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TI Op amp for photodiode application

Martin Kojtal
Intellectual 280 points
Other Parts Discussed in Thread: OPA348

Hi,I'm working with single supply 3.3V because of the MSP430. I would like to ask for help because i want to order OP amp samples. Im browsing TI site and checking op amp's. What should i consider in my case and which op amp would fit for this application?

 

 

  • 0
    Guru 21975 points

    Martin,

    Without knowing details of the application, I think the OPA348 would be a good op amp consider. The 2.2M resistor in the non-inverting input (pin 3) of the transimpedance photodiode amp is not required or desirable. Because the signal is AC-coupled in the next stage, you may want to consider applying an offsetting voltage to this input. This would offset the output of the transimpedance amp by the same voltage and assure that it is not driven to ground with zero signal level. This would avoid a dead band with small signals. This offset is arbitrary, perhaps 0.5V or so. It could come from a resistor voltage divider but should include a large capacitor to ground to thoroughly filter any noise.

    There are other sources of dead band in the subsequent stages, as their outputs may be "stuck" at ground until their offset voltage and output swing range is overcome. Consider offsetting these stages, also.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Intellectual 280 points

    Hey,

    I'm sorry i really havent provided enoguh details. The schematic is part of an pulse oxymeter. It should measure the signal between 0.5-5Hz (there's bandpass to allow only those frequencies). Led's are pulsed 1KHz (1ms).  Ac signal after amplification got amplitude around 1V.

    I'm concern about choosing 2 amplifiers. The first transimpedance and the last one in the same picture with gain 52 (34dB). I've read few articles and they all suggest FET input op amp with photodiode. I prefer op amp available in PDIP package. I'm newbie in op amp, im studying and reading nowadayz about the topic more,that's why i asked for help here. I assume for somebody who deals with it daily, should be trivial .

    I'll add offset as u suggested..Thank you for tip

  • 0 in reply to Martin Kojtal
    Guru 21975 points

    Martin,

    Pulse oximeters generally rapidly alternate between the IR and red LEDs (at 1kHz in your case). The critical information on oxygen concentration is derived from the ratio in amplitude between the two LEDs. It would seem that you would need a bandwidth well in excess on 1kHz to derive this information so I'm curious about the narrow bandwidth of your filter.

    The OPA348 has a FET input (CMOS) and should be good for all the op amps in your application. Sorry, but our modern op amps are not available in DIP versions. SO-8 packages are a bit more difficult but entirely possible to hand solder.

    Regards, Bruce