I'm working on an ultrasonic detection system. We're driving a Piezo element on the end of a long coax cable, and we're driving it with a voltage spike anywhere from 100V to 500V, with a rise time around 15ns. The return signal is an ultrasonic pulse around 5MHz, and relatively weak (0.5mV).
I'm currently working on the receiver circuit, which listens to the same co-ax, next to the driver circuit. I'd like a way of passively surviving the spike.
I initially ran 5K in series with the input, and shunted that to gnd with 1N4148s (see below). The problem is the capacitance of the diodes draws too much current through the resistor and kills the signal. I could reduce the resistors, but then that starts to divert more power from the Piezo. I guess I could look for lower capacitance diodes, or run two in series, but it seems like there's probably a better way.
Anyone have any ideas?
..Piezo 5K ..Input>------////-----*------>LNA .. | .. | .. --- .. / 1N4148 .. --- .. | .. | .. | .. ----- .. --- .. -