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Arduino advanced analogue input
Usually analogue signals either are too small or too large or too negative to measure directly with the internal AD converter. Sometimes the internal converter performance is not good enough, or the signal needs heavy isolation for safety or sensitivity reasons.
The most common performance upgrade from 3.3V supplied single-supply opamp prescaler is to supply the prescaler circuitry with larger voltages, possibly double-sided. Care must be given that the different supplies are connected at the intended 0V point of each supply. This opens up the whole spectrum of amplifier circuits. (several pictures of still simple input stages)
Most common opamp supply design uses +15V and -15V leaving the 0V connection for signal reference ground. This has a lot of history and most circuits found on the Internet are of this type. As the Elduino system is powered from PoE, a low current +-15V supply is easily realised using very traditional 7805/7905 setup. (pic of so)
In order to fight cable induced noise a system of balanced transmission was invented. The signal is transmitted in its normal and complementary mirrored form on a pair of wires. Any noise induced in the cable is induced to both conductors and said noise is cancelled out at a balanced input stage. This scheme can be used to supply a input device its supply power with a phantom supply, as the signals are allowed to float away from zero. (several pictures of differential input stages using single rail and double rail supplies)
At times the input signals are far too large for a prescaler circuit to provide safety. Very small and delicate signals require a prescaler to be located very near the source and the signal could still be disturbed by digital devices. In cases like this, galvanic isolation is used for either safety or performance. Signals can be isolated using a suitable transformer, an optical isolator, capacitively coupling or even magnetoresistively. (several pictures of isolated input stages)