4-10
Cabling and Wiring
Note:
Bias currents are very small but finite currents drawn from the
input terminals of an amplifier. The magnitude of a bias current can range
from a few femtoamperes to a few microamperes, depending on amplifier
design.
Therefore, a resistance path must be defined for the bias currents via a
common mode connection. It must be defined either externally by the user
or internally on the data acquisition board, as follows:
●
Your DAS-TC/B board provides 10k
Ω
resistance paths internally.
The 10k
Ω
resistors are appropriate when the source resistance of the
signal is low—as with thermocouples—and the signal is isolated.
These resistors are connected between the analog signal ground
(B GND) and each of the sixteen Low inputs. See Figure 4-6.
●
However, when the signal source is grounded, the internal 10k
Ω
resistor should be overridden by an external low-resistance path from
the source ground to the analog signal ground (B GND). Refer to the
following sections “Connecting a Grounded Thermocouple” on page
4-12 and “Connecting a Grounded Signal Source” on page 4-14.
Connecting an Ungrounded Thermocouple
In many applications, either the junction of a thermocouple is electrically
isolated from the object being monitored for temperature or the object
being monitored is isolated from ground. If any of your thermocouples are
completely isolated, connect them as shown in Figure 4-7.
