I built a circuit using 2 opamps, 3 CR filters (hi/lo pass), a half-wave rectifier, and a smoothing capacitor that gives me a 0-4.5v output that is proportional to the sine wave I get from a piezo ultrasonic transducer that is receiving a 40khz signal. Basically, it's a measure of distance from the transmitter.
I'm driving it with a 9v battery, and it works like a charm. The transducer ouputs 20-200mV and I get a clean signal all the way through, as confirmed by my scope.
However, when I hook up the Netduino 2 plus to the same battery, it adds a horrendous amount of ripple, right about at 40KHz, so I can't filter it out. I tried up to 1000uf capacitor across the battery, and it helped, but it's still too much - it's drowning my signal at distance. I want to read the voltage on one of the analog input pins and drive a 7-segment display. All that is working - I just need to stop stomping on my signal.
I think it's due to the power regulator on the Netduino, but I can't be sure.
I was thinking I could use an optocoupler and separate power supplies, but I discovered that, like op-amps, optoisolators have their own entire set of concepts to learn. It took 6 months just to get this far ;-}
Any hints on how to proceed? all I need is to take 0-4.5v from one circuit and map it to 0-3.3v in another circuit with good linearity. I'd love to stick with one battery, but I can live with two.