I use ada4817-1 and adg1414bcpz to convert current to voltage.
But the amplifier self-oscillate at frequency about 80MHz.
How can i cancel the unexpected frequency?
In attchment, ADA4817-1 convert current in DUT to voltage. ADG1414BCPZ used to
select feedback resistor to make gain variable.
Thank you for using ADI parts! I just need some more information about your setup to help you better.
Are you seeing the oscillation at all feedback configurations? The usual reason for seeing oscillations from the part is due to capacitive loading. What load is the ADA4817 driving? Am I interpreting your circuit correctly that you are also using the ADG1414 to connect the output of the amplifier to your load?
The ADA4817 is a very high speed part, so it needs extreme care. i would try the following:
Firstly, I would have the highest impedance feedback network (C221 and R88) permanently in circuit and not switched by the ADG1414. The other feedback networks would need some adjustment to component values to compensate for the fact that they are now in parallel with C221 and R88. C221 and R88 should be routed with the shortest possible tracks to the ADA4817.
Secondly I would add additional decouplers to the ADA4817 to work at frequencies above 10MHz. Your schematic does not indicate the dielectric type or chip size for C21 and C22, but their resonant frequency is probably below 10MHz. You should add something like a 470pF NPO capacitor in an 0402 size to decouple the supplies at frequencies above 10MHz.
"Are you seeing the oscillation at all feedback configurations?"
Yes, i do.
"What load is the ADA4817 driving? Am I interpreting your circuit correctly that you are also using the ADG1414 to connect the output of the amplifier to your load?"
The output of ADA4817 is connected to +IN of AD8065.
"I would take a fixed low gain in the first stage and do your scaling
in a second stage."
I use ADG1414 to select different range of DUT.
Is SNR ok if i use a fixed gain in the fist stage.
i will try your advise.
thank all of you.