# Measuring Photodiode current

#### Aishwarya0210

Joined Jul 21, 2022
9
Hi. Can someone please explain to me clearly the difference between a transimpedance amplifier and a precision instrumentation amplifier.

My goal is to measure the current through a photodiode connected after an RC low pass filter to a precision instrumentation amplifier.

#### Ya’akov

Joined Jan 27, 2019
6,260
Welcome to AAC.

A transimpedance amplifier is used to when current to voltage conversion is needed. That is, if you have a sensor with a relatively flat current response but a nonlinear voltage response, a TIA will let you amplify the current as voltage for measurement.

An instrumentation amplifier is a highly buffered differential amplifier designed to have the smallest possible effect on the source at its input. It has.a very high input impedance, and also things that will contribute to its transparency related to measurement like low noise and low DC offset while having very high open loop gain and high common mode rejection.

It sounds like you need a TIA for your application. I think others might have more practical experience with such measurements so I look forward to their replies.

#### Aishwarya0210

Joined Jul 21, 2022
9
The confusion rose when the IC chip on the module I was using had JLA written on it and nothing else. So when I googled JLA, AD8221, a precision instrumentation amplifier popped up first. With the pin diagram matching the connections, we thought this was it and are now taking the output of this amplifier with the input to the diode and trying to calibrate the current measurement.

#### ericgibbs

Joined Jan 29, 2010
15,976
hi 0210,
If you post your circuit we can use LTspice to simulate the AD8221 amp etc, compare results.
E

#### Aishwarya0210

Joined Jul 21, 2022
9
Here's the image of the circuit.

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#### ericgibbs

Joined Jan 29, 2010
15,976
hi 0210,
This is the basic circuit in LTspice, we need some initial component values and the expected specification details.
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#### Aishwarya0210

Joined Jul 21, 2022
9
The inductances could not be found since the circuit was already integrated into the chip and some of the capacitances are in nF range, which the multimeter could not sense (I don't understand why).

Again, the goal is the measure how much current the photo diode is drawing in by measuring the output voltage of the JLA.

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#### ericgibbs

Joined Jan 29, 2010
15,976
hi 0210,
Look through this PDF.
The diagram you have posted is very difficult to read, don't you have a PC drawing program.?

Which type of Photo diode are you using.?
E

Added: video sim of a TIA @Aishwarya0210

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#### ag-123

Joined Apr 28, 2017
259
for photodiode, normally all it takes is this

Code:
vcc +---- R ----+----|<----+ gnd
^     ^
measure at the mid point with adc etc. use a good phototransistor with optics (simple lens) if possible, the amplification provided by the transistor and the lens is useful. or use a rather big one like BPW34 https://www.vishay.com/product/81521/
I'd think these days even solar cells can be used for the same purpose, a 'big' photodiode if you'd like.

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#### ericgibbs

Joined Jan 29, 2010
15,976
hi ag-123
Have you actually read the TS posted query.??

E

#### ag-123

Joined Apr 28, 2017
259
with op amps, there is an example on wikipedia
https://en.wikipedia.org/wiki/Transimpedance_amplifier

My guess is an
https://en.wikipedia.org/wiki/Instrumentation_amplifier
is probably not necessary.

Another thing about the proposed circuit which I don't fully understand is the photodiode part.
It'd seem the op amp would see a differential voltage only if there is some voltage change in the photodiode, e.g. going from dark to light, etc.
If it is static i.e. no changes, the v+, v- differential voltages at the op amp would settle to some fixed same voltage?

For photodiode circuits, if I aren't too particular about the linearity (it is mostly linear), I'd do it the basic reverse voltage way
and use a larger photodiode, phototransistor etc. I'd work the currents out after measuring the voltage.

Using an op amp, I'd guess is convenient as then an analog voltmeter can be used in the Transimpedance_amplifier configuration shown above.

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#### Aishwarya0210

Joined Jul 21, 2022
9
Hi gibbs

I am quite confident that the circuit under study does not have a TIA.
I could not simulate a photodiode and hence am just adding a normal Si diode.
Here is the circuit diagram. With the some ambiguity in he capacitances and inductances.
The answer I am looking for is not in equations but the concept; " Can I model the output voltage of the amplifier as a function of the current drawn by the photodiode?"

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#### ag-123

Joined Apr 28, 2017
259
just IMHO, as I'm not too sure as well, I'd sometimes substitute an op amp in place of an instrument amp as an equivalent simplified block. That makes it 'easier' to understand the circuit at least. If that it the case, then it goes back to the Transimpedance_amplifier concept, where it is still necessary to configure it that way to do that current to voltage translation, even with an instrument amp.

#### MrChips

Joined Oct 2, 2009
26,774
In other words, are they using an INA configured as a TIA?
You can use an opamp configured as an INA or TIA.

#### Aishwarya0210

Joined Jul 21, 2022
9
INA configured as TIA, I think that is what it is.