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Below is a transimpedance amplifier (OPA380) that I'm using to convert a pulsing current signal from a photodiode in (SFH7050).

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Everything works as expected when I was using 2.5V and -2.5V as supply, eg. output voltage and frequency response.

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When I change the supply voltage to 3.3V and GND as supply, everything just changed. In this case, the frequency response slide downwards from 1kHz onwards and the output voltage is not the same as before. The output voltage drop from V to mV.

I can't figured out what's wrong as the datasheet mentioned it could operate in a single supply from 2.7V - 5.5V. I can't post more than 2 links currently, so I'll try to provide more info in the comments.

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  • \$\begingroup\$ How does the circuit with the single supply look like? \$\endgroup\$
    – Mario
    Mar 5, 2016 at 10:28
  • \$\begingroup\$ @Mario the circuit looks the same just that V- is replaced with a GND at the negative rail of op-amp and V+ is increased to 3.3V. \$\endgroup\$
    – Jerry
    Mar 6, 2016 at 6:08

1 Answer 1

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When you had split +/- supplies the current source injected its signal relative to 0V i.e. halfway between the power rails and of course the op-amp will prefer this.

In your 2nd setup you are injecting you current source relative to the most negative rail (again 0V) but how can the op-amp produce a negative voltage under these circumstances? The data sheet indicates that the op-amp cannot be relied upon to produce a sensible output voltage within 100 mV of the most negative rail.

Try biasing the non-inverting terminal to mid rail by using a resistor potential divider.

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  • \$\begingroup\$ I've tried biasing the +ve terminal to mid rail which in this case is 1.65V. The frequency response and output voltage is back to normal but there's an unexpected spike at my frequency response. But tuning the bias voltage to 1V are able to make it back to normal. \$\endgroup\$
    – Jerry
    Mar 6, 2016 at 6:37
  • \$\begingroup\$ Do you know of anything that might have caused this spike? Another than the spike the frequency response is also reduced to 10kHz instead of the original 1MHz. \$\endgroup\$
    – Jerry
    Mar 6, 2016 at 6:51
  • \$\begingroup\$ Try a 100n cap midpoint to negative rail. \$\endgroup\$
    – Andy aka
    Mar 6, 2016 at 10:03
  • \$\begingroup\$ Also, make sure that the photodiode is reverse biased by this new configuration. \$\endgroup\$
    – Andy aka
    Mar 6, 2016 at 10:23

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