Datasheet

MCP4021/2/3/4
DS21945E-page 50 © 2006 Microchip Technology Inc.
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 (R
1
)
with a transfer function capable of compensating for the
lineararity error in 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 R
W
into the voltage divider
calculation. The MCP4021/2/3/4’s R
AB
temperature
coefficient is 50 ppm (-20°C to +70°C). R
W
’s error is
substantially greater than R
AB
’s error because R
W
varies with V
DD
, wiper setting and temperature. For the
50 kΩ devices, the error introduced by R
W
is, in most
cases, insignificant as long as the wiper setting is > 6.
For the 2 kΩ devices, the error introduced by R
W
is
significant because it is a higher percentage of R
WB
.
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 R
W
from the trimming equation along
with the error associated with R
W
. R
2
is not required,
but can be utilized to reduce the trimming “window” and
reduce variation due to the digital potentiometer’s R
AB
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 a
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
V
DD
MCP4022
V
OUT
Thermistor
R
1
R
2
W
A
NTC
V
DD
MCP4021
V
OUT
Thermistor
R
1
R
2
V
DD
MCP4022
V
OUT
2.1 kΩ
MCP4022
50 kΩ
MCP4022
50 kΩ