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I found the post with circuit diagram I wanted to build: circuit diagram.

The resistors values, gain and filter frequency are enough for me.

I wanted to measure light intensity as DC voltage, so bandwidth doesn't matter for me. My light levels are about hundreds of nx (nanolux).

How can I use in this diagram my amplifier TLE2072CP and photodiode BPW21R?

If yes, the question is how to properly power my amplifier TLE2072CP in this circuit? Amplifier TLE2072CP is recommended to use as dual power supply.

In which direction photodiode is connected in this circuit?

Edit:

I found also second circuit diagram:

circuitdiagram2

The second circuit diagram is more suitable for my application?

How should I modify its components values?


I've built this circuit with photodiode BPW21R and TLE2072CP amplifier:

enter image description here

I've added 100nF and 4.7μF connecting +15V (it's +12V in the picture) with ground and 0V (it's -12V in the picture) with ground too. The ground I connected to the shield box.

The circuit works fine but on oscilloscope the output contain 50 Hz sine with amplitude 150 mV.

What causes this sine signal to appear? I was expecting flat DC line with some noise.

How to remove this sine signal?

The filter at the output has a cutoff frequency ~159 Hz, modification of this filter to lower cutoff is the right solution of 50 Hz problem?

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    \$\begingroup\$ I have a feeling you have bitten off more than you can chew and have no idea what you're doing. It seems like you're grabbing random circuits online and not even knowing that they are called transimpedance amplifiers, let alone having an basic concept about what the circuit is actually doing. Can a photodiode even detect 100s of nLx? If a photodiode can actually do it, would take >1GOhm resistors which requires careful low-leakage PCB design...if it can even be done on a PCB to begin with and not need some special on-die or in-package integration. \$\endgroup\$
    – DKNguyen
    Sep 20, 2022 at 3:04
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    \$\begingroup\$ But then you said UV, and UV can't even be measured in lux since it is not visible light. You ignored comments in your previous question pointing out that you were trying to detect UV with a photodiode that is not UV sensitive, indicating you don't know how to read a datasheet. Furthermore the the TLE2072CP isn't even appropriate for high sensitivty photodiode TIAs. And even if the BPW21R was UV sensitive and you could measured UV in lux, extreme extrapolation of Fig.3 in datasheet says photocurrent would be 1fA which is unworkable without expertise and can't even be done on a PCB. \$\endgroup\$
    – DKNguyen
    Sep 20, 2022 at 3:27
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    \$\begingroup\$ What is it you are actually trying to do? Most likely you have mis-specified your requirements because you don't understand what it is you really need. But it is also possible you have chosen something technically unfeasible, or at least technically unfeasible with photodiodes (photodiodes aren't the only way to detectors out there, nor are they the most sensitive). \$\endgroup\$
    – DKNguyen
    Sep 20, 2022 at 3:27
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    \$\begingroup\$ Now that you have clarified, I completely agree with @DKNguyen. This is an extremely difficult problem. A long time ago, I designed a transimpedance amplifier sensitive to nA. It had a 300M resistor and an expensive opamp. You need to understand bias currents, offset voltages, how to layout with guard rings, and how to shield. Without shielding, the circuit was so sensitive that just being a few feet away would affect the output. \$\endgroup\$
    – Mattman944
    Sep 20, 2022 at 8:12
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    \$\begingroup\$ No. You still don't get it. Shielding is not the point. The first stage is the only one that matters because it is the only one that will increase your SNR. But this doesn't change the fact that your photodiode and opamp won't work for nanolux. And even if you had photodiodes and op-amps that theoretically did work, you wouldn't be able to build a PCB that can amplify 1fA of current. This is the bare basics: ti.com/lit/pdf/tidu535 But it still won't work for your requirements. \$\endgroup\$
    – DKNguyen
    Sep 20, 2022 at 23:50

1 Answer 1

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This is a difficult problem, so you want to do everything possible to maximize the sensitivity. I a building something similar to what you need.

Opamps usually work better with dual supplies, a single supply often has compromises.

The circuit will use really large value resistors, so you want an opamp with really low input bias current. You also want low input offset voltage. After a short search, I found a LMC6001, it seems to meet the needs, although the offset voltage is higher than I would like. Adding a second amplifier stage will only amplifify the offset, so it is not wise. Some of the voltage offset can be nulled with software, although it will vary some with temperature. This opamp has a absolute maximum supply voltage rating of 16V (V+ minus V-), you want to be a few volts below 16V.

You want a guard ring around the sensitive net. The guard ring should be at the same potential as the sensitive net (lowers risk of leakage current). For a dual supply this is easy, since the sensitive net is at 0 volts, the ground fill will be the guard ring. Leave extra space around the sensitive net to reduce the chance of leakage.

You want to avoid wires to the photodiode. You can mount the photodiode on the back of the board so it can be flush with your box.

If shielding is necessary on the top of the board, I will use metal standoffs and a thin aluminum sheet ("flashing" from a home improvement store).

Large value resistors have inherent noise that can be an issue ( https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise ), so it is good that you don't need high bandwidth.

There are some extra features that you may not need. I have a small regulator to power my low-power TI MSP430 MCU. I also have a dim LED power indicator (yes, modern LEDs work at < 0.1 mA). If it affects the photodiode, I will disable it, or cover it.

I will run it from one 9-volt and one AA battery. The expected current draw is a few milliamps.

The parts are on order, they will be here in a few days. I just completed the PWB design, I will be ordering it soon.

I will update the answer after I build and test the circuit.

enter image description here

enter image description here

enter image description here

Edit: I built the PWB with the LMC6001. I doesn't work properly, the opamp may be bad or maybe I damaged it. They are expensive, so I only have one. I will order another.

I substituted a TLV2760. This is a low-voltage opamp, max supply voltage of only 4 V (I used 3.3 V, no negative supply, it is rail-to-rail). It has a higher bias current, so I changed R3 to 10 M, and C7 to 0.01 uF. Now, it works as expected. The output still has considerable noise, so a large cap at C8 will be necessary (I will use a 4.7 uF ceramic).

Here is a summary of the key issues to building a high-gain transimpedance amplifier:

You need an ultra-low input/bias current opamp. It needs to be much lower than the current that you are attempting to measure. The bias current of the TLE2072CP is not low enough to meet your goal.

Any offset voltage error of the opamp will show up in the output. Adding a second stage will amplify this error, that is why you want to do most or all of the amplification in the first stage.

Large resistors have a lot of Johnson noise, you want a large feedback capacitor to help filter it out.

The PWB layout is critical. You need to keep the connections short and avoid leakage paths.

The 100 M resistor that I ordered is physically larger than I expected, I used the wrong footprint.

enter image description here

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  • \$\begingroup\$ @MalumWolfram - The circuit is not working well with 100M feedback. I will make some careful measurements and post a separate question. \$\endgroup\$
    – Mattman944
    Nov 30, 2022 at 17:24
  • \$\begingroup\$ Thank you for your answer, I'm looking forward for your post, you can add link here. \$\endgroup\$ Nov 30, 2022 at 23:42
  • \$\begingroup\$ @MalumWolfram - Link to follow up: electronics.stackexchange.com/questions/644934/… \$\endgroup\$
    – Mattman944
    Dec 3, 2022 at 15:05

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