电压检测MAX110ACPE[1](19)

Low-Cost, 2-Channel, ±14-Bit Serial ADCs

Figure 12. Thermocouple Circuit with Software Compensation

unloaded, and subtracting this value from actual weightwhere αis the Seebeck constant for the type of thermo-measurements. The lowpass filtering action of thecouple, T1 is the temperature being measured, andMAX111’s sigma-delta converter helps minimize noise.TThe resolution of the weigh scale can be furtherone method to obtain TREFis the temperature of the junction block. Althoughincreased by averaging several conversions.

to a known temperature (0°C), a more popularREFis to force the junction blockThermocouple Circuit with Software

approach is to measure TREFdirectly using a thermistorCompensation

or PN junction voltage.

A thermocouple is created by the junction of dissimilarThe circuit in Figure 12 shows a k-type thermocouplemetals, and generates a voltage proportional to temper-going through a 54dB gain stage to channel 1 of theature (Seebeck voltage), making it useful for tempera-MAX110. A MAX874 voltage reference provides bothture-measurement instruments. When a thermocouplethe 3V reference voltage and reference junction tem-probe is connected to a measurement instrument, otherperature information to the MAX110. Armed with thethermoelectric potentials are created between the alloystemperature information provided by the MAX874, theof the probe and the copper connectors of the instru-thermocouple voltage created at the junction block canment. These potentials introduce a temperature-depen-be subtracted out in software. The TEMP output of thedent error that must be subtracted from the temperatureMAX874 is nominally 690mV at room temperature, andmeasurement to obtain an accurate result. According toincreases with temperature at about 2.3mV/°C. Placethe law of intermediate metals, the junction of the ther-the MAX874 as close as possible to the terminal block,mocouple-probe alloys with the copper of the instrumentand ensure good thermal contact between them. Thisjunction block can be treated as another thermocouplecircuit employs a common k-type thermocouple and,of the same type. The voltage measured by the instru-with the component values shown, can indicate tem-ment can be expressed as:

peratures in the range of -150°C to +125°C.

V = α(T1 - TREF)

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MAX110/MAX111