I have this project to build an instrumentation system to measure the respiratory rate.
We decided to use a thermistor to measure the variation on the temperature of the air inhaled and exhaled. To measure the variation on the thermistor resistance we decided to use a Wheatsone bridge as shown below.
Since the variation was pretty small (the temperature varies between 25ºC and 28ºC) we needed to amplify the signal. For that we thought about using an instrumentation amplifier - INA128. However it wasn't available in the simulator we were using (which is TINA-TI) so we built the equivalent circuit.
I wanted to know if this is a good option to aquire and amplify this signal, and if this circuit is correct.
To be able to simulate the signal, we measured the signals on the points that were connected to the opamps, and used voltage sources with similar signals.
As you can see, the output has a DC component.
I wanted to know if there is something wrong with our Wheatstone bridge that could be causing that.