I am switching an op amp input between two sources, one is ground, the other is a piezo material. I have an issue with switching transients which cause the op amp to saturate since I have a pretty high gain (x50). To keep things short, Here is the switch. I have a piezo element connected to s2, s1 is grounded. op amp's +input connected to D. Is there a way to prevent the transient from reaching the amp's input?
Here is what it looks like.
The hydrophone receives a sinusoid burst while the switch is in a logic 0 state, then once the burst is sent, it is switched to logic 1 and connected to the op amp. The idea is to read any acoustic reflections seen by the hydrophone. This has all been successful, but the transient is the one piece I would like to remove from the output signal. Unfortunately this is as far is my knowledge has gotten me so I have no clue where to go next.
Circuit is actually backwards my apologies. I just put it together in ppt since I don't have my files available with me. The hydrophone side should be S2 and op amp should be D.
Update:
I have some of the signals and here is what it looks like:
Noise from the switch, with and without the hydrophone
Here is what the reflected signal looks like. This is what I'm interested in, but as you can see it is quite low in amplitude, and thats why I need to amplify it.
@sperophany: As for the cable, I'm using Alpha wire 3302 twisted pair with a 50ohm bnc connector. It's about 6' long, 38pF/ft capacitance. more detailed specs can be found here http://media.digikey.com/pdf/Data%20Sheets/Alpha%20Wire%20PDFs/3302.pdf
Another thought I have as I was looking at the signals was to filter the switch's transient instead. It is pretty high frequency and well out of my system's range (<500kHz) so I was wondering if it would be possible to completely remove it with a lp filter. Any thoughts on this? Any drawbacks or new problems?
Overall, I just want to send a pulse and then listen for echos with the same transducer. I'm open to any and all suggestions – even if I have to scrap my current design and start from scratch.
Thanks again for your time, guys.
Best Answer
I don't see how a serious transient that would affect a relatively slow op-amp is being caused directly by that switch, nor do I see any problem the way you're using it in general.
Do you have a resistor directly across the piezo (not from the non inverting input to ground)? If not, then leakage may be causing the voltage to rise across the piezo while it is disconnected, and when the switch closes you get a transient as the piezo capacitance discharges.