TTL-controlled bipolar precision current source?

[awallin]

Hi all,
From an AD-converter for bridge-applications (e.g. AD7195) I get a TTL logic-signal for AC-excited measurements.
In a 4-wire measurement for an RTD temperature sensor (pt100) I want to use this TTL logic-signal to push either +500 uA or -500uA through the RTD (exact current value TBD).

I have a positive voltage reference (either +2.5V or +5.0V) and with a unity-gain inverting op-amp also -2.5V reference.

The AD-conversion will be done ratiometrically, i.e. we push the same RTD-current through a reference resistor that gives the reference-input for the ADC - which somewhat relaxes the precision requirement on the current source, but I'd still prefer it to be quite stable.

What is the easiest way to do this?
My initial idea was to use a simple op-amp circuit with either a high-side or low-side (does not matter much afaik) sensing resistor. Then the problem is how do we use the TTL-signal to easily switch between the +2.5V and -2.5V reference at the op-amp input? (without sacrificing the precision of the reference)

TIA,
Anders

[ve7xen]

An analog switch should be all you need. They have some series resistance, but it should be negligible for your use with high input impedance.

I suppose you could also put a switch in the current path, with two independent feedback loops etc. and just switch in the appropriate current source. Then your feedback loop would compensate any switch effects.

[awallin]

Thanks! should have thought of that 

Here's what I have now:
http://www.anderswallin.net/2013/06/pt100-frontend/

comments?