MCP4562 View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

MCP4562

7-Bit Single I²C™ Digital POT with Volatile Memory in SC70

Microchip Technology 

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

& 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 MCP4017/18/19’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 kΩ devices, the error introduced by RW is, in most

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

For the 2 kΩ devices, the error introduced by RW 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.

VDD

R1

NTC

Thermistor

VOUT

R2

MCP4017/18/19

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.

VDD

R1

NTC

Thermistor

VOUT

MCP4018

FIGURE 8-6:

Thermistor Calibration using

a Digital Potentiometer in a Potentiometer

Configuration.

MCP4017

FIGURE 8-5:

Thermistor Calibration using

a Digital Potentiometer in a Rheostat

Configuration.

© 2009 Microchip Technology Inc.

DS22147A-page 51

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