I have a 4.096V reference for a ADC and I'd like a -4.096V reference for the -VREF on the ADC.
Question would an OpAmp do the trick or do you have a better suggestion / idea?
An op amp connected as a unity gain inverter will work well. Depending on the accuracy you require, you may need a low offset, low noise op amp, and precision resistors (perhaps 0.1% tolerance, or 1% resistors with a trimpot) to accurately generate the voltage level.
There are other methods, such as a flying capacitor switch, to invert the voltage, but that is more complex and has its own accuracy and noise concerns.
Opamp will introduce inaccuracies unless you get something with nice DC specs. If it is a shunt reference I would to what Pommie suggests and just use two.
in order to invert an OpAmp for negative output you must use a negative supply voltage anyway. Use an LM7905L and trim the output to the desired value.