For the precision multimeter reference design, the product page mentions that the built-in current source along with Vin+ and Vin- inputs can be used to make resistance measurements. Is there more information on how to exactly set up the resistance measurements? BTW if Vin+ and Vin- inputs can be used for resistance maybe they should be labeled different??
Thanks for the question! The easiest way is to route one of the current source outputs either, IDAC1 or IDAC2, to the VIN+ connector and tie VIN- to ground. From there connect the Vin+ and Vin- accross the resistance you are trying to measure. Turn the current source on to a known current value and measure the voltage drop across the resistance. From there just apply Ohm's law: R = Measured voltage / IDAC current.
The maxium voltage drop across the resistance that the current source can support is just under 2.5V (internal volt ref used with IDACs). You can match appropriate current values to resistance ranges to maximize the voltage measurement accuracy. For increased accuracy at the tradeoff of longer measurement times, use the IDAC rotation feature. The current source ranges should support a maximum resistance measurement of about 50kOhms. Check out our example code to see the process in action.
Another method you can use if you have the optional 2.048V voltage reference is to use that output instead of the IDAC. In this case you would need to add a precision resistor of a known value in series with the 2.048V voltage reference, the unknown resistance, and ground. Then you make a voltage measurement across the unknown resistance. From there you apply a voltage divider equation based on the voltage reference and known resistance to get the unknown resistance. This approach is nice because you can use it to measure much larger resistance values compared to the current source measurement.
Good point on the Vin+ and Vin- labels, we will make a note of that for the next update!
