LTspice with wheatstone bridge

Hello!

I'm current sketching a sensor circuit with wheatstone bridge. The resistance can vary between 1 ohm to 1 megaohm depending on the conductivity of liquid.

As the output is ac voltage, i purposely connect a diode to the output so that it can block out the negative voltage reading. The positive voltage reading is important for data processing in A/D converter.

I have been trying to simulate it via LTspice but there is an error. How can I correct the circuit? Is there any way to filter out the negative reading instead of using diode?

I have uploaded my file below. Any suggestion and help would be kindly appreciated.

hi.
You had the diode as a model '1', no such model, also the output of the diode was connected to 0V.

modified asc

EDIT:
This is one option for 'using' the negative half cycle of the signal, full wave precision rectifier, Draft2b.asc

Thanks for your help ericgibbs, it works!

However I've changed the circuit a bit by not including the first part of op amp. Ripple for me is not necessary.

Output voltage appears only the positive half without affecting the peak value. Two op amps will definitely eat up more power.