Measuring Voltage Across a Floating Resistor

Projects General Guidance
1

What's the appropriate method to measuring the voltage across a resistor using the Arduino ADC where I know neither of the two terminals is ground but am sure that the voltage range for the resistor is between my Arduino's voltage capabilities (0V - 5V) since the system I'm measuring from is powered off the Arduino 5V and GND

My quick and dirty method has me taking an ADC measurement of the bottom of the resistor (V-) and an ADC measurement at the top of the resistor (V+) and subtracting to get the total voltage across.

Any suggestions on a better approach?

2

What you are doing is fine if all voltages are >= 0 and <= 5V.

3

Cool, Thank you.

4

whuds:
What's the appropriate method to measuring the voltage across a resistor

Your resistor isn't floating because your resistor is a part of that circuit with a reference node for measurements..... and your devices with the varying voltage are probably not open circuits.

As mentioned by jrem... if voltages on either terminal is in the range 0V to 5V... then you could use an A/D channel to measure voltage at 1 pin relative to gnd. Then measure the voltage at the other pin with a different channel. Then .... as you planned already .... take the difference in the measured voltages.

5

Other options 'd be to switch adc into differential mode, if its supported. Or use external differential/ instrumental amplifier to transform measurements from differential to single ended.