LTC2484CDD datasheet(37/40 Pages) LINER

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LTC2484

2484fa

APPLICATIO S I FOR ATIO

rejection identical as that for the 1x speed mode. The

averaging operation still keeps the output rate with the

following algorithm:

Result 1 = average (sample 0, sample 1)

Result 2 = average (sample 1, sample 2)

……

Result n = average (sample n – 1, sample n)

The main advantage of the running average is that it

achieves simultaneous 50Hz/60Hz rejection at twice the

effective output rate, as shown in Figure 42. The raw

output data provides a better than 70dB rejection over

48Hz to 62.4Hz, which covers both 50Hz

±2% and 60Hz

±2%. With running average on, the rejection is better than

87dB for both 50Hz

±2% and 60Hz ±2%.

Complete Thermocouple Measurement System with

Cold Junction Compensation

The LTC2484 is ideal for direct digitization of thermocouples

and other low voltage output sensors. The input has a typical

offset error of 500nV (2.5

µV max) offset drift of 10nV/°C

and a noise level of 600nVRMS.

Figure 44 (last page of this data sheet) is a complete type

K thermocouple meter. The only signal conditioning is a

simple surge protection network. In any thermocouple

meter, the cold junction temperature sensor must be at the

Figure 43. Calibration Setup

same temperature as the junction between the thermo-

couple materials and the copper printed circuit board

traces. The tiny LTC2484 can be tucked neatly underneath

an Omega MPJ-K-F thermocouple socket ensuring close

thermal coupling.

The LTC2484’s 1.4mV/

°C PTAT circuit measures the cold

junction temperature. Once the thermocouple voltage and

cold junction temperature are known, there are many

ways of calculating the thermocouple temperature includ-

ing a straight-line approximation, lookup tables or a

polynomial curve fit. Calibration is performed by applying

an accurate 500mV to the ADC input derived from an

®1236 reference and measuring the local temperature

with an accurate thermometer as shown in Figure 43. In

calibration mode, the up and down buttons are used to

adjust the local temperature reading until it matches an

accurate thermometer. Both the voltage and temperature

calibration are easily automated.

The complete microcontroller code for this application is

available on the LTC2484 product webpage at:

http://www.linear.com

It can be used as a template for may different instruments

and it illustrates how to generate calibration coefficients

for the onboard temperature sensor. Extensive comments

detail the operation of the program. The read_LTC2484()

function controls the operation of the LTC2484 and is

listed below for reference.

SCK

SDO

SDI

VCC

LTC2484

REF

GND

IN–

IN+

ISOTHERMAL

0.1

µF

2484 F43

26.3C

TYPE K

THERMOCOUPLE

JACK

(OMEGA MPJ-K-F)

GND

IN OUT

NC1M4V0

TRIM

GND

LT1236

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