If a simple attenuator is suitable for your design then a circuit like this would do the trick. I threw this together very quickly with LTSpice to show the concept.



With a sine wave excitation input that has a peak-peak swing of -10V to +10V the output of the attenuator (for the resistor values shown above) is about 1mV to 2.49V.



So some minor tweaking may be required if you want -10V -> 0V and +10V -> 2.5V exactly. For the circuit shown the VREF source is sourcing current up to about 1mA when the VIN is at -10V and so this needs to be taken into account when selecting the VREF component. Sometimes it is desirable to buffer the VREF from the regulator with a unity gain buffer to the attenuator.

Note that an attenuator like this puts a non-symmetric current draw on the signal source and is one of the reasons an opamp input may be desirable.

Michael Karas