Differential Amplifier

Usage:

int output = analogDiffRead(A0, A1, GAIN_8);
// output = (A1 - A0) * 8
int invalid_output = analogDiffRead(A0, A5, GAIN_8);
// invalid_output = -1

Also see the examples in the differential_amplifier library provided with this core.

The differential amplifier (DAP) consists of

where

output = gain * (non_inverting - inverting)

And the output can be read through the ADC (like a normal analog read).

Both inputs and output must be between gnd and vcc.

All information was extracted from this translation of the datasheet: (English) LGT8FX8P_databook_V1.04 (thanks to metallurge for the translated doc here)

Setup the inputs and gain and turn DAO on through the DAPRC (Differential amplifier control register):

DAPCR = bit(DAPEN) | gain | inverting | noninverting;
- Enable: DAPEN
- Gain: GA[1:0]
- Inverting input: DNS[2:0]
- Non inverting input: DPS[1:0] Important: Only one input can use the multiplexer. This means that not all combinations of pins can be compared (See table in the library source)
Configure the ADC input to use the Differential amplifier instead of reading directly from the analog pins

ADCSRC |= DIFS_DIFFAMP;
- ADC input switch: DIFS
Setup the multiplexer if necessary and make the reading

int res = analogRead(inverting == INVERTING_MXER ? pin1 : pin2);
- For this, I'm just reusing the standard analogRead(pin) function which also configures the multiplexer.
- Note: for some pin combinations it is not necessary to use the multiplexer, but for the sake of simplicity I still use the normal analogRead(pin) function. In this case it doesn't matter which pin is passed to the function, as the DAO won't use the output of the multiplexer.
Cleanup:
- Turn DAO off:
  
  DAPCR &= ~bit(DAPEN);
- Undo step (3) (reconnect ADC to multiplexer for future normal analogRead calls)
  
  ADCSRC &= ~DIFS_DIFFAMP;