Datasheet
LTC2485
32
2485fc
APPLICATIONS INFORMATION
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 magnifi ed details near DC and f
S
= 256f
N
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 rejec-
tion of the LTC2485 operating with the internal oscillator
and 2x speed mode is shown in Figure 42.
When the LTC2485 is confi gured in 2x speed mode, by
performing a running average, a SINC
1
notch is combined
with the SINC
4
digital fi lter, 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 (2.5µV max) offset drift of
10nV/°C and a noise level of 600nV
RMS
.
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 ther-
mocouple 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 volt-
age 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 fi t. Calibration is performed by apply-
ing an accurate 500mV to the ADC input derived from an
LT
®
1236 reference and measuring the local temperature
with an accurate 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 thermometer. 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 coeffi cients
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.