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PIN photometer: double opamp

Discussion in 'Electronic Basics' started by Dominic-Luc Webb, Feb 28, 2004.

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  1. Latest adventure is a minimum component PIN
    photometer. Sensitivity is an issue (measuring
    from faint stars on telescope). I am thinking
    to try two opamps to increase weak signals.
    Perhaps more amplification is self defeating,
    since this is not a PMT. For feedback, I chose
    an 0.82 pF capacitor rather than 10 GOhm resistor
    as these are very hard to find and expensive. I
    think this circuit should work, but would be very
    grateful for suggestions for improvements. Please
    consider this is a light meter so it should have
    a linear response to photon flux.

    Dominic
     
  2. Dominic-Luc Webb wrote...
    __________________________________________________________
    | |
    | high-sensitivity (10mV/fA) current-sensing amplifier |
    | by Winfield Hill, 21 Feb 2004 |
    | |
    | __A1 ,--10.0M---, |
    | o means gnd--|+ \ 100nF | __A2 | |
    | connect | >--+---||--33k--+--|- \ | |
    | in air ,-10k--o-|-_/ | | >---+--- |
    | | | G=~20 gnd --|+_/ |
    | - - - (o)---||--+-100k-' |
    | 2pF | (75k + 50k pot |
    | mica 5k for calibration) |
    | | G = 10^13 V/A |
    | gnd A1 = LMC6061 precision gain |
    |________________________________________________________|

    Dominic, if you want to experiment with capacitance opamp
    feedback, try this circuit I posted, and read the next 40
    messages in Roy McCammon's "Mosfet Gate leakage" thread.
    http://www.google.com/groups?as_umsgid=

    This is an integrator followed by a differentiator, and it
    acts like an opamp with a 10,000 Gig-ohm feedback resistor.
    __________________________________________________________
    | |
    | modest-sensitivity (100mV/pA) current amplifier |
    | by Winfield Hill, 2 Mar 2004 |
    | |
    | __A1 ,--10.0M---, |
    | o means gnd--|+ \ 0.02uF | __A2 | |
    | connect | >--+---||--33k--+--|- \ | |
    | in air ,-10k--o-|-_/ | | >---+--- |
    | | | gnd --|+_/ |
    | - - - (o)---||---------' |
    | 2 pF mica |
    | G = 10^11 V/A |
    | A1 = LMC6061 approximately |
    |________________________________________________________|

    It'll surely be too sensitive, but we can lower its gain to
    act like a 100G resistor. Read the thread to learn a cute
    way to reset the 2pF capacitor without changing its value.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  3. John Larkin

    John Larkin Guest

    Win,

    There's a circuit in the latest RSI that's nice. The first stage
    feedback is a capacitor shunted by a high-value resistor, and the
    second stage is a DC coupled amp with an R-C lead network to
    compensate for the frequency response of the first stage. It's a nice
    compromise: less Johnson noise in the front end, but no reset
    requirement.

    John
     
  4. Thanks bunches. I have plenty of homework to do, but I checked
    the specs on the LMC6061 (clearly made for my application). May
    I ask if the A2 in the schematics are the second half of a
    LMC6062 or a second LMC6061 or some other opamp altogether?

    Dominic
     
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