Microchip MCP9801 Handleiding

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2010-2015 Microchip Technology Inc. DS00001001C-page 1
AN1001
INTRODUCTION
Microchip Technology Inc. provides a number of analog
and serial output Integrated Circuit (IC) temperature
sensors. Typically, these sensors are accurate at room
temperature within one degree Celsius (±1°C). How-
ever, at hot or cold temperature extremes, the accuracy
decreases nonlinearly. Normally, that nonlinearity has a
parabolic shape.
This application note derives an equation to describe
the typical nonlinear characteristics of a sensor, which
is used to determine compensation for the sensor's
accuracy error over a specified range of operating tem-
peratures. A PIC® microcontroller unit (MCU) can
compute the equation and provide a temperature read-
ing with higher accuracy.
This application note is based on MCP9700 and
MCP9701 analog-output temperature sensors and
MCP9800 serial-output temperature sensors.
SOLUTION APPROACH
Silicon characterization data is used to determine the
nonlinear sensor characteristics. From this data, an
equation is derived that describes the typical perfor-
mance of a sensor. When the corresponding coeffi-
cients for the equation are determined, the coefficients
are used to compute the compensation for the typical
sensor’s nonlinearity.
The error distribution is provided using an average and
±1 standard deviation (± before and after compensa-
tion. A total of 100 devices were used as representative
for the MCP9700 and MCP9701, while 160 devices
were used for the MCP9800.
Figure 1 shows the typical sensor accuracy before and
after compensation. It illustrates that the compensation
provides an accurate and linear temperature reading
over the sensor operating temperature range.
A PIC MCU is used to compute the equation and
compensate the sensor output to provide a linear
temperature reading.
FIGURE 1: Typical Sensor Accuracy Before and After Compensation.
Author: Ezana Haile
Microchip Technology Inc.
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
-55 -35 -15 5 25 45 65 85 105 125
Temperature (°C)
Typical Accuracy (°C)
Sensor Accuracy
Compensated Sensor Accuracy
IC Temperature Sensor Accuracy Compensation
with a PIC® Microcontroller
AN1001
DS00001001C-page 2 2010-2015 Microchip Technology Inc.
SENSOR ACCURACY
The typical sensor accuracy over the operating tem-
perature range has an accuracy error curve. At hot and
cold temperatures, the magnitude of the error increases
exponentially, resulting in a parabolic-shaped error
curve. The following figures show the average and ±1°C
standard deviation of the sensor accuracy curve for the
MCP9800, MCP9700 and MCP9701 sensors.
FIGURE 2: MCP9800 Accuracy
(160 parts).
FIGURE 3: MCP9700 Accuracy
(100 parts).
FIGURE 4: MCP9701 Accuracy
(100 parts).
The accuracy specification limits for these sensors are
published in the corresponding data sheets as plotted
in Figure 2, Figure 3 and Figure 4. Note that due to the
sensor nonlinearity at temperature extremes, the accu-
racy specification limits are widened. The reduced
accuracy at temperature extremes can be
compensated to improve sensor accuracy over the
range of operating temperatures.
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
-55 -35 -15 5 25 45 65 85 105 125
Temperature (°C)
Accuracy (°C)
+
V
V
V
VV
Average
-
V
V
V
VV
Spec. Limit
-4.0
-2.0
0.0
2.0
4.0
6.0
-55 -35 -15 5 25 45 65 85 105 125
Temperature (°C)
Accuracy (°C)
+
V
V
V
VV
Average
-
V
V
V
VV
Spec. Limit
-4.0
-2.0
0.0
2.0
4.0
6.0
-15 5 25 45 65 85 105 125
Temperature (°C)
Accuracy (°C)
+
V
V
V
VV
Average
-
V
V
V
VV
Spec. Limit
2010-2015 Microchip Technology Inc. DS00001001C-page 3
AN1001
SENSOR THEORY
Temperature sensors use a fully turned-on PNP tran-
sistor to sense the ambient temperature. The voltage
drop across the base-emitter junction has the charac-
teristics of a diode. The junction drop is temperature
dependent, which is used to measure the ambient tem-
perature. Equation 1 shows a simplified equation that
describes the diode forward voltage.
EQUATION 1: DIODE FORWARD
VOLTAGE
IS is a constant variable defined by the transistor size.
A constant forward current (IF) is used to bias the
diode, which makes the temperature T
A the only
changing variable in the equation. However, I
S varies
significantly over process and temperature. The varia-
tion makes it impossible to reliably measure the
ambient temperature using a single transistor.
To minimize IS dependency, a two-diode solution is
used. If both diodes are biased with constant forward
currents of IF1 and IF2
, and the currents have a ratio of
N (IF2/IF1 = N), the difference between the forward volt-
ages (V
F) has no dependency on the saturation cur-
rents of the two diodes, as shown in Equation 2. V
F is
also called Voltage Proportional to Absolute
Temperature (VPTAT).
EQUATION 2: VPTAT
VPTAT provides a linear voltage change with a slope of
(86 µV/°C)*ln(N)|N=10 = 200 µV/°C. The voltage is either
amplified for analog output sensors or is interfaced to an
Analog-to-Digital Converter (ADC) for digital sensors.
The accuracy of VPTAT over the specified temperature
range depends on the matching of both forward current
(IF
) and saturation current (IS) of the two sensors [1].
Any mismatch in these variables creates inaccuracy in
the temperature measurement. The mismatch contrib-
utes to the temperature error or nonlinearity. The non-
linearity is described using a 2nd order polynomial
equation.
VF
kTA
q
--------- IF
IS
-----
 
 
ln=
Where:
k = Boltzmann’s Constant (1.3807 x 10-23 J/K)
q = Electron Charge (1.602 x 10-19 coulombs)
TA= Ambient Temperature
IF= Forward Current
IS= Saturation Current
,IFIS
»
VFVF1 VF2
–=
VF
kT A
q
--------- ln
IF1
IS
-------
N IF1
IS
-----------------
-----------------=
VF
kTA
q
--------- ln
N =
VFVPTAT
=
Where:
VF= Forward Voltages
IF= Forward Currents
VPTAT = Voltage Proportional to Absolute
Temperature

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: MCP9801

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