I have an analogue signal that's 0-4 V that I want to measure using a micro-controller that only measures up to 3.3 V. I want to reduce the analogue signal by 1 V. I've seen designs using voltage splitters, but the voltage reduction is a function of the input voltage. I need something that's a constant 1V.
While a little lazy for me to use 2 op amps when it can be reduced to only 1, this will accomplish what you are after.
It subtracts 1V from your signal, I presume your circuit does not have negative rails, so will clip anything that falls below 1V on the input
The left most op amp is just giving a buffered 1V ouput, the right op amp is a differential amplifier with a gain of 1.