Datasheet MAX6675 (Maxim) - 5

MAX6675 Cold-Junction-Compensated K-Thermocouple- to-Digital Converter (0°C to +1024°C)
Applications Information
mounting technique, and the effects of airflow. Use a
Serial Interface
large ground plane to improve the temperature mea- surement accuracy of the MAX6675. The Typical Application Circuit shows the MAX6675 The accuracy of a thermocouple system can also be interfaced with a microcontroller. In this example, the improved by following these precautions: MAX6675 processes the reading from the thermocou- ple and transmits the data through a serial interface. ● Use the largest wire possible that does not shunt Force CS low and apply a clock signal at SCK to read heat away from the measurement area. the results at SO. Forcing CS low immediately stops ● If small wire is required, use it only in the region of any conversion process. Initiate a new conversion the measurement and use extension wire for the process by forcing CS high. region with no temperature gradient. Force CS low to output the first bit on the SO pin. A ● Avoid mechanical stress and vibration, which could complete serial interface read requires 16 clock cycles. strain the wires. Read the 16 output bits on the falling edge of the clock. ● When using long thermocouple wires, use a twisted- The first bit, D15, is a dummy sign bit and is always pair extension wire. zero. Bits D14–D3 contain the converted temperature in the order of MSB to LSB. Bit D2 is normally low and ● Avoid steep temperature gradients. goes high when the thermocouple input is open. D1 is ● Try to use the thermocouple wire well within its tem- low to provide a device ID for the MAX6675 and bit D0 perature rating. is three-state. ● Use the proper sheathing material in hostile environ- Figure 1a is the serial interface protocol and Figure 1b ments to protect the thermocouple wire. shows the serial interface timing. Figure 2 is the SO out- ● Use extension wire only at low temperatures and put. only in regions of small gradients.
Open Thermocouple
● Keep an event log and a continuous record of ther- Bit D2 is normally low and goes high if the thermocou- mocouple resistance. ple input is open. In order to allow the operation of the
Reducing Effects of Pick-Up Noise
open thermocouple detector, T- must be grounded. Make the ground connection as close to the GND pin The input amplifier (A1) is a low-noise amplifier as possible. designed to enable high-precision input sensing. Keep the thermocouple and connecting wires away from elec-
Noise Considerations
trical noise sources. The accuracy of the MAX6675 is susceptible to power- supply coupled noise. The effects of power-supply noise can be minimized by placing a 0.1µF ceramic
Chip Information
bypass capacitor close to the supply pin of the device. TRANSISTOR COUNT: 6720
Thermal Considerations
PROCESS: BiCMOS Self-heating degrades the temperature measurement accuracy of the MAX6675 in some applications. The magnitude of the temperature errors depends on the thermal conductivity of the MAX6675 package, the www.maximintegrated.com Maxim Integrated │ 5