Digital Transmission for Thermocouple Improves Noise Immunity
By Rend at 18 February, 2010, 10:54 pm
The signal-processing block of thermocouple is sensitive to EMI, just like other low-level circuitry. It’s wires are often exposed to EMI. It will increase undermines the accuracy of the acquired temperature data and uncertainty in the received signal. To solve this problem, usually we use special thermocouple cable but it will expensive and if we replace with another cable, the resulting situation will be difficult to diagnose. Therefor, we can use digital transmission to solve the problem. Here is the circuit :
The circuit will minimize noise by moving the controlling circuitry and adding a remote board with local intelligence near the sensing point. A local pulse-sequence generator (IC3 and IC2) drives the MAX6675/MAX6674′s SPI interface. Four characters per second: 1 stop bit, 1 start bit, and 11 data bits (13 data bits for the MAX6675) and a train of asynchronous serial characters at 4800 bauds are generated by the MAX6674/MAX6675 that is forced by IC2 and IC3.
The 11 data bits for the MAX6674 are 1 bit to warn of an open thermocouple and 10 straight-binary data bits for temperature (MSB first). 12 data bits and 1 bit are provided by the MAX6675 for the alarm. [Source: maxim-ic.com]
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