I'm using an inverting opamp architecture with an AD8534 to sum the ac currents from two physically orthogonal sensing inductors. The magnetic sensors are for low frequencies 200Hz to 5kHz and the source impedance into the OP AMP is low - a few hundred ohms. The circuit is suffering RF rectification in the few hundred MHz region. I can't use a simple capacitor across the non-inverting input since the amp goes unstable. I can't add much series resistance and a capacitor because the passband response will be affected and at low resistance values, the amp is still unstable. I've tried ferrite beads on the inputs but that hasn't worked. I can't completely screen the sensors and that would probably stop them doing their prime job anyway.
I've looked at the EMI hardening app notes MT-070 and MT-096 but those suggestions don't work on the current summing input. Can anyone help with a solution or suggest an EMI hardened amp that I can try? The circuit works at 4V. Thanks.