AN897

DS00897B-page 2

 2004 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

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

+25°C value (10.0 k

Ω) 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 k

Ω

10-bit

PIC16F684

CH0

3

SPI™ Bus

ADC

Firmware

MCP6SX2

PGA

100 k

Ω

1µF

CH1

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)

Design # 1

G = +1

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)

Design # 1

G = +1