Microchip TC7650 Handleiding

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© 2006 Microchip Technology Inc. DS00897C-page 1
AN897
INTRODUCTION
This application note shows two designs that use a
precise, negative temperature coefficient (NTC)
thermistor for temperature measurement. The
thermistor is placed in a resistive divider to linearize the
temperature-to-voltage conversion. The voltage is
processed in the analog domain by the MCP6SX2
(MCP6S22 or MCP6S92) Programmable Gain
Amplifier (PGA) before conversion to the digital
domain.
The first design is simpler and has a smaller
temperature range. The second design changes the
PGA’s gain to achieve a greater temperature range.
Both designs use a piece-wise linear interpolation table
to correct the remaining non-linearity and convert
voltage into degrees Celsius. The design trade-offs
between these approaches will be discussed.
These circuits take advantage of the MCP6SX2’s input
multiplexer (MUX). The PGA is used to process
multiple signals and/or temperatures and digitally sets
the most appropriate gain for each input. This reduces
overall design complexity and allows for temperature
correction of other sensors.
THERMISTOR
The thermistor used in the application note is part
number 2322 640 55103 from BC Components
®; see
Figure 1 and Figure 2. This part is selected for its
accuracy and cost. The thermistor’s temperature is
TTH, while the rest of the circuit is at ambient
temperature TA.
FIGURE 1: Thermistor Response.
Key specifications include [1, 2]:
• Resistance at +25°C: 10 kΩ ± 1%
• B25/85 tolerance: ±0.75%
• Operating temperature range: -40°C to +125°C
(to +150°C for short periods)
• Maximum power
- 100 mW, TTH = 0°C to +55°C
- 100% de-rated at TTH = -40°C and +85°C
• Thermal dissipation factor: 2.2 mW/°C
• Response time: 1.7 s (in oil)
FIGURE 2: Thermistor Accuracy.
Thermistors with different price and accuracy trade-offs
may also be used in this application. It is simple to
modify the circuits to match the desired accuracy.
CIRCUIT
The circuit shown in Figure 3 is used for both designs
described later. It is implemented on the MCP6SX2
PGA Thermistor PICtailâ„¢ Demo Board; see reference
[12].
The resistor RA makes the voltage vs. temperature
response reasonably linear. RB and CB reduce the
noise and act as an anti-aliasing filter for the ADC. The
MCP6SX2 PGA (MCP6S22 [5] or MCP6S92 [6])
buffers the voltage VDIV
. The PGA can be digitally
controlled to change its gain or channel (input).
The PIC16F684 [8] is on the Signal Analysis PICtailâ„¢
Daughter Board; see reference [11]. It has an internal
10-bit ADC that converts VOUT to the digital domain. It
can further process VOUT (e.g., averaging) and convert
it to temperature. It communicates with the PGA via the
SPI serial bus.
Author: Kumen Blake and Steven Bible
Microchip Technology Inc.
100
1000
10000
100000
1000000
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
Thermistor Resistance (
:
:
:
::
)
100
1k
10k
100k
1M
BC Components®
# 2322 640 55103
10 k
:
:
:
::
@ +25°C
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
Thermistor Error Magnitude
(°C)
BC Components
®
# 2322 640 55103
10 kΩ
Ω
Ω
ΩΩ @ +25°C
Thermistor Temperature Sensing with MCP6SX2 PGAs
AN897
DS00897C-page 2 © 2006 Microchip Technology Inc.
FIGURE 3: Thermistor PGA Circuit.
The ADC’s voltage reference is powered from the
same voltage as the voltage divider, giving a
ratiometric circuit; errors in VDD will be automatically
corrected at the ADC.
FIRST DESIGN
This design emphasizes simplicity and uses a standard
approach to designing the thermistor circuit. The
traditional op amp is replaced with a PGA so that it can
multiplex multiple inputs.
Analog Design
The first design keeps the PGA at a gain of +1 V/V for
design simplicity. The resistor RA
is set to its nominal
+25°C value (10.0 kΩ) for best performance at room
temperature; this is a very common design choice.
While this is a simpler design, its accuracy is relatively
low, as will be seen. Notice that Figure 4 shows a much
more linear response than Figure 1.
FIGURE 4: Voltage Divider and PGA
Outputs.
Temperatures between +125°C and +150°C can be
included in the design for overtemperature indication
where accuracy is not as important.
The thermistor power dissipation causes a self-heating
temperature error. Calculating the thermistor’s power
dissipation across temperature, and then dividing by
the specified 2.2 mW/°C thermal dissipation factor,
gives the self-heating temperature error shown in
Figure 5. This is a small, consistent error. It is simple to
adjust for this error using the piece-wise linear
interpolation table in firmware.
FIGURE 5: Thermistor Self-heating
Error.
Analog Error Analysis
Figure 6 displays the ADC’s temperature resolution,
while Figure 7 shows the expected worst-case analog
circuit errors. Both plots are based on these
assumptions:
• ADC’s DC Error ≤ ±3.5 LSb
• PGA’s gain error ≤ ±0.1% (G = +1)
• PGA’s input offset error ≤ ±1 mV (including PSRR
and temperature drift)
• Specified thermistor accuracy
This design achieves an ADC temperature resolution of
0.25°C over the -25°C to +73°C temperature range.
The analog circuit accuracy is better than 1.2°C over
the same range. Other temperature ranges will have
different resolutions and accuracies.
RA
R
TH
10 kΩ
VDD = 5.0V
RB
CB
VDD
10-bit
VIN
VREF
PIC16F684
CH0
3
SPI Bus
ADC Firmware
MCP6SX2
PGA
VOUT
100 kΩ
1 µF
VDIV
VDD
CH1
VREF
Other Input
1%
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
VDIV = VOUT (V)
Design # 1
G = +1
RA = 10 k
:
:
:
::
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
Self-heating Error (°C)
Design # 1
G = +1
RA = 10 k
:
:
:
::
© 2006 Microchip Technology Inc. DS00897C-page 3
AN897
FIGURE 6: ADC’s Temperature
Resolution.
FIGURE 7: Analog Circuit Errors.
Digital Design
The PIC16F684 microcontroller [11, 12] handles
several important tasks. It communicates with the PGA
to set its input channel, can average the measured
signal to reduce noise and converts the result into the
temperature at the thermistor using a piece-wise linear
interpolation table. The microcontroller can either have
a SPI port built in or the SPI interface can be
implemented in software on the microcontroller [7].
FLOWCHART
The flowchart in Figure 8 shows the program flow for
the first design. The firmware is available in 00897
Source Code.zip file in the “00028 - MCP6SX2 PGA
Thermistor PICtail Demo Board“directory. The
firmware was written in relocatable assembly code.
main.asm controls the overall program flow. The PGA
routines are in pga.inc and pga.asm. The thermistor
routines are located in Therm_PGA1.inc and
Therm_PGA1.asm.
The Signal Analysis PC Program commands the
PIC16F684 firmware to perform a real-time sample.
The firmware reads the ADC value and passes it to the
Piece-wise Linear Interpolation (PwLI) routine. The
PwLI routine converts the 10-bit ADC value into a 16-
bit fixed decimal point degrees Celsius value. The fixed
decimal point format reports degrees Celsius in tenths
of a degree. Performing the piece-wise linear
interpolation in tenths of a degree provides better
resolution of degrees Celsius. Finally, the 16-bit
degrees Celsius value is sent to the Signal Analysis PC
Program for display on the real-time strip chart graph.
In the final design, the designer can elect to report in
tenths of a degree or round up in whole degrees.
FIGURE 8: Flowchart for First Design.
PIECE-WISE LINEAR INTERPOLATION TABLE
A piece-wise linear interpolation table [9] is used to
convert ADC codes to estimated temperature. The
ADC’s codes were divided into 64 segments, with 16
codes per segment. The codes in the table are at end
points between segments. Table 1 shows the end
points chosen for this design.
TABLE 1: INTERPOLATION TABLE END
POINTS.
Values of RTH outside the thermistor’s specified
temperature range (-40°C to +150°C) are estimates
only; they are not given by the manufacturer. The
thermistor self-heating error correction has been
included in Table 1.
The table’s entries go outside of the -40°C to +150°C
range to ensure proper functioning of the piece-wise
linear interpolation table when the reading overflows. In
this algorithm, the table values outside the valid range
take on the nearest valid value. This means that when
ADC code > 1008, the table returns a value of -49.3°C.
When ADC code < 16, the table returns a value of
156.1°C.
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
ADC Temperature Resolution
(°C/LSb)
10-bit ADC
DC Error ≤
≤
≤
≤≤ 3.5 LSb
Design # 1
G = +1
RA = 10 kΩ
Ω
Ω
ΩΩ
0
1
2
3
4
5
6
7
8
9
10
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
Analog Circuit Errors
Magnitude (°C)
ADC Erro
r
RAErro
r
PGA Erro
r
Design # 1
G = +1
RA = 10 k
:
:
:
::
Gain
(V/V)
ADC Code
(LSb)
TTH
(°C)
RTH
( )Ω
VOUT
(V)
1 1008 -49.4 630 k 4.922
16 156.1 159 0.078
Start
Read ADC
Perform PwLI
Send Temperature in
°C to Strip Chart
End
Get Real Time Sample

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: TC7650

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