I'm designing a circuit that requires me to measure three low value (8-60 ohms) resistors with a hard wired common connection on one end. My ADC doesn't have enough inputs to measure all three at once along with everything else it needs to do.
My two questions are:
What's the best method of current measurement? A Wheatstone bridge will draw way too much power, so I'm leaning towards a constant current source, and measuring the (amplified) voltage drop over the resistors. At 20mA (I don't want to go much higher as the resistors may warm up and change resistance) this gives me a voltage drop range of 0.16-1.2V.
How should I switch between resistors? Automatic switching is required as this device needs to be a one-button operation - plug in sample, press button, receive information label. Is there a better option than two SPDT relays?