Microchip TC7650 Manual

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Side 1/14

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 PICtail™ 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 PICtail™

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

10k

100k

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

ΩΩ @ +25°C

Thermistor Temperature Sensing with MCP6SX2 PGAs

AN897

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.

10 kΩ

VDD = 5.0V

VDD

10-bit

VIN

VREF

PIC16F684

CH0

SPI Bus

ADC Firmware

MCP6SX2

PGA

VOUT

100 kΩ

1 µF

VDIV

VDD

CH1

VREF

Other Input

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

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	TC7650

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