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TIDA-00826: Input impedance change with offset change

Owen Yan
Intellectual 2030 points
Part Number: TIDA-00826
Other Parts Discussed in Thread: LMH6559

Hi team,

Customer use Fluke9500 to monitor the input impedance of LMH6559, while the it is only 50ohm when offset is 0. the input impedance change with offset change, do we have methods to keep it as 50ohm over offset changing?

B R,

Owen

  • +1
    TI__Genius 9650 points

    Hi Owen,

    I was hoping the customer could provide more information on what exactly they're measuring. The input impedance of the LMH6559 will not be 50-ohms.  The goal however is for the input impedance looking from the perspective of the signal source to be 50-ohms. The goal is to then ensure the two input branches of the FDA have the same impedance which is why the R229 resistor is added at the output of the LMH6559. The potentiometer is meant to tune the dc bias input so that the offset at the output of the FDAs going into the ADC is minimized. The input impedance of the branches is set by the network and resistors like shown in Figure 4 in the reference design. Section 9.2 also highlights the purpose of the potentiometer.

    Best Regards,

    Ignacio