MCP4023 Microchip Technology, MCP4023 Datasheet - Page 50

• Manufacturer Part Number: MCP4023
• Description: Low-Cost NV Digital POT with WiperLock Technology
• Manufacturer: Microchip Technology

MCP4021/2/3/4
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
lineararity
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 MCP4021/2/3/4’s R
coefficient is 50 ppm (-20°C to +70°C). R
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 8-6:
a Digital Potentiometer in a Rheostat
Configuration.
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 potentiometer’s R
part-to-part variability.
DS21945E-page 50
Temperature Sensor Applications
error
R
DD
1
, wiper setting and temperature. For the
W
in
from the trimming equation along
Thermistor Calibration using
V
the
DD
A
W
W
Figure
Figure 8-7
Figure 8-6
R
NTC
Thermistor
2
into the voltage divider
AB
V
Negative
MCP4022
OUT
’s error because R
W
8-6, utilizes a digital
. R
2
utilizes a digital
AB
is not required,
and
W
Temperature
Figure 8-6
temperature
W
is, in most
Figure 8-7
’s error is
W
WB
AB
1
W
is
is
)
.
FIGURE 8-7:
a Digital Potentiometer in a Potentiometer
Configuration.
8.4
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
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:
Trimming.
MCP4022
Wheatstone Bridge Trimming
50 kΩ
R
MCP4022
1
output.
Thermistor Calibration using
Wheatstone Bridge
V
© 2006 Microchip Technology Inc.
DD
V
V
OUT
Figure 8-8
DD
R
NTC
Thermistor
2
MCP4021
2.1 kΩ
V
OUT
MCP4022
50 kΩ
illustrates
a