Datasheet

© 2009 Microchip Technology Inc. DS22003E-page 31
MCP3421
6.2.5 TEMPERATURE MEASUREMENT
Figure 6-10 shows an example of temperature
measurement using a thermocouple sensor and the
MCP9800 silicon temperature sensor. The MCP9800 is
a high accuracy temperature sensor that can detect the
temperature in the range of -55°C to 125°C with 1°C
accuracy.
The type K thermocouple sensor senses the
temperature at the hot junction (T
HJ
) with respect to the
cold junction temperature (reference, T
CJ
). The
temperature difference between the hot and cold
junctions is represented by the voltage V1. This voltage
is then converted to digital codes by the MCP3421.
In the circuit, the MCP9800 is used for cold junction
compensation. The MCU computes the difference of
the hot and cold junction temperatures, which is
proportional to the hot junction temperature (T
HJ
).
With Type K thermocouple, it can measure
temperature from 0°C to 1250°C degrees. The full
scale output range of the Type K thermocouple is
about 50 mV. This provides 40 µV/°C
(= 50 mV/1250°C) of measurement resolution.
Equation 6-2 shows the measurement budget for ther-
mocouple sensor signal using the MCP3421 device
with 18 bits and PGA = 8 settings. With this configura-
tion, it can detect the input signal level as low as
approximately 2 µV. The internal PGA boosts the input
signal level eight times. The 40 µV/°C input from the
thermocouple is amplified internally to 320 µV/°C
before the conversion takes place. This results in
20.48 LSB/°C output codes. This means there are
about 20 LSB output codes (or about 4.32 bits) per 1°C
of change in temperature.
EQUATION 6-2: MEASUREMENT BUDGET
FOR THERMOCOUPLE
SENSOR
FIGURE 6-10: Example of Temperature
Measurement.
Equation 6-3 shows an example of calculating the
expected number of output code with various PGA gain
settings for Type K thermocouple output.
EQUATION 6-3: EXPECTED NUMBER OF
OUTPUT CODE FOR
TYPE K THERMOCOUPLE
Input Signal Level after gain of 8:
Where:
1 LSB = 15.625 µV with 18-bit configuration
Detectable Input Signal Level 15.625
μ V/PGA=
1.953125 μ V for PGA 8==
40
μ V/°C()8320 μ V/°C= • =
No. of LSB/°C
320
μ V/°C
15.625
μ V
------------------------- 20.48 Codes/°C==
~ 40 µV°C
Thermocouple Sensor
MCP9800
Isothermal Block
Heat
Cold Junction (T
CJ
)
Hot Junction
(T
HJ
)
SCL
SDA
V1
V
DD
V
DD
To MCU
(MASTER)
MCP3421
10 kΩ
10 kΩ
1µF
10 µF
log
2
50 mV
15.625
μ V
PGA
------------------------
------------------------
⎝⎠
⎜⎟
⎜⎟
⎛⎞
Expected
Number of Output Code =
Where:
1 LSB = 15.625 µV with 18-bit configuration.
=
11.6 bits for PGA
=
1
=
12.6 bits for PGA
=
2
=
13.6 bits for PGA
=
4
=
14.6 bits for PGA
=
8