# A Simple Thermistor Interface to an ADC

### Abstract

This article describes a simple and cost effective method of measuring temperature using a thermistor connected in a half-bridge configuration. The goal is to perform a ratiometric measurement such that the V_{REF} source voltage to the divider is the same as the reference to the analog-to-digital converter (ADC) used to measure the voltage at V_{T}.

There are many circuits and measurement methods that can be used with a thermistor to determine the temperature. The simplest approach is to use a half-bridge circuit also known as a resistor divider, shown in Figure 1. The goal is to perform a ratiometric measurement such that the V_{REF} source voltage to the divider is the same as the reference to the ADC used to measure the voltage at V_{T}. The R_{1} resistance is known.

The equation for V_{T} is shown in Equation 1.

The equation for the ADC result is shown in Equation 2.

where ADC is the ADC result and N = the ADC resolution.

Substituting Equation 1 into Equation 2 yields Equation 3 and the V_{REF} term is cancelled out. This leaves the R_{1} value, which is known, and the ADC result, which is measured. The R_{1} resistor should be a temperature stable resistor otherwise it will affect the accuracy of the temperature measurement.

Rearranging Equation 3 and solving for R_{T} yields Equation 4.

After the value for R_{T} is calculated, the temperature can then be calculated by using the equation provided by the thermistor vendor. An example of an equation is shown in Equation 5, which is for a NTC thermistor.

The thermistor vendor would provide the value for the coefficients b0, b1, and b3. The equation can be solved for directly or a lookup table can be used if easier. Simple linear interpolation between the table data points is required to gain the proper resolution. A plot of the NTC thermistor is shown below in Figure 2.