MCP3421A3T-E/CH 데이터 시트보기 (PDF) - Microchip Technology

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MCP3421A3T-E/CH

18-Bit Analog-to-Digital Converter with I2C Interface and On-Board Reference

Microchip Technology 

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 (THJ) with respect to the

cold junction temperature (reference, TCJ). 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 (THJ).

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

Detectable Input Signal Level = 15.625μV/PGA

= 1.953125μV for PGA = 8

Input Signal Level after gain of 8:

= (40μV/°C) • 8 = 320μV/°C

No. of LSB/°C = 3-1--2-5--0-.-6--μ-2---V5----/μ-°---VC-- = 20.48 Codes/°C

Where:

1 LSB = 15.625 µV with 18-bit configuration

MCP3421

Hot Junction

VDD

(THJ)

Isothermal Block

Heat

Cold Junction (TCJ)

V1

MCP3421

MCP9800

Thermocouple Sensor

~ 40 µV°C

VDD

SDA SCL

10 kΩ

10 kΩ

To MCU

(MASTER)

FIGURE 6-10:

Measurement.

Example of Temperature

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

Expected

⎛

⎞

Number

of

Output

Code

=

lo

g2

⎜

⎜

⎝

-1------5-5-----.P-0--6------G2----m--5--A----μ--V------V----⎠⎟⎟

Where:

= 11.6 bits for PGA = 1

= 12.6 bits for PGA = 2

= 13.6 bits for PGA = 4

= 14.6 bits for PGA = 8

1 LSB = 15.625 µV with 18-bit configuration.

© 2009 Microchip Technology Inc.

DS22003E-page 31

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