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help me findout better solution for the photodiode ,the current is from 8.7nA to 600uA

zumao xu
Prodigy 60 points
Other Parts Discussed in Thread: OPA376, LOG114

i used the opa2376 for the photodiode circuit , the current is from 8.7nA to 600uA and the reference voltage of AD is 1.2V.  the range is very huge .so i used two stage amp circuit for the signal. the first stage is current -to voltage  ,the ciruict is normal circuit for photodiode , and V1=I*R,  R=1k, the second  stage  is voltage amplification V2=A*V1，A=150 .  but when the circuit is on power and the input current is 8.7nA , the output voltage  V1 is about 0.6mV, i  do know the why the output is 0.6mv ?  the input  offset votage is  just 25uV and the nput Bias current is 10pA  on the OPA2376 datasheet , but the theory voltage is equal to 8.7uV(8.7na*1000).  the output voltage 0.6mV is greater than the theory votage 8.7uV. Pls help me , if you have better solution to  ampilfy  the signal current from 8.7nA to 600uA ,PLS recommend to me, thanks very much.

  • 0
    TI__Guru 67541 points

    Zumao,

    I do not see any issue with OPA376 photodiode application you describe - with zero input current, IS1, the output is at -25uV while with IS2 of 8.7nA the output voltage is at -16.6uV as it should (please see below).  I lifted the input common-mode voltage by 1.2V off the ground (Vcc=3.7V, Vee=-1.3V) so the output is not saturated (in non-linear range).  If you continue to have a problem, please attached the circuit schematic so to avoid any ambiguities with your precise application.

  • 0
    TI__Intellectual 2770 points

    Hello Zumao,

    Some people are using Logarithmic Amplifiers to cover a wide input range. The LOG114 was developed especially for converting the wide range of input current from a photodiode to a voltage. The input range is from 100pA to 10mA. For a change from 8.7nA to 870uA the output would change by 5x0.375V=1.875V (this default scaling factor can be adjusted).

    Maybe it's interesting to have a closer look at the datasheet: http://www.ti.com/lit/ds/sbos301a/sbos301a.pdf

    Best regards,

    Thomas