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DS22152B-page 51

MCP40D17/18/19

8.3

Temperature Sensor Applications

Thermistors

are

resistors

with

very

predictable

variation with temperature. Thermistors are a popular

sensor choice when a low-cost temperature-sensing

solution is desired. Unfortunately, thermistors have

non-linear characteristics that are undesirable, typically

requiring trimming in an application to achieve greater

accuracy. There are several common solutions to trim

and linearize thermistors. Figure 8-5 and Figure 8-6

are simple methods for linearizing a 3-terminal NTC

thermistor. Both are simple voltage dividers using a

Positive Temperature Coefficient (PTC) resistor (R1)

with a transfer function capable of compensating for the

linearity error in the Negative Temperature Coefficient

(NTC) thermistor.

The circuit, illustrated by Figure 8-5, utilizes a digital

rheostat for trimming the offset error caused by the

thermistor’s part-to-part variation. This solution puts the

digital potentiometer’s RW into the voltage divider

calculation. The MCP40D17/18/19’s RAB temperature

coefficient is a low 50 ppm (-20°C to +70°C). RW’s error

is substantially greater than RAB’s error because RW

varies with VDD, wiper setting and temperature. For the

50 k

Ω devices, the error introduced by R

W is, in most

cases, insignificant as long as the wiper setting is > 6.

For the 2 k

Ω devices, the error introduced by R

W is

significant because it is a higher percentage of RWB.

For these reasons, the circuit illustrated in Figure 8-5 is

not the most optimum method for “exciting” and

linearizing a thermistor.

FIGURE 8-5:

Thermistor Calibration using

a Digital Potentiometer in a Rheostat

Configuration.

The circuit illustrated by Figure 8-6 utilizes a digital

potentiometer for trimming the offset error. This

solution removes RW from the trimming equation along

with the error associated with RW. R2 is not required,

but can be utilized to reduce the trimming “window” and

reduce variation due to the digital pot’s RAB part-to-part

variability.

FIGURE 8-6:

Thermistor Calibration using

a Digital Potentiometer in a Potentiometer

Configuration.

NTC

VDD

VOUT

Thermistor

MCP40D17

NTC

VDD

VOUT

Thermistor

MCP40D18

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