Re: Low drift OP amp for photodiode circuit

Robert Latest wrote:
Phil Hobbs wrote:

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.

A 500G resistor has about 0.18fA/rtHz noise, that's about as much noise as a 0.1pA (leakage) current has intrinsically. So if your opamp is (and needs to be) better than that, use the integrator method.

How does the LED reset method work? Using the LED as a photoswitch?

I also like the supply reversal method conceptually but have never used it.

robert

It's a bit more complicated but very low leakage--comparable or superior to an unprotected MOSFET, i.e. below 100 fA from -5V to +0.5V bias.

It's just a two-LED diode clipper--when reset is asserted, A2 forces the output of A1 to equal its input, within an offset of I_in R1 (inconsequential in my case). You have to choose R1 and C1 so that the composite amp is stable when RESET is active, but there are other ways of doing that, e.g. running A2 at a higher noise gain. D1/R3/R4 forces the LEDs to be biased off reliably during integration. There's a bit of a hold step, but we need to do correlated double sampling to get the noise improvement anyway, so that isn't a big item.

(In my circuit, I actually use a BJT inverter instead of the extra reset line + diode, and the charge was being dumped into the integrator by another LED switch rather than being continuously integrated. This is from the second generation of my Footprints sensor, which was a $10 thermal infrared imager with 100 pixels and ~0.1K NETD. The second version might have been $20, plus another $40 for the PIC and the box--the system is about 15 dB lower in price than the next lowest, which doesn't work as well.)

Cheers,

Phil Hobbs


D1
|\ | R3
RESET o-----| >|-------RRRRR-----------------------------+
|/ | 1/4 LMC6034 |
(+5 = reset, |
0 = integrate) /| |
LED 1 LED 2 / | |
/ +|-------+
R2 | /| | /| / | |
RESET' o---+----RRRRR--|< |--*---|< |------< | A2 |
| \| | | \| \ | R
(+5 = integrate, | \ | R
0 = reset) | \-|--+ R R4
| \| | R
| | R
*---------------------+ |
R |
R1 R || C1 |
+--------||------+ |
| || | |
| | |
I_in | |\ 1/4 LMC | |
| | \ 6034 | |
o------*--|- \ A1 | |
| \ | |
| >--------*---------+--O
| / Output
| /
+----´+/
| |/
|
-----
\ /
V
.

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