Hi there,

We have a high precision multichannel photodiode frontend prototype up and running and are now facing a new customer requirement: They want the whole thing even smaller. Since our current design uses an opamp that is only available in 2 channels per package variants we need to redesign with a new one that has a much smaller footprint. In general the specs of the AD8643 could be a good compromise, but we need more specs. Particularly the input capacitance and the input current noise spectrum would be of help. We definetely need the input capacitance for estimating the circuit performance and I can´t seem to find it in the datasheet.

Thanks in advance,

Sebastian.

Hi, Sebastian.

I simulated AD8643 to give you the rough estimate value of the input capacitance. The -3dB frequency is 41kHz based on the Bode plot. So to get the input capacitance, I substitute the values of R=1Mohms and f=41kHz to the formula f =1/(2*pi*R*C). This will give the result of 3.88pF or approximately 4pF but, I'll still verify this to the product applications engineer. Below are the schematic and Bode plot of the simulation.

Thanks,

Anna