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LMH5401: CM Pin Input Impedance

Shoaf Robinson
Prodigy 20 points
Part Number: LMH5401

Tool/software:

I am using this FDA in a circuit interfacing with an ADC that provides a common mode pin to tie to the LMH5401's CM pin. The ADC's datasheet specifies that its common mode is designed to drive a high impedance load of ≥ 10 KΩ . If a lower impedance load is required, a buffer should be used on the common mode signal.

The datasheet for the LMH5401 provides this:

I just wanted to see if anyone could verify the FDA's CM pin input impedance is high enough that I would not need to include a buffer? Any insight is appreciated - thanks!

  • +1
    Guru 316780 points

    The datasheet does not specify this, but if we assume that it's similar to the opamp inputs, the impedance is larger than 33 kΩ. And "bias current is not critical" implies that it's actually lower than 150 µA.

  • +1 in reply to Clemens Ladisch
    TI__Intellectual 1110 points

    Yes, no buffer is required. Looking at the LMH5401 datasheet, it looks like it is a connection directly to the gate of a small FET which controls the common mode voltage. It also mentions that there is no internal bias network, so it can be assumed that the input impedance is similar to a FET gate DC impedance. Some guidance can be had from looking at the LMH5401EVM, where the pin is driven by two 1k resistors to V+ and V-, holding the pin at midsupply.

    Gideon