Can it be used as a normal op amp, as a subtractor?
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You can use an FDA as a subtractor. Unlike an op amp used in the differencing configuration, which utilizes the nature of the inverting and noninverting inputs to perform the subtraction, with an FDA you must provide an opposite polarity signal into one of the inputs. This effectively 'swaps' that input signal path into a method for subtracting.
As an example, here is the subtractor circuit from the TI Analog Engineers Circuit Cookbook :
As you can observe, the output signal Vo is referenced to the signal Vref, which can be set to 0V or GND. The non-inverting terminal takes the signal to be subtracted from, and the inverting terminal takes the subtracted signal. This is V2diff - V1diff, shown in the image as Vo.
For an FDA setup, the resulting voltage will be the differential voltage between Vout+ and Vout- of the THS4131.
For an FDA circuit:
As you can see above, the image shows that the signal V1 is matched to the noninverting input of the FDA for its positive signal, and the negative output of V1 goes to the inverting terminal of the FDA. For signal V2, this polarity must be switched to perform subtraction, otherwise the signals will sum instead of subtract. Observe how signal V2 has the positive output go the inverting terminal of the FDA, and the negative output to the noninverting terminal of the FDA.
Utilizing the FDA as shown will result in comparable behavior to a normal op-amp difference circuit. I encourage you to perform a simulation and confirm this behavior for yourself and your application need.