Ideas on causes of excess noise in transimpedance pre-amp?

[Winfield Hill]

[email protected] wrote...

Come on Win - pay attention. In his original post Bret had already
assigned a value of 0.15pF to the parallel capacitance of his 10M
resistor, based on the frequency dependence of the closed loop
gain of the op amp.

OK, I missed that. Sorry I can't pay good attention, I'm too
busy elsewhere. But that doesn't keep me from doing some fast
keyboard typing every now and then.

 

[Bret Cannon]

I'm too buried to go through all the posts on this topic, so apologies if
this has been suggested before....but a 60 kHz peak can be caused by a
linear voltage regulator with the wrong bypass cap. The output of a 7805
looks inductive, and the bigger the bypass, the bigger (and lower in
frequency) the noise peak is.

You might try putting a 5-ohm resistor in series with the regulator output
and see if that makes life better.

Cheers,

Phil Hobbs

Thanks!!!!

That solved the excess noise problem! With a small resistor added to the
output pin of each voltage regulator, the excess noise that peaked around 60
kHz is gone on both circuit board versions of the transimpedance amp. Now
the output noise is at the Johnson noise limit of the feedback resistor
until well past the 33 kHz signal frequency. It seems to beautifully match
the theory.

Thanks for your suggestion and to everyone who contributed suggestions.

Bret Cannon

 

[[email protected]]

Thanks!!!!

That solved the excess noise problem! With a small resistor added to the
output pin of each voltage regulator, the excess noise that peaked around 60
kHz is gone on both circuit board versions of the transimpedance amp. Now
the output noise is at the Johnson noise limit of the feedback resistor
until well past the 33 kHz signal frequency. It seems to beautifully match
the theory.

Congratulations - both to you, for digging out the good advice that did
solve your problem, and to Phil Hobbes for identifying the noise source
that did make the difference.

For anyone who really needs a quiet 5V regulator - try looking at 5V
reference sources. They tend to be expensive, but some of them are
remarkably quiet.