MCP3421 datasheet(31/42 Pages) MICROCHIP

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DS22003E-page 31

MCP3421

6.2.5

TEMPERATURE MEASUREMENT

Figure 6-10

shows

example

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

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

thermocouple,

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)

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

μV/PGA

1.953125

μV for PGA

μV/°C

() 8320

μV/°C

•

No. of LSB/°C

320

μV/°C

15.625

μV

-------------------------

20.48 Codes/°C

~ 40 µV°C

Thermocouple Sensor

MCP9800

Isothermal Block

Heat

Cold Junction (TCJ)

Hot Junction

(THJ)

SCL

SDA

VDD

To MCU

(MASTER)

MCP3421

10 k

log

50 mV

15.625

μV

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

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