EMC Issue with op-amp ckt

[umesh49]

Hi,
I am having an interesting issue with an op-amp circuit during radiated immunity test.
It is happening within 1GHZ to 2 GHZ frequency. I have designed this circuit for the gain of 1.34 and it world good, but at some frequencies between 1GHZ – 2HGZ it seems to be showing a different gain.
With my input was 2.26V my output (without RF applied) was 0.83V, but after RF was applied it became : 0.70V. I had checked the voltage at + input of op-amp and it does not change much.
Looked at op-amp bandwidth data and gain should always reduce at higher frequency but I am not able to figure out was is making my gain to increase.
All the voltage measurement I did was using high end scope and output and input voltages were DC with no noise on it.

Any suggestion will surely be very helpful. I am attaching a picture of circuit.

Regards,
Umesh

 

[crutschow]

It's probably not a gain change, it's more likely the radiation is being rectified by some of the internal circuitry generating an additional (subtractive) output voltage. What is the output voltage with the radiation applied and no input?

Is the circuit shielded in any way? If not you may need to add some or place it in a metal housing.

 

[FvM]

Instead of/in addition to a filter capcitor between both OP inputs, you'll want filter capacitors against ground, also supply bypass if not already present.

This is a slow amplifier, it shouldn't be too complicated to immunize the circuit against GHz signals, even without metal screening.

 

[umesh49]

FVM, Crutshow
Thanks for your reply.
There is .01uF cap at supply line of op-amp.
I tried shielding the whole board and grounded the shield but it does not improved the performance much. I have one more op-amp on circuit (it is inside an ASIC) and its output also shows an offset in voltage at those frequencies. Both the op-amp are having 1MHZ Gain bandwidth product.

 

[crutschow]

So it's an offset voltage and not a gain change?