Electronic – Multiplexing of resistance measurements

measurementmicrocontrollerresistance

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:

  1. 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.

  2. 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?

Thanks!

Best Answer

Get a dual 3:1 analog multiplexor. (Or 4:1)

Wire the first 4:1 to each of the 4 resistors, with the common node to Current Source.

Wore the 2nd 4:1 also to each of the 4 resistors, with the common node to ADC Vin+.

Tie the ADC Vin- to "GND". I presume you can implement a 10 milliohm Ground connection between the 3 resistors. The value of 10milliOhm * 20mA is an error of 200 microVolts out of 0.16 volt, or error of 0.16/0.0002 = 1/800 or 0.125% error due to GND plane/GND wiring resistance.

Related Topic