unloaded, and subtracting this value from actual weight

measurements. The lowpass filtering action of the

MAX111’s sigma-delta converter helps minimize noise.

The resolution of the weigh scale can be further

increased by averaging several conversions.

Thermocouple Circuit with Software

Compensation

A thermocouple is created by the junction of dissimilar

metals, and generates a voltage proportional to temper-

ature (Seebeck voltage), making it useful for tempera-

ture-measurement instruments. When a thermocouple

probe is connected to a measurement instrument, other

thermoelectric potentials are created between the alloys

of the probe and the copper connectors of the instru-

ment. These potentials introduce a temperature-depen-

dent error that must be subtracted from the temperature

measurement to obtain an accurate result. According to

the law of intermediate metals, the junction of the ther-

mocouple-probe alloys with the copper of the instrument

junction block can be treated as another thermocouple

of the same type. The voltage measured by the instru-

ment can be expressed as:

V =

where

α is the Seebeck constant for the type of thermo-

couple, T1 is the temperature being measured, and

to a known temperature (0°C), a more popular

or PN junction voltage.

The circuit in Figure 12 shows a k-type thermocouple

going through a 54dB gain stage to channel 1 of the

MAX110. A MAX874 voltage reference provides both

the 3V reference voltage and reference junction tem-

perature information to the MAX110. Armed with the

temperature information provided by the MAX874, the

thermocouple voltage created at the junction block can

be subtracted out in software. The TEMP output of the

MAX874 is nominally 690mV at room temperature, and

increases with temperature at about 2.3mV/°C. Place

the MAX874 as close as possible to the terminal block,

and ensure good thermal contact between them. This

circuit employs a common k-type thermocouple and,

with the component values shown, can indicate tem-

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

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

______________________________________________________________________________________

21

243k

1k

1k

10k

1µF

1µF

IN1+

IN1-

REF-

REF+

-5V

CS

DIN

DOUT

SCLK

243k

1M

1k

10k

10k

K-TYPE

+5V

IN2-

IN2+

MAX110

1/4 MAX479

1/4 MAX479

1/4 MAX479

TEMP

OUT

MAX874

+5V

Figure 12. Thermocouple Circuit with Software Compensation