ADC Measure from source with higher voltage and different ground

Question

I would like to measure the voltage of a battery that varies from 4,5V to 3V.

To do so I'm planning to connect this source to a STM32F4 DISCO board and one of its ADCs, which will be set to a VREF+ of 3V. The conversion speed will be very low.

As the voltage from the source is higher than the STM32F4 Vref pin I need to add a voltage divider so that, taking into account I'll use 3V as Vref, the source maximum voltage (4,5V) is converted to 3V, something similar to the following schematic:

^{simulate this circuit – Schematic created using CircuitLab}

I'm trying to understand how to proceed to have a real measurement voltage value at the ADC_IN if I'm dealing with multiple grounds in the signal conditiong path, Battery GND, 12V GND and Microcontroller GND.

So, I have to main questions:

• How should I connect the ground nets to have a measurement as accurate as possible?

• If I have multiple sources to read from and, each source has different ground, could I still have accurate measurements from all sources? How should these grounds be connected?

Answer 1

If you're that concerned about grounds and accuracy, get an instrumentation amplifier like the AD8221: https://www.analog.com/en/products/ad8221.html

This would allow you to do truly differential measurements, and, as a benefit, the datasheet shows you how to set up your bypass capacitors to various supplies and grounds.

You can then use the Vref pin to move the instrumentation amp output to the center of your ADC range.

If you don't need hyper accuracy, I'd probably just tie the battery grounds to the microcontroller ground and then run the V+ terminal into the top of a resistive divider like your R1/R2 (10K/10K or 100K/100K depending upon the required ADC input impedance) above and then run the center directly to the ADC input.

The Beaglebone Black, for example, does this on some of its ADC lines because they are referenced to 1.8V instead of 3.3V.

If you really want to be careful you can probably tie the grounds together through a 100Ohm resistor if you really think you might get weird spikes. A battery probably won't cause any issues like that, though.

Answer 2

You could use a current to convey the quantity measured between two "ground domains". Have a fixed reference positive current source. Convert the battery voltage into a negative current. The leftover current from the reference is then converted to voltage right at the ADC input pin, using a resistor. Then the ground offsets won't matter much, as long as the compliance of the current sources is not exceeded.

Scaling is done by adjusting the load resistance. Offset is done by changing the reference current. Note that the 3.0-4.5V battery voltage is mapped to 3.0V-0.0V at the ADC input.

The current source I1 would typically be an LM334 configured for 450uA of current. D1's exact value is not critical, it can be 6-10V and is used only to extend the compliance of current source U1+M1 down to "ground" from the point of view of the ADC.

R1+C1 is a 10ms time constant lowpass filter. It ensures that the slew rate of the current source I1 is not exceeded (see LM334 datasheet). C3 smooths out the sampling pulses on ADC's input.

The A/D sampling rate should be kept constant and controlled by a timer, so that the average input current is constant as well. That way the sampling current will cause a constant offset on R3, and not noise dependent on the timing of the software loop that includes A/D sampling.

^{simulate this circuit – Schematic created using CircuitLab}