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Need help for Selection of low noise Quad Opamp for Photodiode Transimpedance Amplifier

Faisal Rehman
Prodigy 180 points
Other Parts Discussed in Thread: OPA4354, OPA4277, LMC6082, OPA2320, ADS1258, OPA4376

Dear TI Employee and members,

I am looking to design a Photodiode Transimpedance Amplifier for converting the photocurrents of an ASIC image sensor (4x4 photodiodes) with extremely low currents <1nA and capacitance <1pA  . I will be using  20 MOhm or 50 Mohm Transimpedance so the voltage would be in the range 8mV-20mV. I am bit worried about the noise levels as the photocurrents are too small. As I also have space constraints on the PCB so I will be using Quad Opamps possibly in single or double stage i.e. TIA followed by voltage amplifier. My requirements for a Quad Opamp are as follows:

Low Offset Voltage: 80 µV or less

Single-Supply Operation: 2.5 to 7.5 Volts

Low Noise: 8 nV/sqrt(Hz) or lesser

Bandwidth 20MHz or lesser.As I am getting only the DC currents from photodiode  so High bandwidth is not necessary

I was looking in the meanwhile in the data sheet of OPA4354 with 250MHz bandwidth.But not sure if it could be the right choice as I dont require high bandwidth.

Can someone please help me select a suitable Quad Opamp for my application?

Secondly I am thinking to place the ASIC and opamps close toghether on a small PCB, I would also appreciate if someone suggest tips for better design especially grounding techniques.

Thanks in advance.

Best Regards,

Faisal

  • 0
    Prodigy 120 points

    Hi Faisal,

    The OPA 4354 would have been ok, except that it has really high offset voltage .... 8mV which I think does not match your specs. There are a few Opamps that would match your requirements. The best one seems to be OPA4277. However you could also look at OPA2111 or the LMC6082 - both TI parts.THere is also the OPA2320 to consider, unfortunately it comes in dual package only.

    The Offset voltage plays a major role in the design, so you have rightly limited your offset voltage requirement. However a lower offset voltage is always better since, lower the offset voltage, lower the dark current - which you will need to subtract from every measurement - either in your software or in the hardware itself. If response time is not an issue then the low slew rate of the OPA4277 should suffice. However if you need faster response time, then suggest you go for one having a higher slew rate.

    Yes the photo-diode should be as close to the opamp as possible. You could place a small cap across the feedback resistor. Or better still you should combine the gain stage with a second or third order filter since noise is a major issue, not just from the opamp but from other sources such as the photodiodes and light conditions.

    Hope this helps ...

    Best regards

    Anand

  • 0 in reply to Anand16722
    Prodigy 180 points

    Hi Anand,

    Thank you very much for your recommendations. According to the specs of opa4277, it seems to be a good option  but it comes with dual supply.I am looking for a single supply opamp as I have only dc photocurrents (without any modulation), it would make the things more simpler to use only one supply.

    Do you have any idea how can I reject the 50Hz supply noise? as this was a severe problem with my previous design. As I intend to use ADS1258 24bit ADC after the TIA , a 50Hz noise and a high frequency noise filter would be required.

    Best Regards,

    Faisal

  • 0 in reply to Faisal Rehman
    Prodigy 120 points

    Hi Faisal,

    Seems you are in a catch 22 situation. Your best bet seems to be the OPA2320. Unfortunately you will have to compromise on one of your requirements. Either the Dual supply or Quad opamp. OPA2320 is a Single Supply part but you will need to use two of these.

    Some ADCs come with built in 50 Hz rejection circuitry. I haven't used the ADS1258, so don't know if it does. But you can design a second order butterworth filter using Filterpro from the TI website. It is fairly simple.

    Best regards

    Anand

  • 0 in reply to Anand16722
    Prodigy 120 points

    Hi Faisal,

    I found the OPA4376 that might be a good match. Suggest you check it out.

    Regards

    Anand

  • 0 in reply to Anand16722
    Prodigy 180 points

    Hi Anand,

    Thanks a lot for your recommendation. I think OPA4376 would be well suited for my application due to its 4 channels, single supply (CMOS processs) and lower offset.

    Best Regards,

    Faisal