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.

 

You changed the generic diode "D" to a "1", and there is no model for a diode "1", which is why you are getting the error message.

Consider using a precision rectifier. Rod Elliott shows several examples of precision rectifiers on his site:
http://sound.westhost.com/appnotes/an001.htm

Don't use the "basic" circuit (the 1st one).

 

This cannot work well as shown. The diode will not conduct unless the peak voltage is more than the minimum diode drop, (about 0.6V for silicon).

You are also only obtaining the output from one bridge arm, so you cannot determine the balance point unless you have more circuitry which is not shown.

I would suggest using either a precision rectifier circuit using op-amps to "idealise" the diode response, or a level shifter / amplifier to bring the whole AC voltage into the ADC range.

You might also consider a differential amplifier to sense both bridge arms in order to detect the bridge balance condition - what are your requirements?

 

I need an A/D converter with Vref from 0V to 5V. The voltage sensed from the bridge circuit ranges from 19mV to 1V.

The problem is, is there any A/D converter which can sense the voltage as low as till millivolt and convert it to digital value? Is this the reason to use op amp/rectifier to amplify the voltage till 5V range, or I am not necessary to do that?

The problem with op amp is that offset voltage always exists and this disrupt the reading.

 

Use a dual supply instrumentation amplifier.
An INA128 will likely work for you.