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So I realized that it might become a necessity for me to use a transimpedance amplifier for a pulse watch that I intend to make.

Some background:

The pulse watch shoots light at 940 and 660nm at the wrist and picks up the reflected light in a photodiode (https://dk.farnell.com/hamamatsu/s1223/photodiode-pin-to-5/dp/1495587).

Hence why the signal that the photodiode picks up may be diluted and subject to noise, if I don't treat it correctly. The V_Out signal is supposed to go into my Arduinos analog port from which, I (hopefully) can draw a nice pulse curve.

As I'm not an electrical engineer, the primary part that I'm unsure about is the operational amplifier that I inserted into the circuit. I tried to look around for similar applications, but I'm still unsure about the capacitors, and resistors that I've put in there (if the values are correct).

The op-amp I intend to use is this: https://dk.farnell.com/texas-instruments/op07cd/ic-op-amp-low-offset-smd-soic8/dp/9589929?st=OP07

enter image description here

  1. Is this application of the operational amplifier correct, with correct resistor and capacitor values, i.e. will this provide me with a nice low-noise amplified output?

  2. The loop after the photodiode with R3 still confuses me. Is this just to pull V_Out low when no current is flowing?

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  • \$\begingroup\$ For one thing, the + and - of your op amp are backwards. Just swap them. \$\endgroup\$ – wbeaty Nov 28 '18 at 18:50
  • \$\begingroup\$ I realized. Will I be good then? \$\endgroup\$ – Jeppe Christensen Nov 28 '18 at 18:51
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  1. The front page of the OP07 datasheet says it can run from +/- 3 V supplies, but all specs are for +/- 15 V. And you're only providing 5 V (with a resistor in the negative supply line, why?).

  2. You have your op-amp connected for positive feedback, not negative.

  3. Once you fix the feedback, your circuit will try to produce a negative output voltage proportional to the photocurrent of D7. But the op-amp has no negative supply voltage, so it won't be able to do this.

You need to go back to the drawing board and re-think your op-amp selection, how you power it, and how you make the feedback connection.

Various IC vendors (Analog, TI, etc) have app notes showing how to design an op-amp TIA circuit, so consider using a proven design from one of those.

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  • \$\begingroup\$ So what you suggest, is turning the op-amp around, and use another amp like this: dk.farnell.com/microchip/mcp6041-e-p/… ? \$\endgroup\$ – Jeppe Christensen Nov 28 '18 at 18:47
  • \$\begingroup\$ @JeppeChristensen, Fig 1 here shows a reasonable TIA configuration with a single supply. You might want to use your ADC's reference voltage as the input to the non-inverting pin rather than a resistor divider. \$\endgroup\$ – The Photon Nov 28 '18 at 19:06
  • \$\begingroup\$ I see, but I need more knowledge in order to understand what's going on here. it's extremely intimidating, as I don't have a background in this. \$\endgroup\$ – Jeppe Christensen Nov 28 '18 at 19:44
  • \$\begingroup\$ my.vanderbilt.edu/imaginginstrumentation2016/2016/01/… could something like this work as well? - this, I somewhat understand. \$\endgroup\$ – Jeppe Christensen Nov 28 '18 at 20:02
  • \$\begingroup\$ @JeppeChristensen, yes but you need an op-amp with very good rail-to-rail input and output if you want to be able to measure small optical signals. \$\endgroup\$ – The Photon Nov 28 '18 at 20:46

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