AN897 datasheet(3/16 Pages) MICROCHIP

3 / 16 page

 2004 Microchip Technology Inc.

DS00897B-page 3

AN897

FIGURE 6:

ADC’s Temperature

Resolution.

FIGURE 7:

Analog Circuit Errors.

Digital Design

The

PIC16F684

microcontroller

(discussed

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].

FLOWCHART

The flowchart in Figure 8 shows the program flow for

the

first

design.

The

firmware

available

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

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

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.

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 given for RTH, 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.

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.

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

-50

-25

100

125

150

Thermistor Temperature (°C)

10-bit ADC

DC Error

≤ 3.5 LSb

Design # 1

G = +1

RA = 10 kΩ

-50

-25

100

125

150

Thermistor Temperature (°C)

ADC Error

RA Error

PGA Error

Design # 1

G = +1

RA = 10 k

Gain

(V/V)

ADC Code

(LSb)

TTH

(°C)

RTH

(

Ω)

VOUT

(V)

1008

-49.4

630 k

4.922

156.1

159

0.078

Start

Read ADC

Perform PwLI

Send Temperature in

°C to Strip Chart

End

Get Real Time Sample

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