MCP4013 Microchip Technology, MCP4013 Datasheet - Page 46

• Manufacturer Part Number: MCP4013
• Description: Low-Cost 64-Step Volatile Digital POT
• Manufacturer: Microchip Technology

MCP4011/2/3/4
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
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 MCP4011/2/3/4’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.
DS21978C-page 46
Temperature Sensor Applications
R
DD
1
, wiper setting and temperature. For the
W
from the trimming equation along
Thermistor Calibration using
V
DD
W
Figure
Figure 8-7
A
W
Figure 8-6
R
NTC
Thermistor
2
into the voltage divider
AB
V
MCP4012
OUT
’s error because R
W
8-6, utilizes a digital
. R
2
utilizes a digital
AB
is not required,
and
W
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.
MCP4012
Wheatstone Bridge Trimming
50 kΩ
R
MCP4012
1
output.
Thermistor Calibration using
Wheatstone Bridge
V
© 2006 Microchip Technology Inc.
DD
V
V
OUT
Figure 8-8
DD
R
NTC
Thermistor
2
MCP4011
V
2.1 kΩ
OUT
MCP4012
50 kΩ
illustrates
a