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schematic
how to make the freq spectrum become flat?the cap of the PIN is about 9pF,so two PINs constitude a cap of 18pF.
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zoom in
schematic
how to make the freq spectrum become flat?the cap of the PIN is about 9pF,so two PINs constitude a cap of 18pF.
Hi Baolin,
I would probably do it this way:
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
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
Hi Baolin,
the inductance of copper traces at the input (here 30nH assumed according to the 1nH/mm rule of thumb) results in strong ripple in the frequency response and even instability.
When working with a 4GHz OPAamp each millimeter counts. You cannot do with a 4GHz OPAmp what you can easily do with a 4MHz OPAmp. A 4GHz isn't forgiving even the least mistake!
Kai