AN897 datasheet(9/16 Pages) MICROCHIP

9 / 16 page

 2004 Microchip Technology Inc.

DS00897B-page 9

AN897

FIGURE 24:

Measured Errors,

Design # 2.

Note that it was necessary to add a resistor in series

with Rvar at high temperatures in order to have 5°C

spacing between data points.

Both Figure 23 and Figure 24 agree with the design

results; the second design has much better perfor-

mance. The 1% resistors in Rvar will give roughly the

same error as the thermistor.

The thermistor was then used to measure room tem-

perature using Design # 2. The result was ADC code

281 with a gain of +1, which corresponds to 23.7°C

(74.7°F).

DESIGN ALTERNATIVES

The references in this application note include informa-

tion on other design approaches. AN685 [3] covers

more traditional application circuits using thermistors.

AN867 [4] shows an alternative thermistor circuit using

the PGA; it has greater flexibility, but increased design

cost and complexity.

The following sections discuss modifications to the

designs in this application note.

Increased Accuracy

In order to achieve greater accuracy, the analog

components need to be more precise. 12-bit ADCs,

(e.g., the MCP3201) will increase the resolution. A

0.1% tolerance resistor for RA will reduce the circuit

error.

Calibrating the thermistor [1, 2] will cancel most of its

variation over process. It may be beneficial to also

calibrate the circuit. This will increase firmware

complexity and execution time on the microcontroller

unless the corrections are included in the linear

interpolation table(s).

The piece-wise linear interpolation table may need

more entries, especially for the first design. The calcu-

lations will require more precision, which results in

slower processing time.

Other Gains

The second design can be done with other gains.

Increasing the number of gains has the drawback of

needing more piece-wise linear interpolation tables,

increasing the firmware size.

Adding a gain(s) between +1 and +8 increases the

ADC resolution. The decrease in gain accuracy (from

0.1% at G = +1 to 1% at G

≥ +2) reduces the overall

accuracy, especially at a gain of +2. The tradeoffs

depend on the design specifics.

Adding a gain between +8 and +32 improves both the

accuracy

and

the

ADC

resolution

higher

temperatures. The choice of +16 is a good one.

Removing the gain of +32 may be attractive for designs

that reach a reduced temperature range (e.g., +125°C).

Changing the gain of +32 to +16, instead of removing

it, is one compromise.

When the gains are related by a common multiplier, the

hysteresis algorithm is simplified. When G = 1, 2, 4, 8,

16, and 32, the multiplier is 2. When G = 1, 4 and 16,

the multiplier is 4. The gain increases all occur at one

ADC code, while the gain decreases all occur at

another ADC code. Thus, the hysteresis algorithm only

has to compare the ADC code to two code values and

change the gain based on the result.

More Input Channels

When more than two inputs (including other tempera-

ture sensors) need to be multiplexed into the ADC, the

6-channel MCP6S26 and the 8-channel MCP6S28

PGAs provide additional channels. The thermistor

input can be used to correct other sensors, such as

humidity sensors.

Op Amp Buffer

The MCP6SX2 PGA, shown in Figure 3, can be

replaced with a unity-gain buffer; Microchip’s MCP6001

op amp would be a good choice. The advantages

include simplicity and cost. The disadvantages are the

inability to multiplex multiple input signals and the

improvement in ADC temperature resolution due to

changing the PGA’s gain.

Remote Thermistor Issues

Thermistors that are located remotely from the PGA

(e.g., not on the same PCB) may require design

changes. Possible issues include:

• Shielding sensor pickup wires

• EMI filtering and protection

• Wiring resistance voltage drop

• Mismatch between thermistor ground and PCB

ground

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

-50

-25

100

125

150

Thermistor Temperature (°C)

Design # 2

Thermistor Emulator, Rvar

G = +1

G = +8

G = +32

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