AN897 datasheet(5/16 Pages) MICROCHIP

5 / 16 page

 2004 Microchip Technology Inc.

DS00897B-page 5

AN897

FIGURE 12:

PGA Output Voltage.

The gain change points were chosen to make the

ADC’s resolution as good as possible (see Figure 13)

at a reasonable cost. The number of gains was kept

low to minimize the piece-wise linear interpolation

table’s size in firmware.

The maximum voltage allowed in each range is 300 mV

from VDD. This keeps the PGA in its specified output

range and allows some headroom for noise. The

minimum voltage allowed is well above 300 mV from

ground, which keeps the PGA in its most linear region

of operation.

Random noise can make the PGA’s gain change

frequently. Adding hysteresis to the gain-selection

algorithm (in firmware) reduces this problem. The

hysteresis needs to be large enough to compensate for

the PGA’s maximum gain error (±1%).

Figure 12 and Table 2 show a hysteresis of 1.7°C and

2.0°C at the lower temperature and higher temperature

transistions, respectively. The gain-change points are

separated by 6% of VDIV, which is six times larger than

the PGA’s maximum gain error; this ensures proper

functioning

the

gain-change

algorithm.

The

thermistor self-heating error has been corrected in

Table 2.

TABLE 2:

PGA GAIN-CHANGE POINTS

WITH HYSTERESIS.

Analog Error Analysis

Figure 13 displays the ADC’s temperature resolution

and Figure 14 shows the expected worst-case analog

circuit

errors.

Both

plots

are

based

these

assumptions:

• ADC’s DC Error

≤ ±3.5 LSb

• PGA’s gain error

≤ ±1% (±0.1% at 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

0.27°C over the -40°C to +150°C temperature range.

The analog circuit accuracy is better than 3.0°C over

the same range. Other temperature ranges will have

different resolutions and accuracies.

FIGURE 13:

ADC’s Temperature

Resolution.

FIGURE 14:

Analog Circuit Errors.

Digital Design

The PIC16F684 microcontroller (Appendix A.2.5

“Signal Analysis PICtail Daughter Board”) handles

several important tasks. It communicates with the PGA

to set its gain and input channel, can provide averaging

to reduce the noise and converts the result into the tem-

perature at the thermistor using a piece-wise linear

interpolation table. The microcontroller can have either

a SPI port built in or the SPI interface can be

implemented in software on the microcontroller [7].

Gain

(V/V)

Gain

Change

(V/V)

ADC Code

(LSb)

VDIV

(V)

TTH

(°C)

→ 8

< 113

0.552

50.9

→ 1

> 960

0.586

49.2

→ 32

< 226

0.138

94.6

→ 8

> 960

0.146

92.6

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

-50

-25

100

125

150

Thermistor Temperature (°C)

Hysteresis

Design # 2

RA = 28.0 k

G = +1

G = +32

G = +8

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

-50

-25

100 125 150

Thermistor Temperature (°C)

Design # 2

RA = 28.0 kΩ

10-bit ADC

DC Error

≤ 3.5 LSb

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-50

-25

100

125

150

Thermistor Temperature (°C)

Design # 2

RA = 28.0 k

PGA Error

ADC Error

RA Error

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