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MAX31850

Cold-Junction Compensated, 1-Wire Thermocouple-to-Digital Converters

Maxim Integrated 

MAX31850/MAX31851

Cold-Junction Compensated, 1-Wire

Thermocouple-to-Digital Converters

Detailed Description

The MAX31850/MAX31851 are sophisticated thermo-

couple-to-digital converters with a built-in 14-bit analog-

to-digital converter (ADC), cold-junction compensation

sensing and correction, a digital controller, a 1-Wire

data interface, and associated control logic. The devices

are available in several versions, each optimized and

trimmed for a specific thermocouple type (K, J, N, T, S,

R, or E.). The thermocouple type is indicated in the suffix

of the part number (e.g., MAX31850K). See the Ordering

Information table for all options.

The 1-Wire bus by definition requires only one data line

(and ground) for communication with a central microcon-

troller. The data line requires a weak pullup resistor since

all devices are linked to the bus through a three-state or

open-drain port (i.e., the DQ pin). Four location address

inputs simplify mapping of individual devices to specific

locations.

Each device has a unique 64-bit serial code, allowing

multiple devices to function on the same 1-Wire bus.

Therefore, it is simple to use one microcontroller to con-

trol many devices distributed over a large area. In this

bus system, the microcontroller identifies and addresses

devices on the bus using each device’s unique 64-bit

code. Because each device has a unique code, the

number of devices that can be addressed on one bus

is virtually unlimited. The 1-Wire bus protocol, including

detailed explanations of the commands and time slots, is

described in the 1-Wire Bus System section.

The scratchpad memory contains the 2-byte temperature

register that stores the cold-junction-compensated ther-

mocouple temperature data. A second 2-byte register

stores the local cold-junction temperature. Both of these

registers also store fault data for open thermocouple as

well as shorts to supply and ground.

Power may be obtained either from a power supply con-

nected to VDD, or from the 1-Wire pullup resistor through

the DQ pin when the bus is high. The high bus signal also

charges an internal capacitor (CPP), which then supplies

power to the device when the bus is low. This method

of deriving power from the 1-Wire bus is referred to as

“parasite power.”

Temperature Conversion

The devices include signal-conditioning hardware to

convert the thermocouple’s signal into a voltage com-

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

and T- inputs connect to internal circuitry that reduces

Maxim Integrated

the introduction of noise errors from the thermocouple

wires.

Before converting the thermoelectric voltages into equiva-

lent temperature values, it is necessary to compensate for

the difference between the thermocouple cold-junction

side (device ambient temperature) and a 0NC virtual refer-

ence. For a K-type thermocouple, the voltage changes by

approximately 41FV/NC, which approximates the thermo-

couple characteristic with the following linear equation:

VOUT = (41.276FV/NC) x (TR - TAMB)

where VOUT is the thermocouple output voltage (FV), TR

is the temperature of the remote thermocouple junction

(NC), and TAMB is the temperature of the device (NC).

Other thermocouple types use a similar straight-line

approximation but with different gain terms. Note that

the MAX31850/MAX31851 assume a linear relationship

between temperature and voltage. Because all thermo-

couples exhibit some level of nonlinearity, apply appro-

priate correction to the device’s output data.

Cold-Junction Compensation

The function of the thermocouple is to sense a difference

in temperature between two ends of the thermocouple

wires. The thermocouple’s “hot” junction can be read

across the operating temperature range (Table 1). The

reference junction, or “cold” end (which should be at

the same temperature as the board on which the device

is mounted) can range from -55NC to +125NC. While the

temperature at the cold end fluctuates, the device con-

tinues to accurately sense the temperature difference at

the opposite end.

The device senses and corrects for the changes in

the reference junction temperature with cold-junction

compensation. It does this by first measuring its internal

die temperature, which should be held at the same tem-

perature as the reference junction. It then measures the

voltage from the thermocouple’s output at the reference

junction and converts this to the noncompensated ther-

mocouple temperature value. This value is then added

to the device’s die temperature to calculate the thermo-

couple’s “hot junction” temperature. Note that the “hot

junction” temperature can be lower than the cold junction

(or reference junction) temperature.

Optimal performance from the device is achieved when

the thermocouple cold junction and the device are at

the same temperature. Avoid placing heat-generating

devices or components near the MAX31850/MAX31851

because this could produce cold-junction-related errors.

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