LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 32

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2485
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 f
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 SINC
with the SINC
rejection identical as that for the 1x speed mode. The
averaging operation still keeps the output rate with the
following algorithm:
……
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 (2.5µV max) offset drift of 10nV/°C
and a noise level of 600nV
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
32
Result 1 = average (sample 0, sample 1)
Result 2 = average (sample 1, sample 2)
Result n = average (sample n – 1, sample n)
4
digital filter, yielding the normal mode
U
U
RMS
.
S
= 256f
W
1
N
notch is combined
are different, see
U
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
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:
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.
http://www.linear.com
®
1236 refer-
2485fa