CN0287 Datasheet, PDF (2/9 Pages) Analog Devices – Devices Connected

CN-0287
Circuit Note
+5V
COLD JUNCTION COMPENSATION
+5V
: ANALOG GROUND
0Ω
: DIGITAL GROUND
SCLK CT
CT
INT
INT
ADG738_CS
ADT7310 CS
SCLK
ADT7310_CS
SCLK
SCLK
DIN
DOUT
GND _DIN
DOUT
DIN
+5V
SYNC SCLK DIN DOUT
+5V
ADG738
S1
D
1.69kΩ
+5V 300Ω
V+
AD8603
V–
S2
+4.096V ^VDD
A
W
B
V_SS
AD5201
SHDN
LOGIC
CONTROL CLK
+5V
SCLK
SDI
DIN
PWR-ON
PRESET
CS
GND
+4.096V
R3
1kΩ
C2 C2
C2
+5V
V_OUT
FORCE
SENSE
ADR3440
GND ^FORCE
SENSE
5.6V
ZENER DIODE
+5V
1.69kΩ +5V
300Ω
CH 1
1
2
3
4
CH 4
1
2
3
4
1.69kΩ
1.69kΩ
+5V ^300Ω
300Ω
S7
S8
+5V
JP1
1
2
3
1.69kΩ
+5V ^300Ω
1.69kΩ
THERMOCOUPLE:
RTD 2,3W
300Ω
ADG702
S
D
IN
1nF
27nF
1nF
+5V
JP4
1 RTD 4W
2
3
1.69kΩ
+5V ^300Ω
1.69kΩ
+5V
300Ω
1.69kΩ
300Ω
1nF
27nF
1nF
4.02kΩ
0.1%
10ppm
REFIN1(+) REFIN1(0)
P3
P2
AD7193
AIN1
AIN2
AIN3
AIN4
AIN5
AIN6
AIN7
CS
SCLK
DOUT
DIN
AV_DD
AGND
AIN8
REFIN2(+)
DV_DD
DGND
REFIN2(–) ^AINCOM
+5V
INT
CT
V_DD1
GND₁
V_IA
V_IB
V_DD2
GND₂
V_OA
V_OB
+5V_ISO
GND_ISO
INT_ISO
CT_ISO
ADuM1280
AD7193_CS
+5V
SCLK
V_DD2
DOUT
ADT7310_CS
GND₂
V_OA
DIN
ADG738_CS
V_OB
+5V_ISO
V_DD1
GND₁
V_IA
V_IB
GND_ISO
ADT7310_CS _ISO
ADG738_CS _ISO
ADuM1280
+5V
0.1µF
+5V
10µF
+5V
V_ISO
DIN
SCLK
AD7193_CS
DOUT
GND_ISO
V_OA
V_OA
V_OC
V_ID
+5V_ISO
V_DD1
GND₁
V_IA
V_IB
V_IC
V_OD
GND_ISO
DIN_ISO
SCLK_ISO
AD7193_CS _ISO
DOUT_ISO
ADuM5401
Figure 1. 4-Channel Thermocouple and RTD Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)
CIRCUIT DESCRIPTION
Temperature Measurement Introduction
signal generated is typically from several microvolts to tens of
millivolt depending on the temperature difference.
Thermocouples and RTDs (resistance temperature detectors)
are the most frequently used sensors for temperature
measurement in industrial applications. Thermocouples are
able to measure very high temperatures up to about +2300°C
and also have a fast response time (measured in fractions of a
second). RTDs are capable of higher accuracy and stability than
thermocouples, and the resistance of long wire lengths
(hundreds of meters) to a remote RTD can be compensated for
with 3- or 4-wire connections.
A thermocouple consists of two wires of different metals joined
at one end. This end is placed at the temperature which is to be
measured, refered to as the measurement junction. The other
end is connected to a precision voltage measurement unit, and
this connection is referred to as the reference junction or alternately
the cold junction. The temperature difference between the
measurement junction and the cold junction generates a voltage
(known as the Seebeck effect voltage) that is related to the
difference between the temperatures of the two junctions. The
For example, K-type thermocouples are capable of measuring
−200°C to +1350°C with an output range of approximately −10
mV to +60 mV. It is important for the signal chain to maintain
as high impedance and low leakage as possible to achieve the
highest accuracy for the voltage measurement. In order to
convert this voltage to an absolute temperature, the cold junction
temperature must be accurately known. Traditionally 1°C to 2°C
has been considered sufficient, although since the cold junction
measurement error contributes directly to the absolute
temperature error, a higher accuracy cold junction temperature
measurement is beneficial
An RTD is made from a pure material, such as platinum, nickel
or copper, that has a predictable change in resistance as the
temperature changes.The most widely used RTD is platinum
(Pt100 and Pt1000).
One method used to accurately measure the resistance is to
measure the voltage across the RTD generated by a constant
current source. Errors in the current source can be cancelled by
referring the measurement to the voltage generated across a
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