abnormal oscilloscope behaviour

[kakeh]

About Circuit: The circuit is a simple voltage amplifier of photo diode current drop

before going to the circuit please find the attachments, which are the screen shots of oscilloscope that i am using

the circuit has a photo diode equivalent which in fact has lower current generated, which is causing the output on oscilloscope to look fluctuating its axis at regular intervals

kindly provide me a solution why the oscilloscope is behaving like this in low currnent scenario

in the screen shot attached just by putting a parallel resistance has effected the output and why ??

[mnauss]

Well, I am not sure how the simulator works in your case, but just looking on the data sheet for your Amp (which looks like a very cool part by the way) you have way too much gain, and I am not surpised it oscillates as stated in the data sheet. Below is the recommended Rf resistor values. If you were to actually breadboard this thing you would see a lot of instability with your current RF values. Layout will also be ultra critical. This is a note from the data sheet.

Unlike conventional voltage feedback amplifiers, the feedback
resistor has a direct impact on the closed-loop bandwidth and
stability of the current feedback op amp circuit. Reducing the
resistance below the recommended value can make the amplifier
response peak and can even become unstable. Increasing the
size of the feedback resistor reduces the closed-loop bandwidth.
Table 5 provides a convenient reference for quickly determining
the feedback and gain set resistor values, and the small and
large signal bandwidths for common gain configurations. The
feedback resistors in Table 5 have been optimized for 0.1 dB
flatness frequency response.