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I am designing a modular system that will take 9 or 16 photodiode inputs, depending on its setup, and put them through an op amp. The diodes and the op amps are on different PCBs, connected by pin headers and sockets.

The schematic looks like this: Photodetection schematic

However, some of the time some diodes will not be present (i.e. D1 will be missing), because on the modular diode board there may be only 9 diodes (if configured that way). The remainder of the components will still exist, as they are all on the same PCB.

The datasheet says to do this with the unused op amps: enter image description here

I can see no way to make either of these circuits by only modifying the schematic at the point where D1 is inserted, because the design of the PCB means all the + inputs are always tied to ground.

I effectively have one place to make changes, so the question is what to do with the - input. Leave it floating, connect it to a voltage divider, connect it to ground or supply via a resistor (or no resistor). I realise the situation is not ideal. Any suggestions?

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3 Answers 3

up vote 4 down vote accepted

There is nothing more you need to do. Your circuit without the diode is stable all by itself. R2 provides DC feedback so that the output will be driven to the same as the positive input, which is ground.

Think of it this way. Your circuit is basically a transimpedance amplifier. By disconnecting the diode, you are feeding 0 current into the transimpedance amplifier. 0 current is a valid signal in this case, and no different from the diode being in the dark.

Also, the datasheet is being too specific about what to do with unused opamps. You want to drive them so that they don't pick up noise or oscillate, but otherwise use little current. There are various ways to achieve this. They only show some examples.

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I totally agree, and I think that the datasheet A option is just a waste of power. –  Vladimir Cravero May 26 at 15:02

You can leave the opamp with it's feedback components and with no diode connected. This won't be a problem at all. The ideas suggested by the data sheet are just typical suggestions.

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On your modular board that holds the PD's , if the PD is not present, then substitute it for a resistor. What you are looking to do is to prevent the open input from being coupled to stray fields and causing the op-amp to consume excessive power and to cause cross talk on other channels. Connecting it to something that prevents activity is all that is necessary.

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