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ADL5304 Output Slowly Ramps Up When Light is Blocked on Photodiode Input

Question asked by kaq on Sep 7, 2017
Latest reply on Dec 28, 2017 by kaq

My question is about the ADL5304 Log Amplifier.


I used this Log Amp on a board design and in general it works fine.  I am able to get 8 decades of dynamic range (outstanding!).  The input to this Log Amp is a photodiode, an EPM705, similar or identical to the one that is used in the Application Section of the ADL5304 data sheet.


However, I observe that with no light on the photodiode,  the output of the log amplifer slowly ramps up to about 2 volts.  I am very careful to make sure there is absolutely no light on the photodiode.


Is this due to leakage current in the photodiode?  I know that the ADL5304 has "Adaptive Bias Current" control, and I exploited that feature in my design. 


I have attached the schematic here.