Issues with getting a clear output signal when using a RasperryPi, Sparkfun Minigen, and amplifiers.

Thread Starter

Justin Long

Joined Dec 1, 2015
For my school senior design project we are using THIS BOARD, with the Raspberry Pi 2 Model B. We are having some issues getting a clean output when sending the signal from the RaspberryPi, to the Minigen, then through some programmable gain amplifiers (PGA's DATASHEET). I think it may have something to do with needing a cleaner ground because the signal looks like two sine waves flickering back and forth and that seems like a grounding issue (but that is just a guess and unfortunately I dont have a picture of the scope output to show). We are currently using a 3.3V pin to power our Minigen and then several GPIO's to control the Minigen and the PGA's in the circuit.

This is our current circuit schematic (resistor values still up in the air and power to the op amps is currently coming from lab equipment but will later come from a transformer of some sort) for the project and it has changed a lot so we are open to suggestions. I was hoping someone could point out any problems they see with it or offer suggestions for how we might get rid of the issues we are having. HERE are the op amps we are using after the PGA's and as a summing amplifier, it is the 552 from that datasheet.

Just to explain how the PGA's work if you dont want to read the datasheet above, they are controlled using 3 bits that give it a gain from 0-7. When you send it, "000" it acts as an open circuit, "001" it has a gain of 1, "010" a gain of 2, and so on and so forth up to "111" where it has a gain of 7. Our intent is to use three of them so we can achieve a decently small step size and sum them with the final opamp at the end to achieve a variable output from 0-60Vpp.

Any guidance or help is greatly appreciated and let me know if any additional information is needed.