What is the proper way to measure an external supply like a 9V battery which is not connected to the controlling part (micro-controller) of the system. Take an external 1-Channel SAR ADC with SPI interface like MCP3001 and a micro-controller which has its own supply. I see we must divide the 9V first to 5V so the MCP3001 can properly handle it. But where to connect the VREF pin of MCP3001? Can I connect it to VDD like usually? Also do I need a galvanic isolation?
How to properly measure voltage of external supply
adcbatteriesisolationvoltage measurement
Related Solutions
As long as you can tolerate tying each end of the resistor to ground via a large resistance (500K or so), without disturbing the circuit, you can use an arrangement like this:
where you use two channels of the ADC (or two ADC's), and compute the difference between them.
The voltage dividers as shown allow only 20% (100K/(400K+100K)) of the voltage to appear at the inputs to the microcontroller. Choose other values as needed.
As Olin as pointed out, this arrangement exceeds the maximum recommended impedance for PIC ADCs, due to the time constant of the ADC input capacitance and the source impedance. It might still be okay as long as the voltage across the resistor doesn't change rapidly and you can tolerate a long acquisition time. It might be worth trying and see if the values you obtain are suitable for your application.
That PIC has an internal 1.024V bandgap reference, selected by setting the Channel Select bits to 11111.
So if you set your ADC module's reference to Vdd and then tell it to measure the 1.024V reverence, you can infer back to what your Vdd actually is.
Normally you would work out the voltage on a pin which you've A2D'd with something like:
Vin = (ADCval/ADCrange)*Vref
But in this case its Vin which you know (1.024v) and you want to solve for Vref (your Vdd), so:
Vref = (Vin * ADCrange)/ADCval
or more specifically for your case:
Vdd = (1.024 * 4096)/ADCval
Best Answer
Since the MCP3001 doesn't have an internal reference voltage source, you will have to supply an external reference voltage that your input voltage will be measured against.
Depending on the measurement accuracy required, you can either use your ADC's supply voltage or a dedicated voltage reference like these. Either way, the accuracy of your measurements will depend on both the ADC's accuracy and that of your reference voltage.
Also remember to adjust your input voltage divider to scale the battery voltage to the range between 0 V and Vref (which might be lower than 5 V).