MAX6674ISA+ Datasheet MAX6674ISA+, MAX6674ISA+T | P4-P6

MAX6674

Cold-Junction-Compensated K-Thermocouple-

to-Digital Converter (0°C to +128°C)

4 _______________________________________________________________________________________

Detailed Description

The MAX6674 is a sophisticated thermocouple-to-digi-

tal converter with a built-in 10-bit analog-to-digital con-

verter (ADC). The device also contains cold-junction

compensation sensing and correction, a digital con-

troller, an SPI-compatible interface, and associated

control logic.

The MAX6674 is designed to work in conjunction with

an external microcontroller (µC) or other intelligence in

thermostatic, process-control, or monitoring applica-

tions. The µC is typically a power-management or key-

board controller, generating SPI serial commands by

“bit-banging” general-purpose input-output (GPIO) pins

or through a dedicated SPI interface block.

Temperature Conversion

The MAX6674 includes signal conditioning hardware to

convert the thermocouple’s signal into a voltage that is

compatible with the input channels of the ADC. The T+

and T-inputs connect to internal circuitry that reduces the

introduction of noise errors from the thermocouple wires.

Before converting the thermoelectric voltages into

equivalent temperature values, it is necessary to com-

pensate for the difference between the thermocouple

cold-junction side (MAX6674 ambient temperature) and

a 0°C virtual reference.

For a type-K thermocouple, the voltage changes by

41µV/°C, which approximates the thermocouple char-

acteristic with the following linear equation:

OUT

= (41µV/°C)

- T

AMB

)

where:

OUT

is the thermocouple output voltage (µV).

is the temperature of the remote point (°C).

AMB

is the ambient temperature (°C).

Cold-Junction Compensation

The function of the thermocouple is to sense a differ-

ence in temperature between two ends. The thermo-

couple’s hot junction can be read from 0°C to

+127.875°C. The cold end (ambient temperature of the

board on which the MAX6674 is mounted) can only

range from -20°C to +85°C. While the temperature at

the cold end fluctuates, the MAX6674 continues to

accurately sense the temperature difference at the

opposite end.

The MAX6674 senses and corrects for the changes in

the ambient temperature with cold-junction compensa-

tion. The device converts the ambient temperature

reading into a voltage using a temperature-sensing

diode. To make the actual thermocouple temperature

measurement, the MAX6674 measures the voltage from

the thermocouple’s output and from the sensing diode.

The device’s internal circuitry passes the diode’s volt-

age (sensing ambient temperature) and thermocouple

voltage (sensing remote temperature minus ambient

temperature) to the conversion function stored in the

ADC to calculate the thermocouple’s hot-junction tem-

perature.

Optimal performance from the MAX6674 is achieved

when the thermocouple cold junction and the device

are at the same temperature. Avoid placing heat-gener-

ating devices or components near the MAX6674

because this may produce cold-junction-related errors.

Digitization

The ADC adds the cold-junction diode measurement

with the amplified thermocouple voltage and reads out

the 10-bit sequence onto the S0 pin. A sequence of all

zeros means the thermocouple reading is 0°C. A

sequence of all ones means the thermocouple reading

is +127.875°C.

Pin Description

PIN NAME FUNCTION

1 GND Ground

2T-

Alumel Lead of Type-K Thermocouple.

Should be connected to ground

externally.

3 T+ Chromel Lead of Type-K Thermocouple

Positive Supply. Bypass with a 0.1µF

capacitor to GND.

5 SCK Serial Clock Input

6 CS

Chip Select. Set CS low to enable the

serial interface.

7 S0 Serial Data Output

8 N.C. No Connection

MAX6674

Cold-Junction-Compensated K-Thermocouple-

to-Digital Converter (0°C to +128°C)

_______________________________________________________________________________________ 5

Applications Information

Serial Interface

The Typical Application Circuit shows the MAX6674

interfaced with a microcontroller. In this example, the

MAX6674 processes the reading from the thermocou-

ple and transmits the data through a serial interface.

Force CS low and apply a clock signal at SCK to read

the results at S0. Forcing CS low immediately stops any

conversion process. Initiate a new conversion process

by forcing CS high.

Force CS low to output the first bit on the S0 pin. A

complete serial interface read requires 16 clock cycles.

Read the 16 output bits on the falling edge of the clock.

The first bit, D15, is a dummy sign bit and always zero.

Bits D14–D5 contain the converted temperature in the

order of MSB to LSB. Bit D4 reads a high value when

any of the thermocouple inputs are open. Bit D3 is

always low to provide a device ID for the MAX6674.

Bits D2–D0 are in three-state when CS is high.

Figure 1a is the serial interface protocol and Figure 1b

shows the serial interface timing. Figure 2 is the S0 output.

Open Thermocouple

Bit D4 is normally low and goes high if the thermocou-

ple input is open. The open thermocouple detection cir-

cuit is implemented completely into the MAX6674. In

order to allow the operation of the open thermocouple

detector, T- must be grounded. Make the ground con-

nection as close to the GND pin as possible.

Noise Considerations

The accuracy of the MAX6674 is susceptible to power-

supply coupled noise. The effects of power-supply

noise can be minimized by placing a 0.1µF ceramic

bypass capacitor close to the supply pin of the device.

Thermal Considerations

Self-heating degrades the temperature measurement

accuracy of the MAX6674 in some applications. The

magnitude of the temperature errors depends on the

thermal conductivity of the MAX6674 package, the

mounting technique, and the effects of airflow. Use a

large ground plane to improve the temperature mea-

surement accuracy.

The accuracy of a thermocouple system can also be

improved by following these precautions:

• Use the largest wire possible that does not shunt

heat away from the measurement area.

• If small wire is required, use it only in the region of

the measurement and use extension wire for the

region with no temperature gradient.

• Avoid mechanical stress and vibration that could

strain the wires.

• When using long thermocouple wires, use a twist-

ed-pair extension wire.

• Avoid steep temperature gradients.

• Try to use the thermocouple wire well within its tem-

perature rating.

• Use the proper sheathing material in hostile envi-

ronments to protect the thermocouple wire.

• Use extension wire only at low temperatures and

only in regions of small gradients.

• Keep an event log and a continuous record of ther-

mocouple resistance.

Reducing Effects of Pick-Up Noise

The input amplifier (A1) is a low-noise amplifier

designed to enable high-precision input sensing. Keep

the thermocouple and connecting wires away from

electrical noise sources.

MAX6674

Cold-Junction-Compensated K-Thermocouple-

to-Digital Converter (0°C to +128°C)

6 _______________________________________________________________________________________

SCK

D15

D14

D13 D12

D11

D10

D7 D6

D4 D3

D2 D1 D0

Figure 1a. Serial Interface Protocol

D15 D0D1D2D3

SCK

CSS

Figure 1b. Serial Interface Timing

BIT

DUMMY

SIGN BIT

10-BIT

TEMPERATURE READING

THERMOCOUPLE

INPUT

DEVICE

STATE

Bit 15 14

12 11

10 9

8765 4 3 210

0 MSB LSB 0 Three-state

Figure 2. S0 Output

P1-P3 P4-P6 P7-P8

MAX6674ISA+

Mfr. #:

Buy MAX6674ISA+

Manufacturer:

Maxim Integrated

Description:

Sensor Interface K Thermocouple to Digital Converter

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MAX6674ISA+

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