MCP4021 datasheet(50/64 Pages) MICROCHIP

50 / 64 page

MCP4021/2/3/4

DS21945E-page 50

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 8-6 and Figure 8-7

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

lineararity

error

the

Negative

Temperature

Coefficient (NTC) thermistor.

The circuit, illustrated by Figure 8-6, 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 MCP4021/2/3/4’s RAB temperature

coefficient is 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 RW is, in most

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

For the 2 k

Ω devices, the error introduced by RW is

significant because it is a higher percentage of RWB.

For these reasons, the circuit illustrated in Figure 8-6 is

not the most optimum method for “exciting” and

linearizing a thermistor.

FIGURE 8-6:

Thermistor Calibration using

a Digital Potentiometer in a Rheostat

Configuration.

The circuit illustrated by Figure 8-7 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 potentiometer’s RAB

part-to-part variability.

FIGURE 8-7:

Thermistor Calibration using

a Digital Potentiometer in a Potentiometer

Configuration.

8.4

Wheatstone Bridge Trimming

Another common configuration to “excite” a sensor

(such as a strain gauge, pressure sensor or thermistor)

is the wheatstone bridge configuration. The wheat-

stone bridge provides a differential output instead of a

single-ended

output.

Figure 8-8

illustrates

wheatstone bridge utilizing one to three digital

potentiometers. The digital potentiometers in this

example are used to trim the offset and gain of the

wheatstone bridge.

FIGURE 8-8:

Wheatstone Bridge

Trimming.

NTC

VDD

MCP4022

VOUT

Thermistor

NTC

VDD

MCP4021

VOUT

Thermistor

VDD

MCP4022

VOUT

2.1 k

MCP4022

50 k

MCP4022

50 k

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