Re: Low drift OP amp for photodiode circuit

On Fri, 21 Sep 2007 10:15:09 -0400, Phil Hobbs
<pcdhSpamMeSenseless@xxxxxxxxxxxxxxxxxx> wrote:


Since ultralow light level measurements are so slow anyway, one nice
method is to use a capacitor instead of the feedback resistor, and
integrate the charge coming from the PD. You can reset the cap with a
MOSFET wired in parallel with it (there are other methods too--my
favourite uses LEDs).

There are lots of nice things about this method. One is that it's much
much quieter at very high gains--unlike 500G ohm resistors, capacitors
don't have thermal noise. Thus the slope of the V vs T curve can be
measured much more accurately than the instantaneous voltage across the
feedback resistor. (*)

Another one is that you can do range switching by just changing the
frequency of the reset pulses. A third is that you can add a Schmitt
trigger and make an oscillator--a current-to-frequency converter--that
will make the measurement much easier, and if you have a universal
counter, there's no problem getting lots of digits on a frequency
measurement.

Of course the best thing to do is to use brighter light. ;)

Cheers,

Phil Hobbs

(*) The reset process does have thermal noise, but as long as you
measure the voltage just after reset and subtract that from the
measurement, there's no thermal noise in the photocurrent measurement.


How's this? Since there's nothing to be learned during the integration
period, disconnect the amp.



v+
|
|
pd shielded-
| coil
| reed
| relay
+-------o o------+------amp---adc
| |
cap |
| discharge
| fet
gnd |
gnd

.

Relevant Pages

  • Re: Low drift OP amp for photodiode circuit
    ... You can reset the cap with a MOSFET wired in parallel with it. ... Thus the slope of the V vs T curve can be measured much more accurately than the instantaneous voltage across the feedback resistor. ... will make the measurement much easier, and if you have a universal counter, there's no problem getting lots of digits on a frequency measurement. ... The reset process does have thermal noise, but as long as you measure the voltage just after reset and subtract that from the measurement, there's no thermal noise in the photocurrent measurement. ...
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  • Re: Low drift OP amp for photodiode circuit
    ... This is for a photodiode circuit for low level light measurement. ... voltage of 350uV max. ... feedback resistor like 50 to 200G, ...
    (sci.electronics.design)
  • Re: Very simple multitasking on MSP430
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