Bonus lab: Op Amp and LM 35

Introduction:
In this experiment, we are using the temperature sensor LM 35 to measure temperature. LM 35 is  a device which gives the output voltage proportional to the temperature around it with a output voltage according to the revolution 19.5mV/ degree Celsius. However, we want to measure the temperature more precise with the revolution 0.1 degree Celsius. This is when the op amp application can be used. The lab requires the LM 35 to measure 15 - 35 degree Celsius when the input voltage range from 0 -5V.

Procedure:

The Revolution of the LM35 is 19.5mV/ degree Celsius. However, we want the revolution is 20mV/ 0.1 degree Celsius. The op amp can help us to amplifies the voltage output to measure the temperature precisely.


The picture shown below illustrates how to set it up:

As shown above, the gain for the op amp is 20, and we use the the difference amplifier to accomplish the experiment.

Build the circuit:
We range the gain of the op amp to be 22 by set the ratio of R₂/R₁ = R₄/R₃ = 22 .

Measurement:
Set the voltage come in the non-inverting terminal is 153mV, The voltage output Vo = 1.23V.

Calculation:

V_o = gain (V₁– V₂)

V₁ = V_o/gain  ­+ V₂ = 1.23/22 +153mV = 212 mV.

Temperature: T = V₁/10 = 212/10 = 21.2 ^oC.