Datasheet
© 2009 Microchip Technology Inc. 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 (R
1
)
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 R
W
into the voltage divider
calculation. The MCP40D17/18/19’s R
AB
temperature
coefficient is a low 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 8-5 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 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 pot’s R
AB
part-to-part
variability.
FIGURE 8-6: Thermistor Calibration using
a Digital Potentiometer in a Potentiometer
Configuration.
NTC
V
DD
V
OUT
Thermistor
R
1
R
2
MCP40D17
NTC
V
DD
V
OUT
Thermistor
R
1
MCP40D18