on VOCM1 pin is 1.02V. While it should be 1.57V based on Figure 78, or 1.31V if furtherly considering 5uA of input bias current. The test result seems support 72k of specs more.

    Regards,
    John