Op Amp Application

Purpose
   The purpose of this lab is to use a scaling and level-shifting op-amp circuit which is LM741to process the output signal from an electronic temperature sensor so that the output can show reading in degrees Fahrenheit converting from Centigrade reading.

Background
   The temperature sensor we have used is AS35 which produces an output voltage that is proportional to the air temperature based on a scale factor of 10mV/degree C. Also, AS35 requires an 4 to 20V to operate.

Pin Connection

Figure 1. Front View of AS35

Figure 2. Top View of AS35

Design
   The converting formula between Centigrade and Fahrenheit readings is TF = 1.8TC + 32. So, we can use a single 741 op-amp to scale and shift a signal. our design is shown as following,

Figure  3. Designed Circuit

By assuming the op amp is ideal, so V+ = V-= Vc. Using nodal analysis on the node which is just above R1, we can write (Vc-Vref)/R1 = -(Vc-Vf)/R2 

==> Vf/R2 = (Vc/R2+Vc/R1)-Vref/R1 

==> Vf = R2*(Vc/R2+Vc/R1)-R2/R1*Vref

==> Vf = (1+R2/R1)Vc-R2/R1*Vref. Comparing the equation to TF = 1.8TC + 32, we can get (1+R2/R1)=1.8 and 0.8*Vref = 0.32, which means Vref = 0.4V; however, 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 sheet which is 25 mA for LM741.

Assemble

R1=0.999 M ohms

R2=0.812 M ohms (both readings from multimeter), so that the ratio of R2/R1=0.813 which is close to the ideal value, 0.80. Two power supplies, one used as positive 9V and the other used as negative 9V, must connect to AS35 and LM 741 properly. In addition, we use a variable power supply to provide 0.4V as Vref instead of using a power supply and two resistors as a voltage divider. 

Figure 4. Build-up Circuit Connected to Multimeter and Power Supply

Figure 5. The Build-up Circuit

Data Analysis

Vc=0.228V

Vf=0.711V(readings from multimeter)

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%