slow ramp on output of op amp - help

Thread Starter


Joined Mar 25, 2009
I have a small circuit that is amplifying a very small signal from a photo-transistor. Everything seems to work fine except for the second stage of amplification. The output of stage two is going to a I2C ADC (Delta Sigma type) and I picked up the signal at the amp output and it has a slow 1+ second ramp up time.

I came across a app note from microchip about capacitative loads and op amps. I implemented a series resistor on the output and it improved the response but I can not remedy the issue (added R9).

I have attached the oscilloscope image and schematic.

Op amp used:
A/D used:

A side note: If I pick up the analog signal at C1 or the output of the first amp then there is no ramp.

Much thanks to anyone that has an idea. Analog design definitely is not my strength even for this basic circuit.

P.S. Link to the Microchip App note i mentioned