Op Amp Applications

In this experiment, we will use AS35 , which is an electronic temperature sensor that will  produce output voltage that is proportional to the temperature around it to a scale factor of 10mV/degree C. The device only operate at 4-20V and an external resistor to provide an appropriate load.






As shown above, we chose R1 to be 5000 ohms while R2 was chosen to be 4000 ohms . This was so that the ratio R2/R1 was 4/5 and Vo was multiplied by a factor of 1.8. To provide the correct level-shifting, Vref was solved for so that (4/5)Vref was equal to the amount of shift required, 320mV. It turned out to be -400mV, so a voltage divider was used to obtain -400mV from a 4.5V source.

Vref has to be negative if the overall shift is to be positive. The design also has constraint which the output current from the op-amp is much less than the maximum allowable output current specified in the data sheep which is 25 mA for LM741.

The initial circuit output unusually large values and the sensor smelled burnt. When the simple circuit on the first page was built, it still output over 2V. We realized that the manufacturer's diagram was wrong after we disassembled the circuit, so we had to use new values for some resistors. The right column still lists the actual values


Data Collect:



Vc=0.228V
Vf=0.711V
Theoretical Value of Tf = (1.8*0.228+0.32)*100=73.04 degree Fahrenheit
% error = (73.04-71.1)/73.04*100% = 2.66%
The % error between the two values is 2.66%. The error may be come from the measurement was taken not too long after it was overheated due to the incorrect diagram