I have an MCP4921 DAC that is controlled by a Raspberry Pi.
Then, I have built a Howland current pump circuit that is able to regulate current depending on the input voltage:
Both of these work beautifully in the lab. I am also able to use the DAC to control the V3 voltage source on the circuit. Again, this works fine.
The last piece of the puzzle was to be able to pulsate the output current to have it look like a square wave at a given frequency. To do this, I have decided to try and use a CPC1150N solid-state relay.
I am able to use the PWM module on the Raspberry Pi to control the relay. What I tried to do was put the solid-state relay in between the DAC and the R2 resistance. However, this did not work. For some reason, I have a 4V voltage drop across the solid-state relay. Setting one end of the optical relay to 2V with the DAC, I measure 6V at the other end.
I am not very knowledgable with solid-state relays to understand why this is happening. My question is: Why is this happening? Would it even be possible to use a solid-state relay at that location and if not, what are the alternatives?
I have thought of purchasing a new DAC with switching capabilities, but wanted to know if there was another way before doing so.