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OPA320: Photodiode

Part Number: OPA320

Hello TI community, 

I am trying to implement a simple current to voltage conversion using OPA320, photodiode BPW34 (input current 50u) and feedback resistor (1Mohm). The frequency of my signal is just 60 Hz, so not using any feedback capacitor. I am giving Vcc of 5V and attached a schematic here. 

Unfortunately I am not getting any output on the oscilloscope at pin 1. 

Could you please let me know what could be the issue. 

Thanks for your time. 

  • these designs always require the source capacitance and often that requires a negative bias on the photodiode, looks here like you have zero volts across it? Once you get that source C, then we do need to add a feedback C for stability

    as for no output, maybe there is no input. Also, you are showing a positive signal on the negative supply? and ground on the positive supply? perhaps you should just put a positive 5V on the +supply and ground on the negative, 

  • Hi Anubha,

    your circuit is instable:

    I would do it this way:

    anubha_opa320.TSC

    Kai

  • The circuit is unstable because without CF the loop-gain phase margin is zero - see below.

    Asssuming photodiode junction capacitance of Cj=72pF, adding CF of 10pF is enough to stabilize the circuit - see below.

    R1 and R2 used by Kai lifts the output by 50mV into OPA320 linear region so you may measure correctly photodiode current down to zero current - see below.

    anubha_opa320 AC.TSC