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Question asked by brianvg on May 20, 2015
Latest reply on May 21, 2015 by brianvg



I am working on a project which requires detecting very small modulations of photo-current from a photodiode.


The anticipated photodiode will have a 3pF capacitance when fully reverse biased. The expected variation in the photocurrent will be 290nA dropping periodically down to around 284 nA. The signal change is vanishingly small, but the hope is to be able to detect this minor change, and characterize its waveform using an ADC.


I came across the EVAL-CN0272 when looking for possible solutions. From reading the description, I see that it has been designed for a maximum current of 500uA, with a 12 bit converter, so I can only expect to see a bit resolution of about 122nA. This clearly wont work. Since this is a small scale, proof of concept, if I was able to alter the circuit this would be great. I considered that it might be possible to increase the size of the feedback resistor, sacrificing bandwidth for a smaller dynamic range and smaller bit resolution. A 10Mohm resistor should be able to reduce the bit resolution to 122pA, and reduce the bandwidth to around 330kHz.


My question is, have I missed anything? Does this have the potential to work? Even with the increased bit resolution, the number of bits which would change is small, but the hope is to condition the signal later digitally.


If this is impractical, are there any suggestions for a different evaluation hardware? The logarithmic amplifier probably does not make any sense because the signal has very little dynamic range.


Thanks for any guidance,