AN929 Datasheet, PDF (16/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
V_REF = 5V
R_S
28 kΩ
100 kΩ
R_T
10 kΩ
1 µF
V_DD
MCP6S21
V_OUT
FIGURE 23:
V_OUT vs. Temperature.
FIGURE 24:
∆V_OUT vs. Temperature.
TABLE 2: ADC RESOLUTION
ADC
(N-bits)
2^N
8 bits
9 bits
10 bits
11 bits
12 bits
256
512
1024
2048
4096
Ideal ADC
Volts/bit
(V_FS = 5V)
19.5 mV
9.77 mV
4.88 mV
2.44 mV
1.22 mV
ENOB
(N - 1)
7-bits
8-bits
9-bits
10-bits
11-bits
ENOB
Volts/bit
(V_FS = 5V)
39.0 mV
19.5 mV
9.77 mV
4.88 mV
2.44 mV
PROGRAMMABLE GAIN AMPLIFIER (PGA)
CIRCUIT
A PGA circuit can be used to increase the gain of the
amplifier at the temperatures where a small change in
the output voltage is difficult to detect. Increasing the
output voltage as a function of temperature allows a
lower bit ADC to accurately resolve the relative small
differences in the thermistor’s resistance at cold and
hot temperatures. The circuit shown in Figure 25 uses
the MCP6S21, which uses a SPI interface to select a
gain of 1, 2, 4, 8, 16 or 32 V/V.
Gain Adjustment
Input Selection
SPI
V_OUT
=


R^----_S^----R+----^T-R^----_T^{---}
(
V_R
EF⁾
(Amplifier Gain)
FIGURE 25:
PGA Thermistor Circuit.
TABLE 3:
GAIN CHANGE POINTS WITH
HYSTERESIS
Gain Change Temperature
(V/V)
(°C)
V_OUT
(V)
1→2
2→4
4→8
8 → 16
16 → 32
16 ← 32
8 ← 16
4←8
2←4
1←2
37.0
57.0
77.0
97.0
118.5
114.5
93.0
73.0
53.0
33.0
0.885
0.451
0.236
0.1288
0.0708
0.0789
0.1448
0.268
0.516
1.012
The output voltage of the PGA circuit is shown in
Figure 26. The gain of the amplifier is adjusted as a
function of temperature with the values shown in
Table 3. The advantage of the PGA circuit is shown by
comparing the V_OUT slope plots of Figure 24 and
Figure 27. The V_OUT slope for the PGA circuit has a
minimum value of 30 mV for temperatures greater than
35°C, which means that only a 9-bit ADC is required. In
contrast, a voltage divider with a gain of one will require
an 11-bit or higher ADC to provide an equivalent
temperature resolution. The resolution of a thermistor
circuit is important in applications such as
overtemperature shutdown circuits.
DS00929A-page 16
 2004 Microchip Technology Inc.