LTC2400 Linear Technology, LTC2400 Datasheet - Page 33

• Manufacturer Part Number: LTC2400
• Description: 24-Bit uPower No Latency ADC in SO-8
• Manufacturer: Linear Technology

TYPICAL APPLICATIONS
Thermocouple Applications
Figure 35 shows a thermocouple interface circuit that
demonstrates the practicality of direct connection to the
LTC2400 using even the lowest output thermocouples (in
this case, a type S thermocouple, with a full-scale output
of 18mV).
This topology is the least costly solution for thermocouple
sensing. As shown, it is capable of resolving approxi-
mately 0.25 C without averaging. Since the LTC2400 does
not exhibit any easily discernible quantization effects,
averaging can significantly extend the resolution for slow
changing processes.
In this circuit, a 1N4148 diode provides cold junction
compensation by producing, at the positive terminal of the
thermocouple, an approximation of the average Seebeck
coefficient for a type S thermocouple over the temperature
range expected at the cold junction (0 C to 40 C). If the
operating range is less, the coefficient can be adjusted to
produce a better match for the range anticipated. This
basic circuit can be used with other thermocouples by
changing the divide ratio to suit the Seebeck coefficient of
the type chosen (see table).
THERMOCOUPLE
U
–
+
COLD JUNCTION
ISOTHERMAL
Figure 35. Diode Cold Junction Compensation
1N4148
*20 C T
THERMOCOUPLE
Cu
Cu
TYPE
K
S
J
A
50 C
R
1M
TC
COEFFICIENT*
5V
50.2 V/ C
39.2 V/ C
6.15 V/ C
SEEBECK
R2*
R3*
100
R1
43.2k
– 2mV/ C
– SB
SINGLE POINT GROUND
This circuit produces a DC offset at the cold junction
reference point, of 1mV to 15mV, which must be nulled out
in software. This DC offset, resulting from the forward
voltage of the diode, is variable from device to device and
must be calibrated for each unit.
Since the temperature coefficient of the 1N4148 diode is
not guaranteed, a trim should be provided to accommo-
date a coefficient from 1.7mV/ C to 2.3mV/ C. Alterna-
tively, a transistor can be used as a sensor with Omega
Engineering thermocouple circuit board connectors that
are available with TO-92 transistor retainer clips, placing
the transistor in physical contact with the cold junction.
The 1M resistor R
detection scheme, producing full scale at the input of the
LTC2400. Note that this resistor contributes to the offset
and must have low TC, as should the resistors R2 and R3.
Since R1 provides forward bias for the diode, its tempera-
ture coefficient is not as critical.
The circuit in Figure 35 uses only 12% of the LTC2400’s
input range and is able to accommodate the full-scale
output of all thermocouple types. The commonly used
3.83k
4.99k
32.4k
R2
2400 F35
3
2
V
IN
V
*25ppm, 1% TOLERANCE
REF
1
GND
V
TC
CC
LTC2400
4
60Hz
shown is intended as an open-circuit
0.1 F
F
O
8
SDO
SCK
50Hz
CS
10k
5
6
7
5V
LTC2400
33