Problem with coupling Op-amp???......

[karthickb3e]

hi all...,,,

i am using photo diode trans impedance amplifier in first stage using op amp LMV794.....

in second stage it is amplified to my required gain...
my doubt is

1.) the first stage output is clear with out noise...
2.) by coupling the second stage, there is noise of 10khz, what is the problem,,,, but direct input is applied to second stage there is no noise .. my doubt i coupling of two stages is problem ?????? please help..... to solve problem!


I attached the circuit in pdf...

 

[steppermotor]

Well for one as you may know the noise of the total system in this case:

F = F1 +F2-1/Gain1

From this equation you can see that the Noise Figure (F) is dominated by the first stage. This mean you have to make sure that the gain of the first stage is sufficient to support the second. Also you can also experience loss due to unmatched inputs and cable loss.

Also consider adding 0.1uF bypass capacitors on the power supply rails.

 

[crutschow]

You appear to have an oscillation in the op amps. Weren't you told to always decouple op amps with a 0.1uF capacitor directly from each power pin to ground?

 

[FvM]

In a short, the circuit of the second stage ignores the requirements for the decompensated OP. At least, C4 must be reduced. There's a detailed discussion of LMV794 feedback options in the datasheet.

As a minor problem, you're apparently continuing your previously presented idea of operating photo diodes without suitable DC bias.