Measurement Computing DBK Part 2 - Manual (Page 163)

through the use of shielded thermocouples and/or averaging.
You can minimize the effect of noise by (1) using shielded thermocouples,
(2) averaging readings, or (3) employing both of these practices.
To accommodate shielding, grounded connections, labeled “Analog Common,” are provided. A typical
use of the connection would be to attach the shield of a shielded thermocouple.
If a thermocouple shield is connected to the DBK84 module, leave the shield
unconnected at the other end of the thermocouple.
Open Thermocouple Detection
The DBK84 is equipped with open thermocouple detection for each channel. This means that a broken
thermocouple wire [or otherwise unconnected input] that is measured will result in an off-scale reading.
This is accomplished by applying a small bias current to each of the channel inputs. Whenever a valid
input is absent, the bias current saturates the input amplifier, resulting in the off-scale reading. When in
this
“off-scale” state, however, the input amplifier draws more current from the power supply. Specifically, the
power draw of the module from
±15 V will increase by 0.75 mA for each open channel.
If available power is limited, insert shorted TC plugs into unused channels. This will
minimize power consumption. Note that it is not enough to simply avoid scanning unused
channels; to minimize power consumption the channels must be physically shorted in the
hardware.
The power requirements, detailed in the product specification, assume worst case
connection conditions.
DBK Option Cards and Module 989494 DBK84 pg. 3