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LMP7718: High gain trasnimpedance amplifier

user5010808
Prodigy 30 points
Part Number: LMP7718
Other Parts Discussed in Thread: TINA-TI

Hi everybody,
I am currently designing a high gain transimpedance amplifier to amplify the signal of a pyroelectric sensor.
Already read stuff like tidu535.
I chose the LMP7718 mainly because of the low input bias current.
We need a very high value for the feedback resistor (>1Gohm). I connected the IN+ to a virtual ground in order to see negative pulses.
The problem is that the offset at the output of the amp depends on the value of the feedback resistance. With a 1G resistor the amp is saturated.
Also when we connect the amp as a follower we measure 10 mV at the output.
Any idea of what could be wrong here?

Thank you very much for your help.
Herve

  • 0
    TI__Genius 15975 points
    Hi Herve,

    Is there a reason to use such a high value of feedback resistor for the TIA? As you have noted, the output offset would be huge which could cause the amplifier to saturate.

    Instead, would it be possible for you to use a lower value feedback resistor and then cascade the TIA's output subsequently with two or threes stages of voltage amplification? It might also be good to simulate your circuit in TINA-TI with the appropriate input photo-diode capacitance as noted in tidu535, to make sure the circuit is working as expected.

    Best Regards,
    Rohit