Bootstrap technique to reduce noise in TIA

Our sensor(capacitance is around 1300pF) output current is very low(low light detection) around 1 to 10 micro amperes.. 

I tried with standard TIA design but due to very low level of current, it is mixing up with noise and I am unable to differentiate with noise..

I have tried the circuit referred in LTC6244 datasheet(bootstrap technique.. Fig 6a)..  But the opamp is oscillating or saturating.. No signal is seen.. 

OPAmp used is OPA656

Feedback capacitor replaced with 0.5pF. 

What could be the problem.???.

Observations:

OPAmp inverting pin is not at virtual ground. At Gate of BF862, 5V is observed.. (At this time opamp is saturating)



Sensor capacitance is around 1300pF. How I can analyse the circuit?? Please provide me the detailed analysis of the circuit... How to calculate feedback capacitance when capacitance of sensor varies or If feedback resistor varies??
[edited by: bhavani at 11:16 AM (GMT -5) on 9 Feb 2021]
Parents Reply
  • Hii 

    Thank u for the response.. Yes, I have LT spice and the circuit is simulated as well. There is no voltage of 5V is observed at the gate of BF862 during simulation.

    I have tested the circuit with AD8038 on Breadboard. At gate terminal of BF862, voltage of 5V is observed..

    Bias applied to photodiode is 55.3V..

    Schematic is Fig 6a in LTC6244 datasheet.. There are no other modifications in the circuit except 55.3V as bias and Photo diode capacitance is of 1300pF, IC used is AD8038

    Another observation is that, Bias without load it is 55.3V and when bias is connected to the circuit, it reduced to 39V approx.

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