I designed a piezo amplifier with the AD8331 as LNA and PGA. The piezo element is very close to the amplifier, so instead of impedance matching I try to keep the input impedance as high as possible to avoid voltage drop. (Piezo Impedance is approx 40 Ohms in resonance). Therefore I do not use the active termination network and leave it open.
The input is DC decoupled with a 100nF capacitor. Since the piezo is practically a capacitor, the input is an open circuit. (true only without signal of course)
I experience the LNA gets unstable when the input is open, independent if the piezo is connected or not. Applying active termination does not help. Also the suggested ferrit bead on the input does not make it better. The only solution is to insert a resistor in the kOhms range or lower from the input to ground. This makes the LNA stable, but the resistor adds noise of course. But making the resistor smaller will make my signal smaller also.
Is there any non-resistor solution to this problem?