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LTC2400CS8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P39P40-P40
34
LTC2400
TYPICAL APPLICATIONS
U
thermocouple with the highest output is type E, at about
70mV. This circuit does not provide curvature correction
for the Seebeck effect at the cold junction. If the applica-
tion requires very high accuracy, the temperature of the
cold junction should be determined via a separate input
to the A/D, using an RTD for example. The cold junction
compensation can be performed by implementing the
thermocouple’s NBS polynominal curvature correction
in software. (The input to the LTC2400 can be multi-
plexed using the LTC1391 with little degradation.) If a
separate temperature sensor is used to monitor the cold
junction, the connection from the thermocouple to the
LTC2400 can be direct. The junctions formed at the point
where the thermocouple leads meet different metal (e.g.,
copper traces) must be equal in temperature, and the
cold junction sensor must be mounted at that point. Any
temperature differential between the leads, or any differ-
ential between the leads and the temperature sensor will
introduce an error into the reading.
Figure 36 shows an inexpensive circuit with removal of the
DC offset. The output of the LT
®
1077 is attenuated in order
to produce the required coefficient, as well as reduce the
noise and offset error contribution. If used with a ther-
mistor, this circuit can be modified to produce curvature
correction. The removal of the offset associated with diode
forward voltage, or the 273°K overhead on some mono-
lithic temperature sensors, simplifies the use of substan-
tial gain after the thermocouple. Chopper amplifiers such
as the LTC1050 can extend the noise floor of the LTC2400
by as much as a factor of 10 to 20. The use of a gain of 20
in front of the LTC2400 can extend the resolution of a
thermocouple application to 0.02°C or better.
If absolute accuracy is not important, the use of a low
noise bipolar amplifier, such as the LT1028, can extend
the resolution an additional order of magnitude.
Note that achieving high accuracy in the circuit in Figure 36
requires a calibration sequence for circuit offset and gain
correction.
V
IN
SDO
SCK
CS
36.1µV/°C
R5
1k
6
3
R
1mV/°C
2
7
4
5V
+–
2
1
R2
174k*
V
+
V
–
4
*RECOMMENDED 0.1%, ±5ppm IRC AFD SERIES CHIP RESISTORS
5
6
7
5V
10k
60Hz
SELECT R3 FOR
THERMOCOUPLE TYPE
S: 6.19Ω
K: 39.2Ω
J: 49.9Ω
E: 61.9Ω
50Hz
8
V
REF
V
CC
0.1µF
5V
GND
LTC2400
F
O
2400 F35
R3
1k*
R6
6.19Ω
R1
226Ω*
R4
10k*
–
+
LT1077
LM334
SO-8
Figure 36. Inexpensive Amplifier Improves Cold Junction Compensation
35
LTC2400
TYPICAL APPLICATIONS
U
V
IN
SDO
SCK
CS
3
TYPE
S
6
2
54
5V
+–
2
1
4
5
6
7
5V
10k
60Hz 50Hz
8
V
REF
V
CC
0.1µF
GND
LTC2400
F
O
2400 F36
V
IN
S
LT1025
R
–
GND
Figure 37. The LT1025 Complete Cold Junction Solution
A simpler, and potentially less expensive solution is the
use of the LT1025 as shown in Figure 37.
The LT1025 incorporates the functions of temperature
sensor, a precision divider chain required to produce the
appropriate correction for five different types of thermo-
couples, as well as curvature correction. The LT1025 must
be located at the cold junction. The use of a thermal mass
around the cold junction, as well as protection from air
currents, is advisable.
Simple Platinum RTD Interface
If high temperature resolution is required over a more
limited range, Figure 38 can resolve approximately
0.01°C without additional amplification. The resistance of
a platinum RTD changes by approximately 0.31Ω/°C at
T
A
= 25°C. The 100Ω to 300Ω source impedance of this
circuit does not compromise the stability, accuracy or
noise level of the LTC2400.
V
IN
SDO
SCK
CS
3
FS
5V
2
R1*
12.1k
Pt RTD
100Ω
1
4
*VISHAY S102 OR EQUIVALENT
5
6
7
5V
10k
60Hz 50Hz
8
V
REF
V
CC
0.1µF5V
GND
LTC2400
F
O
2400 F37
Figure 38. Simplest Platinum RTD Interface
36
LTC2400
TYPICAL APPLICATIONS
U
The 12.1k resistor should be a precision resistor such as
a Vishay S102 series, or must be temperature stabilized.
The excitation current is low enough for most sensors that
the self-heating effect is near the noise floor of the LTC2400.
The use of a bipolar amplifier configuration shown in
Figure 39 offers a potential resolution of 0.001°C
In order to achieve these results, the following effects
must be considered. Variation in the self-heating of the
RTD element due to air currents is the most difficult
challenge. If the RTD is mounted in a sealed glass enclo-
sure and painted black, the LTC2400 can detect the arrival
of a person in the room. This is also true of infrared
thermocouple sensors (thermopiles) that can also be used
directly with the LTC2400. A variation of this circuit with
two RTDs can detect small differential temperatures in
order to determine heat inflow or outflow from a process.
In order for this circuit to be practical, the ambient tem-
perature of the amplifier and resistors must be controlled
or the resistors must exhibit very low temperature coeffi-
cients. Precision resistor networks are always a good
alternative and are available from Vishay or Caddock.
Half-Bridge Strain Gauge
The circuit in Figure 40 is a ratiometric half-bridge circuit
with direct connection to the LTC2400. The use of two
thin-film strain gauges in a half-bridge configuration can
produce 2mV/V output and approximately 12-bit resolu-
tion. The 175Ω source impedance seen by the LTC2400
does not compromise operation.
The optional resistor shown can be up to 5k and will
provide surge and transient protection for the LTC2400
if the strain gauges are located some distance from the
LTC2400, or if the strain bearing member is not well
grounded and may be subject to ESD discharge. Thin-
film strain elements form coupling capacitance to the
strain bearing member to which they are bonded. If noise
–
+
LT1028
6
TO
LTC2400
3
2
300Ω
1k
5V
–5V
V
REF
R3*
9.09k
R4**
100Ω
R2*
9.09k
Pt RTD
100Ω
MUST BE 5ppm/°C OR BETTER,
AN ARRAY IS RECOMMENDED
MUST BE VERY STABLE <5ppm/°C
*
**
R1*
9.09k
0.1µF
2400 F38
FS
V
IN
SDO
SCK
CS
3
5V
2
350Ω
STRAIN
ELEMENT
R1
5k
OPTIONAL
350Ω
STRAIN
ELEMENT
1
4
5
6
7
8
V
REF
V
CC
0.1µF
GND
LTC2400
F
O
2400 F39
5V
10k
60Hz 50Hz
Figure 39. Extremely High Resolution RTD Interface Figure 40. Half-Bridge Connection for Strain Gauges
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LTC2400CS8#PBF

Mfr. #:
Buy LTC2400CS8#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog to Digital Converters - ADC 24-B Pwr No Lat Delta-Sigma ADC in SO-
Lifecycle:
New from this manufacturer.
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