Datasheet
LTC2480
38
2480fd
applicaTions inForMaTion
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 thermo-
couples 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
. 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 ther-
mocouple 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 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 fit. 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 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 LTC2480 product Web page 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.
CS
SCK
SDO
SDI
F
O
6
9
7
1
10
V
CC
5V
LTC2480
REF
GND
IN
–
IN
+
3
ISOTHERMAL
2
C7
0.1µF
C8
1µF
4
R2
2k
R7
8k
62
5
4
R8
1k
5
2480 F43
26.3C
TYPE K
THERMOCOUPLE
JACK
(OMEGA MPJ-K-F)
118
GND
IN OUT
G1
NC1M4V0
TRIM
GND
LT1236
+
Figure 43. Calibration Setup