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I am designing a trans-impedance amplifier using a TEMD7000X01 photodiode and an AD8551ARZ op-amp. RF is 100 kΩ, CF is 150 pF, and VCC is +3.3 V. The output of the op-amp is connected to an ADC pin of a microcontroller.

This circuit works as intended when the op-amp is on, but when it is off, because the diode is forward-biased, the voltage at the output is negative, which is an issue for our microcontroller. Adding a small signal diode at the output does help limit the negative voltage, but there is some leakage current which is not ideal.

I had thought that if the photodiode was reverse-biased instead, it would fix the negative voltage issue when the op-amp is off. This does fix the negative voltage issue when the op-amp is off, but when it is on, the output is stuck at 0. Any advice on how to manipulate the circuit to fix the negative voltage issue?

enter image description here

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  • \$\begingroup\$ What is the value of RF? \$\endgroup\$
    – Voltage Spike
    Feb 12 at 21:07
  • \$\begingroup\$ What do you mean by "when the OPAMP is off"? Is the Vcc rail disconnected from the supply, while also leaving R1 and R2 active? \$\endgroup\$ Feb 12 at 21:38
  • \$\begingroup\$ Out of curiosity, how do you get a situation when an op-amp is off (powered down) while the microcontroller is on? \$\endgroup\$ Feb 12 at 21:39
  • \$\begingroup\$ If you want to reverse bias the photodiode you need to add a negative rail to the opamp or else use a negative voltage on the reverse bias. If you just flip the photodiode around then the output will be a negative voltage. However, I don't see any problem with a negative voltage here given the 100k ohm resistor. \$\endgroup\$ Feb 12 at 21:44
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    \$\begingroup\$ Why is it a problem? A small negative voltage into the microcontroller will not cause any problems. \$\endgroup\$ Feb 12 at 23:26

3 Answers 3

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This shouldn't be an issue, the diode and resistor will be high impedance. At worst a negative high impedance input will rerun in the protection diodes and almost no current >uA will flow.

If you are having issues, it's likely that there is a problem with the opamp input being low impedance while the opamp is off.

The best thing to do if the input protection diodes on the micro aren't working is add external protection diodes.

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However, when it is off, because the diode is forward-biased, the voltage at the output is negative, which is an issue for our microcontroller.

It is not, unless it's a very large area photodiode (say 1in2) illuminated with very strong light sources.

Input currents from most photodiodes will be safely shunted by the input protection diodes in both the op-amp and the MCU input pin structure. You've probably noticed that the negative input voltage is not below 0.7V. That shows that the protection diodes do their job. You can measure the current if you wish, but I bet it won't be anywhere near the recommended maximum input currents on the pins of the op-amp and MCU.

The op-amp "minds" the negative voltages just as much as the MCU (i.e. not so much), even when it's off, since it does not lose the substrate connection to ground, and the protection diodes between pins and ground are still active.

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The photodiode current will pass throught the load resistance creating the negative voltage when VCC is zero. A diode placed as shown will limit the voltage to -1 diode drop. The diode in this case is forward biassed.

When VCC is applied, the diode will be reversed biassed. Any reverse leakaged will easily be handled by the now functional op-amp, so a Schottky dioide could be used to keep the negtive voltage small.

schematic

simulate this circuit – Schematic created using CircuitLab

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