Differential analog signal conversion using Arduino Uno

Question

I need to perform a A/D conversion for a differential signal using Arduino Uno. I have tried a software method where I connect one input to A0 and another to A1, read ADC for both channels and then subtract it in the code. But the 100 us time to convert the analog signals is causing some errors as it is not perfectly a differential signal conversion (i.e. there is some delay between the two samples taken and hence it is not exactly differential signal).

Now I am not sure about this, but can I connect one of the analog inputs to the Arduino Uno GND pin and then perform a conversion for the other signal i.e. one input to ground of Arduino another to A0 of Arduino and then perform A/D conversion for A0. Would this be correct? Would it cause damage from a circuit perspective?

Answer 1

Given your requirement to measure a 20mV signal on top of 2.5V, one solution to this is to use an instrumentation amplifier. To get 10-bit-ish accuracy out of a 20mV signal you need 20uV-ish stability. One choice, only available in a rather tiny MSOP8 package, would be the AD8237. Use the maximum gain possible that will not result in any possible saturation (of the amplifier or overrange of the ADC). That way you can get about 10 bits resolution rather than just a few bits- maybe 200-250 times better.

You could use a lower performance difference amplifier, but it would greatly compromise the measurement.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

If you look at the ATMega328p datasheet you'll see that the ADC only supports single ended voltage measurements. For some other Atmel chips (e.g. the ATTiny45) you can change the negative pin used by the ADC by setting the ADMUX register. For instance

ADMUX |= _BV(MUX0);

should set ADC1 as the negative input.

If you look at the Arduino source for analogRead (wiring_analog.c) you'll see that this method resets the ADMUX register so you'd have to write your own function if you wanted to use a differential measurement using a chip that supported it.

Answer 3

You can use Arduino Leonardo, which is based on AtMega32U4, which supports differential ADC input. However, the Arduino library doesn't seem to support this mode, so you may need to manipulate registers directly.

There is a small example here: https://forum.arduino.cc/index.php?topic=276903.0

Answer 4

You should use one of these (hardware) solutions

• op-amp in differential circuit (LM358 - cheap and popular rail-to-rail op-amp)
• external differential ADC (MCP3421 - i2c, differential, slow, accurrate, easy to use but slow)

to measure differential signal with ATmega328, because this microcontroller ADC has no differential feature.

Trying to connect your signal source may be very bad idea (depending on your signal source).

If your signal can be sampled slower - you can add some RC filters to make voltage changes slow enough to get difference by measuring 2 channels one after another.