Measuring pin capacitance

Discussion in 'General Electronics Chat' started by Distort10n, May 16, 2008.

  1. Distort10n

    Thread Starter Active Member

    Dec 25, 2006
    429
    1
    I am trying to measure input capacitance on the IN pin for the PI5V330S:

    [​IMG]

    The yellow top trace is my input signal from an Agilent 80 MHz arbitrary generator: 1 MHz square wave, 5Vpp @ 2.5Vdc.

    The three bottom traces are the results of my measurements under these conditions:

    1) Open - the trace is actually cut so a piece of wire is shorting the trace back together. No part is in the socket so the trace is an open.

    2) A 10kΩ resistor is soldered across the cut trace. No part is in the socket. Measured at the 'open' end of the resistor.

    3) A 100kΩ resistor is soldered across the cut trace. no part is in the socket. Measured at the 'open' end of the resistor.

    It is obvious that there is parasitic capacitances on the board. I calculate in the ~8 pF. I was hoping that is good news because then I would be able to place the part in the socket, and back calculate the pin capacitance since the part specifies 6 pF MAX. ASSUMING 6pF, then that is double the amount of capacitance so I should see that reflected in the rising edge of my square wave.

    But I do not.

    The waveform barely changes, so I must conclude that the pin capacitance is really, really, low or something else is amiss.

    Any other suggestions (rather than an LCR meter - sucks BTW) to measure pin capacitance?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Just to get it straight:
    M1 - Yel, top display - 1MHz in
    M2 - Pnk, trace cut
    M3 - Blu, 100k resistor
    M4 - Grn, 10k resistor

    6pF is the MAX specified for the input; there is no "typical" spec. If it works, then you're good to go.
     
  3. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    If you really need that detail find out how the manufacturer tests it. I know some tests are in the datasheet but some either are not or are 'industry standard'.
     
  4. Distort10n

    Thread Starter Active Member

    Dec 25, 2006
    429
    1
    Correct.

    Good point about the MAX specification. What I could do is solder a capacitor from the trace to ground and still leave the trace open. I could see what a 1.2pF cap does to the waveform.
    The conclusion could very well be that the input capacitance is <1pF.
     
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