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OPA657 in die form
I am using OPA657 as TIA in the die form.
On my PCB, I have a TIA followed by a unity gain OPA657 amplifier to drive the Load.
Initially, I am using the 2 stages as voltage amplifier on PCB. APD is not connected yet.
Out of the 5 PCB's that we mounted the components on, 3 PCB outputs are sensitive when I shine Red light on the OPA657 die.
Is OPA657 in die form vulnerable to light? If yes, why only on 3/5 PCB outputs? The output starts jumping and becomes unstable.
The OPA657 is a decompensated amplifier, meaning that it is not unity gain stable, so I am somewhat puzzled when you mentioned that you are using it as a buffer to drive the load. For unity gain stable FET amplifier, consider using the OPA656 or the OPA659. A FET buffer optimize for gain of 2V/V is the OPA653. In that stage, the noise of the amplifier will not add significant contribution to the overall noise, so that spec should be relaxed.
You should see a voltage on the output if you shine a light on a die, but I would not expect an oscillation.
Also for the TIA, the APD parasitic capacitance is part of the compensation as at high frequency, the gain will be set by 1+Cf/Cs, so a lower input capacitance than what the amplifier expects for stability will yield an unstable amplifier.
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