# Op-amp differential amplifier circuit for inputs greater than positive rail

I'm trying to get the difference of two DC signals using an op-amp difference amplifier circuit. The gain will be 1. The difference will be around 1-5v. The inputs could be as high as 50v though.

Examples:

1. Vin+ = 50v, Vin- = 48v, Vout = 2v
2. Vin+ = 2v,  Vin- = 0v,  Vout = 2v


The problem is this would need the op-amp's inputs to be capable of going up to 25v. Is there a way round this? I have absolutely no idea how to solve this!

Edit: Using two potential dividers before the op-amp circuit that reduce the signals by 90%, followed by the difference amplifier with a gain of 10 would work but this would degrade the accuracy of the result by a factor of 10? Edit 2: And those dividers would have to be very stiff compared to those in the differential circuit

simulate this circuit – Schematic created using CircuitLab

• Why does the op-amp have to measure those inputs directly? Why don'y you just connect equivalent voltage dividers on both inputs? – krb686 Feb 11 '14 at 12:24

• @ScottSeidman There are other kinds of noise besides Johnson-Nyquist. If you assume an op-amp voltage noise $e_n$ (including 1/f noise/drift), the presence of dividers at the inputs increases that noise (when referred to the inputs of the dividers) by the ratio. So Dreistein is not wrong on that point (though the AD8479 is not magic, as you point out, the same thing happens with it, so a bit wrong). The resistor ratio does not actually reduce the CMRR (Common Mode Rejection Ratio), rather this is an application that requires a large CMRR, a bit different. – Spehro Pefhany Feb 11 '14 at 13:48