LTC2485 datasheet(32/40 Pages) LINER

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LTC2485

2485fa

Using the 2x speed mode of the LTC2485, the device

bypasses the digital offset calibration operation to double

the output data rate. The superior normal mode rejection

is maintained as shown in Figures 31 and 32. However, the

magnified details near DC and fS = 256fN are different, see

Figures 40 and 41. In 2x speed mode, the bandwidth is

11.4Hz for the 50Hz rejection mode, 13.6Hz for the 60Hz

rejection mode and 12.4Hz for the 50Hz/60Hz rejection

mode. Typical measured values of the normal mode

rejection of the LTC2485 operating with the internal oscil-

lator and 2x speed mode is shown in Figure 42.

When the LTC2485 is configured in 2x speed mode, by

performing a running average, a SINC1 notch is combined

with the SINC4 digital filter, yielding the normal mode

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 43. 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 LTC2485 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 45 (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 LTC2485 can be tucked neatly underneath

an Omega MPJ-K-F thermocouple socket ensuring close

thermal coupling.

The LTC2485’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 44. 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 LTC2485 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_LTC2485()

function controls the operation of the LTC2485 and is

listed below for reference.

APPLICATIO S I FOR ATIO

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