MODIFY THE TITLE:unflatness of the freq spectrum of tia output when there is NO input light.

Hi Baolin,

I would probably do it this way:

baolin_opa847.TSC

Don't forget to put caps across R3 and R6.

Kai

thanks anyway.Where is R3 AND R6? in simulation the freq maybe flat , in real test it's not .

Hi Baolin,

R3 and R6 of your schematic

Add a small series resistance to the output pin of OPA847 to provide isolation of capacitive loads. Or do you connect a scope with 50R input to the output of OPA847? Then add a 50R resistor to the output pin of OPAmp.

Can you show your layout and a photo of your setup?

Kai

Forgot to ask: What about C4=100pF? What is the purpose of this cap?

Kai

layout:

100p 1kΩ: CR  10M low filter circuit.

Add a small series resistance to the output pin of OPA847 to provide isolation of capacitive loads. 

can you tell me why ?

Hi Baolin,

capacitive loads erode the phase margin and decrease stability. Please read the section "Driving Capacitive Loads" of datasheet.

Kai

i know that ,i will remove the cap tomorrow. and do you have any suggestion about the layout?

Don't forget to put caps across R3 and R6. why should i do this ?in my view,this will decrease the input cap!

Hi Baolin,

Is this the same board that you have since redesigned and posted in another thread?

Best,
Sam

no ,they are the same version . In another thread, i give two designes.

Baolin,

why do you start two threads about the same circuit?

Kai

That's way too much inductance for a 4GHz OPAmp:

Kai

Kai,

maybe want to discuss more!concentrate on the unflatness! 

thanks!

Baolin