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Switching small capacitive loads

Discussion in 'Electronic Design' started by Andrew Holme, Aug 14, 2009.

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  1. Andrew Holme

    Andrew Holme Guest

    A colleauge of mine is building an automated test jig for measuring rise
    times under different capacitive loads ranging from 5 to 70pF. Switching
    small capacitances with relays is difficult/impossible and I proposed he use
    electronic switching. Here are the schemes we've considered:

    .-- 1V step from 100-ohm source impedance
    | 10ns rise time (unloaded)
    | -------o-----------------o----------------o----> To 'scope
    | | | |
    --' --- --- ---
    C1 --- C2 --- C3 --- R3 10k
    | | | ___
    .-----o | o----|___|-> +10V
    | | | |
    L1 C| | PIN |/ |/
    1uH C| V Diode .----| Q1 .----| Q2
    C| - D1 | |> | |>
    | | | | | |
    .-. | .-. | .-. |
    R1 | | === R2 | | === R3 | | ===
    120 | | GND 1k | | GND 1k | | GND
    '-' '-' '-'
    | | |
    | | |
    3.3V Level CMOS Logic Control Lines

    C1/C2/C3 in 15 - 50pF range
    Q1/Q2 = 2SC4774 or BFS20W

    Our first thought was to use PIN diode switching; but we see problems with
    the above circuit in simulation when the diode is supposed to be off. Large
    negative DC bias is required to stop the fast rise time of the signal
    turning on the diode; and, unless an impractically large inductance is
    specified, choke L1 passes a sizeable AC current, causing severe waveform

    The bipolar transistor circuit seems much better. There is no waveform
    distortion except for the expected RC low-pass filtering effect of the
    selected load capacitance. The output capacitances of the 2SC4774 and
    BFS20W are both specified as about 1pF at Vcb = 10V; however, it seems to
    work equally well in simulation with or without DC collector bias.

    Any comments or advice?

  2. Pickering do a low capacitance SIL relay (<0.1pF):

  3. Jamie

    Jamie Guest

    Q1 seems to be more practical if you add a discharge circuit at the
    source when it's goes low. You may already have that in mind? I can't
    tell if your source signal is actually pulling to common when off. This
    is keeping in mind, that Q1 remains selected so that discharge can
    take place.

    Q2, will introduce voltage on the source line in the deselected
    transition. You may want to think about that.

    Those are my observations at first glance.
  4. It may have <0.1pF between open contacts, but each terminal
    still has >1pF to GND!

    I have the measurements somewhere, but I'd have to dig them out.

    Jeroen Belleman
  5. The "Guard screen only" (103G) version is speced at 0.1pF contact to coil
    capacitance and <0.1pF open contact capacitance.
    The magnetic screen version (103M) is the worst at 0.45pF per contact to
    coil, so close to your claimed 1pF figure.

    So it's possible to get the correct version with only 0.2pF total
    capacitance to the coil. Unless you are saying they don't meet their specs?

  6. They *do* meet their specs. With the (grounded) guard screen
    in between, the coil-to-contact capacitance is indeed very small.
    It's the contact-to-guard capacitances I was thinking of. I dug up
    my measurements: It's about 2.8pF for each contact.

    Anyway, in view of the intended application, the measurement of
    rise time with varying capacitive loads, it may be useful to
    keep in mind that the open contacts of the relay with guard
    screen look like a short piece of open-ended transmission line.
    It's up to the OP to judge if that's relevant.

    Jeroen Belleman
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