AN897 Datasheet, PDF (9/16 Pages) Microchip Technology – Thermistor Temperature Sensing with MCP6SX2 PGAs

0.5
0.4 ^{Design # 2}
0.3 ^{Thermistor Emulator, Rvar}
0.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
G = +1
G = +8
-0.5
G = +32
-50 -25 0 25 50 75 100 125 150
Thermistor Temperature (°C)
FIGURE 24:
Design # 2.
Measured Errors,
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 R_A 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.
AN897
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 at 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
 2004 Microchip Technology Inc.
DS00897B-page 9