MCP4011T-103E/CH View Datasheet(PDF) - Microchip Technology
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MCP4011T-103E/CH Datasheet PDF : 60 Pages
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MCP4011/2/3/4
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
linearity 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 RW into the voltage divider
calculation. The MCP4011/2/3/4’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 kΩ devices, the error introduced by RW is, in most
cases, insignificant as long as the wiper setting is > 6.
For the 2 kΩ 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.
VDD
R1
NTC
Thermistor
VOUT
R2
A
MCP4012
W
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.
DS21978C-page 46
VDD
R1
NTC
Thermistor
MCP4011
VOUT
R2
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.
VDD
MCP4012
2.1 kΩ
VOUT
MCP4012
50 kΩ
MCP4012
50 kΩ
FIGURE 8-8:
Trimming.
Wheatstone Bridge
© 2006 Microchip Technology Inc.
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