LTC2480CDD datasheet(37/40 Pages) LINER

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LTC2480

2480f

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 LTC2480 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. The input span may be

optimized for various sensors by setting the gain of the

PGA. Using an external 5V reference with a PGA gain of 64

gives a

±78mV input range—perfect for thermocouples.

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

same temperature as the junction between the thermo-

couple materials and the copper printed circuit board

traces. The tiny LTC2480 can be tucked neatly underneath

an Omega MPJ-K-F thermocouple socket ensuring close

thermal coupling.

The LTC2480’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 including 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 LT

®1236 refer-

ence and measuring the local temperature with an accu-

rate 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 ther-

mometer. Both the voltage and temperature calibration

are easily automated.

The complete microcontroller code for this application is

available on the LTC2480 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_LTC2480()

function controls the operation of the LTC2480 and is

listed below for reference.

Figure 43. Calibration Setup

SCK

SDO

SDI

VCC

LTC2480

REF

GND

IN–

IN+

ISOTHERMAL

0.1

µF

2480 F43

26.3C

TYPE K

THERMOCOUPLE

JACK

(OMEGA MPJ-K-F)

GND

IN OUT

NC1M4V0

TRIM

GND

LT1236

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