Datasheet MAX6674 (Maxim) - 5

MAX6674 Cold-Junction-Compensated K-Thermocouple- to-Digital Converter (0°C to +128°C)
Applications Information Thermal Considerations Serial Interface
Self-heating degrades the temperature measurement accuracy of the MAX6674 in some applications. The The Typical Application Circuit shows the MAX6674 magnitude of the temperature errors depends on the ther- interfaced with a microcontroller. In this example, the mal conductivity of the MAX6674 package, the mounting MAX6674 processes the reading from the thermocou- technique, and the effects of airflow. Use a large ground ple and transmits the data through a serial interface. plane to improve the temperature measurement accuracy. Force CS low and apply a clock signal at SCK to read the results at S0. Forcing CS low immediately stops any The accuracy of a thermocouple system can also be conversion process. Initiate a new conversion process improved by following these precautions: by forcing CS high. ● Use the largest wire possible that does not shunt Force CS low to output the first bit on the S0 pin. A heat away from the measurement area. complete serial interface read requires 16 clock cycles. ● If small wire is required, use it only in the region of Read the 16 output bits on the falling edge of the clock. the measurement and use extension wire for the The first bit, D15, is a dummy sign bit and always zero. region with no temperature gradient. Bits D14–D5 contain the converted temperature in the ● Avoid mechanical stress and vibration that could order of MSB to LSB. Bit D4 reads a high value when strain the wires. any of the thermocouple inputs are open. Bit D3 is always low to provide a device ID for the MAX6674. ● When using long thermocouple wires, use a twist- Bits D2–D0 are in three-state when CS is high. ed-pair extension wire. Figure 1a is the serial interface protocol and Figure 1b ● Avoid steep temperature gradients. shows the serial interface timing. Figure 2 is the S0 output. ● Try to use the thermocouple wire well within its tem-
Open Thermocouple
perature rating. Bit D4 is normally low and goes high if the thermocou- ● Use the proper sheathing material in hostile environ- ple input is open. The open thermocouple detection cir- ments to protect the thermocouple wire. cuit is implemented completely into the MAX6674. In ● Use extension wire only at low temperatures and order to allow the operation of the open thermocouple only in regions of small gradients. detector, T- must be grounded. Make the ground con- ● Keep an event log and a continuous record of ther- nection as close to the GND pin as possible. mocouple resistance.
Noise Considerations Reducing Effects of Pick-Up Noise
The accuracy of the MAX6674 is susceptible to power- The input amplifier (A1) is a low-noise amplifier supply coupled noise. The effects of power-supply designed to enable high-precision input sensing. Keep noise can be minimized by placing a 0.1µF ceramic the thermocouple and connecting wires away from bypass capacitor close to the supply pin of the device. electrical noise sources. www.maximintegrated.com Maxim Integrated │ 5