MCP40D17 Microchip Technology Inc., MCP40D17 Datasheet - Page 51

• Manufacturer Part Number: MCP40D17
• Description: 7-bit Single I 2 C? With Command Code Digital Pot With Volatile Memory In Sc70
• Manufacturer: Microchip Technology Inc.

8.3
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.
are simple methods for linearizing a 3-terminal NTC
thermistor. Both are simple voltage dividers using a
Positive Temperature Coefficient (PTC) resistor (R
with a transfer function capable of compensating for the
linearity error in the Negative Temperature Coefficient
(NTC) thermistor.
The circuit, illustrated by
rheostat for trimming the offset error caused by the
thermistor’s part-to-part variation. This solution puts the
digital potentiometer’s R
calculation. The MCP40D17/18/19’s R
coefficient is a low 50 ppm (-20°C to +70°C). R
is substantially greater than R
varies with V
50 kΩ devices, the error introduced by R
cases, insignificant as long as the wiper setting is > 6.
For the 2 kΩ devices, the error introduced by R
significant because it is a higher percentage of R
For these reasons, the circuit illustrated in
not the most optimum method for “exciting” and
linearizing a thermistor.
FIGURE 8-5:
a Digital Potentiometer in a Rheostat
Configuration.
© 2009 Microchip Technology Inc.
Temperature Sensor Applications
R
DD
1
, wiper setting and temperature. For the
V
Thermistor Calibration using
DD
W
R
Figure
NTC
Thermistor
Figure 8-5
2
V
into the voltage divider
MCP40D17
OUT
AB
’s error because R
8-5, utilizes a digital
AB
and
W
Figure 8-5
temperature
is, in most
Figure 8-6
W
’s error
W
WB
1
W
is
is
)
.
The circuit illustrated by
potentiometer for trimming the offset error. This
solution removes R
with the error associated with R
but can be utilized to reduce the trimming “window” and
reduce variation due to the digital pot’s R
variability.
FIGURE 8-6:
a Digital Potentiometer in a Potentiometer
Configuration.
MCP40D17/18/19
R
1
W
from the trimming equation along
V
Thermistor Calibration using
DD
Figure 8-6
NTC
Thermistor
MCP40D18
V
W
OUT
. R
DS22152B-page 51
2
utilizes a digital
is not required,
AB
part-to-part