4-12
Cabling and Wiring
Connecting a Grounded Thermocouple
A thermocouple junction is sometimes joined to an object that is directly
or indirectly grounded, usually to optimize heat transfer and response
time. In such cases, connect the ground of the object being monitored
directly to the analog ground (B GND), as shown in Figure 4-8.
Figure 4-8. Connecting a Grounded Thermocouple
This external low-resistance common-mode connection essentially
overrides the internal 10k
Ω
resistor of the DAS-TC/B.
Note:
Connect the analog ground (B GND) to the ground of the
grounded object. Do not connect analog ground (B GND) to the Low
input connection at the board. Connecting the analog ground (B GND) to
the Low input at the board—or allowing the internal 10k
Ω
resistor to
make such a connection by default—causes any noise current between the
grounded object and the analog ground to flow across the Low signal
lead, resulting in a noise voltage across the lead. The input amplifier
accepts this noise voltage as part of the Low input signal, instead of
rejecting the noise voltage as a common-mode signal.
Thermocouple
Channel n High
Channel n Low
Optional Shield
Es
STA-TC/B
or
STP-TC/B
Chassis
GND
Input
Amplifier
(To chassis via S-1800
cable for STA-TC/B)
B GND
Grounded
Object
i
R wire
V
g1
-
V
g1
V
g2
V =
cm
V
g2
