I'm trying to amplify a faint sine wave (+/ 100mv) from an ultrasonic transducer and then get the RMS into 03.3v range, so I can read it on a Netduino.
I'm a software guy by trade, so please excuse me if I use some terms incorrectly.
I have an ultrasonic transducer, which when I put it near the transmitter, gives a perfect sine wave centered on 0v at up to 100mv.
I used an LM386 and a coupling capacitor to to amplify that to a perfect sine wave up to the supply voltage (5v) where it clips, but I'm OK with that. My scope shows it as I expected.
I tried to use a true RMS chip (AD736JNZ) to get the RMS, and ran into problems  nothing works as expected. I simplified the circuit to test my assumptions, so I hooked it up as just a precision rectifier like on page 6 of their applications guide *PDF and got closer, but now things got even more confusing.
If my singal is low, the chip output flat lines at around +1.75v. If I increase my signal, it starts to output something that looks like a halfrectified version of my input, slightly out of phase, with amplitude inversely proportional to the input.
I'm stumped. I hope you all can help point me in the right direction so I can get on to the rest of my robot ;} Here's a diagram of my breadboard.
I'm a software guy by trade, so please excuse me if I use some terms incorrectly.
I have an ultrasonic transducer, which when I put it near the transmitter, gives a perfect sine wave centered on 0v at up to 100mv.
I used an LM386 and a coupling capacitor to to amplify that to a perfect sine wave up to the supply voltage (5v) where it clips, but I'm OK with that. My scope shows it as I expected.
I tried to use a true RMS chip (AD736JNZ) to get the RMS, and ran into problems  nothing works as expected. I simplified the circuit to test my assumptions, so I hooked it up as just a precision rectifier like on page 6 of their applications guide *PDF and got closer, but now things got even more confusing.
If my singal is low, the chip output flat lines at around +1.75v. If I increase my signal, it starts to output something that looks like a halfrectified version of my input, slightly out of phase, with amplitude inversely proportional to the input.
I'm stumped. I hope you all can help point me in the right direction so I can get on to the rest of my robot ;} Here's a diagram of my breadboard.
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