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photodiode EPD-440-0-1.4 & OPA129

Jiri Pospisil
Prodigy 50 points


Hello.
I would like to ask.
I got this photodiode "EPD-440-0-1.4" and join it to an operational amplifier OPA129. But I do not know how I calculate resistor Rf.

I tried the following formula: Rf = Uout / Iph
where: Uout = output voltage "AMP"
Iph = diode current

Diode Datasheet: http://www.roithner-laser.com/datasheets/pd/EPD-440-0_1_4.pdf

I think I'll measure currents Imax = 1.55 uA. But how do I know what will be the lower limit of the current? I expect the unit or hundreds nA or pA?

So if I figured it right, so "5/1.55e-6 = Rf = 3.2 Mohm"

But for the current 10pA resistance is Rf = 500Gohm. And it is a very large value.

operational amplifier is supplied with + /-15V (connector "Power"). And the output amplifier is applied to the AD converter (MCU connector).
MAX4051 circuit will be used for range switching.

  • 0
    Guru 21975 points

    Jiri,

    Your calculation would seem to indicate that you expect a full-scale (5V) output with 10pA diode current. While you may want to detect 10pA-level inputs, it is not necessary that you have a full-scale 5V output at this current input.

    Assuming you have a 12-bit a/d converter, you can detect and measure signals of a few millivolts at the low end of your detection range.

    To avoid oscillations, you will probably need a capacitor from the op amp output to the inverting input.

    Regards, Bruce.