LM35 temperature sensor signal processing and LabVIEW interfacing

Semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about –55°C to +150°C. The output is scaled to give sensitivity of 10mV/°C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples.

All semiconductor temperature sensors make use of the relationship between a bipolar junction transistor's (BJT) base-emitter voltage to its collector current:

where k is Boltzmann's constant, T is the absolute temperature, q is the charge of an electron, and Is is a current related to the geometry and the temperature of the transistors.

The LM35/36/37 are voltage output temperature sensors with a 10mV/°C (LM35/36) or 20mV/°C (LM37) scale factor. Supply current is below 50µA, providing very low self-heating (less than 0.1°C in still air). The LM35 provides a 250mV output at +25°C and reads temperature from +10°C to +125°C. LM35 have an output scale factor of +10mV/°C.

LT1167 Single Resistor Gain Programmable, Precision Instrumentation Amplifier

As the output from LM35 is very low (typically 300 mV at 30°C), it is susceptible to distortion due to noise. The signal is amplified using an amplifier to a suitable level so as to utilize the full input range of ADC of Arduino. It improves the measurement sensitivity.

We are using LT1167 precision In-amp from Linear Technologies (now Analog Devices).

Supply voltage: ±15 VDC

Setup and connection

REF pin (5) is to be connected to Ground.

We have selected Rg=8.2kΩ. So the gain is 7.02.

The amplified output is fed to analog input channel of an Arduino UNO board. The data is read on LabVIEW using Makerhub example program for Single Channel AI read. I tweaked the program to display temperature in degree C.