Datasheet > AN897

AN897

更新时间： 2022-04-23 23:00:11

品牌	Logo	应用领域
美国微芯 - MICROCHIP		温度传感
页数	文件大小	规格书
16页	430K
描述
Thermistor Temperature Sensing with MCP6SX2 PGAs

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AN897 数据手册

AN897

fixed decimal point degrees Celsius value. The fixed

decimal point format reports degrees Celsius in tenths

of a degree. Performing the piece-wise linear interpola-

tion 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.

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

10-bit ADC

DC Error ≤ 3.5 LSb

In the final design, the designer can elect to report in

tenths of a degree or round up in whole degrees.

Design # 1

G = +1

R_A= 10 kΩ

Start

-50 -25

75 100 125 150

Get Real Time Sample

Thermistor Temperature (°C)

FIGURE 6:

Resolution.

ADC’s Temperature

Read ADC

Perform PwLI

Design # 1

G = +1

R_A= 10 k:

Send Temperature in

°C to Strip Chart

ADC Error

R_AError

PGA Error

End

FIGURE 8:

Flowchart for First Design.

-50 -25

75 100 125 150

PIECE-WISE LINEAR INTERPOLATION TABLE

Thermistor Temperature (°C)

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.

FIGURE 7:

Analog Circuit Errors.

Digital Design

The PIC16F684 microcontroller (discussed in

Appendix A.2.5 “Signal Analysis PICtail Daughter

Board”) handles several important tasks. It communi-

cates with the PGA to set its input channel, can aver-

age the measured signal to reduce noise and converts

the result into the temperature at the thermistor using a

piece-wise linear interpolation table. The microcontrol-

ler can either have a SPI™ port built in or the SPI

interface can be implemented in software on the

microcontroller [7].

TABLE 1:

INTERPOLATION TABLE END

POINTS.

Gain

(V/V)

ADC Code

(LSb)

T_TH

(°C)

R_TH

(Ω)

V_OUT

(V)

1008

-49.4

156.1

630 k

159

4.922

0.078

Values given for R_TH, when the temperature is outside

the thermistor’s specified range (-40°C to +150°C), are

estimates only; they are not supported by actual data.

The thermistor self-heating error correction has been

included in Table 1.

FLOWCHART

The flowchart in Figure 8 shows the program flow for

the first design. The firmware is available in

00897.zip file in the Design 1 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

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 (Appendix A.2.6 “Firmware for the

Signal Analysis PICtail Daughter Board”), 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.

are

located

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

 2004 Microchip Technology Inc.

DS00897B-page 3

1 234 5 6 7...16

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