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Input capacitance of LMH7322

Per Magnusson
Intellectual 500 points

Other Parts Discussed in Thread: LMH7322

What is the input capacitance of the LMH7322 comparator? I do not see it mentioned in the datasheet and on a board I designed it seems to be way higher (maybe 8 pF) than what I expected from a high-speed comparator.

I use it to listen to a wide-band high-frequency (up to ~500 MHz) analog signal to detect if its peaks go above a certain level. To not disturb or load the signal, it does this through a 1k resistor, but the 1k in conjunction with the unexpectedly high input capacitance seems to limit the 3 dB bandwidth to about 20 MHz.


Per

  • 0
    TI__Mastermind 22825 points

    Hi Per,

    Regarding the 20MHz -3dB bandwidth limit that you measure with the LMH7322 as a peak detect: I am curious as to how you measure the -3dB bandwidth? Are you adding a scope probe to the LMH7322 (after the series 1k) and looking at its roll-off? If so, you must use something like a FET probe (~0.2pF to make sure you don't overwhelm the measurement with a passive probe capacitance).

    Also, have you made sure you have a low capacitance layout / board which is not contributing to the input parasitic capacitance?

    Here is some old data taken on the LMH7322 EVM board using a similar technique as yours to come up with the input capacitance:

    LMH7322 Input Resistance Capacitance Measurement from 7_20_12 Hooman 12_11_15.xlsx

    So this measurement points to an input resistance of 28 kohm and an input capacitance of 0.9pF.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Intellectual 500 points

    Hi Hooman,

    Thanks for your reply. And thanks for pointing out my stupid mistake, namely to ignore the parasitics of the passive oscilloscope probe I was using. I should have known better. Also thanks for the missing datasheet figure of 0.9 pF input capacitance. It would be great if that figure were included in future versions of the datasheet.

    The layout is very compact. The trace length from the center of the 0402 pad of the 1 k resistor to the center of the LMH7322 pad is 1.6 mm. So layout parasitics is not dominant in this case.

    Even with 0.9 pF of input capacitance I might have to rethink the value of the series resistor to not have too little bandwidth, although the extra loading of the signal at the other end of the series resistor might become an issue with a much lower resistance value.

    Cheers,

    Per

  • 0 in reply to Per Magnusson
    TI__Mastermind 22825 points

    Hi Per,

    Thanks.

    I've already noted to myself to make sure the input capacitance (0.9 pF) and input resistance (28 kohm) are added to the LMH7322 datasheet on the next revision.

    Regards,

    Hooman