Measurement Computing DBK81 - Manual (Page 3)

It should be noted that thermocouples output very small voltages and that long thermocouple leads can
pickup a large amount of noise. However, the DBK81, DBK82, and DBK83 inherently provide a high
level of noise immunity via their 4 Hz signal bandwidth and input filtering. If desired, further noise
reduction can be achieved 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 “SHIELD,” are provided. A typical use of the
connection would be the attachment of the shield to a shielded thermocouple.
If a thermocouple shield is connected on the DBK card, leave the shield
unconnected at the other end of the thermocouple.
Open Thermocouple Detection
The DBK81, DBK82, and DBK83 are 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 a card from
±15 V will increase by 0.75 mA for each open
channel.
If available power is limited, short unused channels by connecting a short length of wire
between the H and L terminals. 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.
Installing the DBK82 in the DBK41 Enclosure
Because of its physical size, the DBK82 will not fit into 1-slot enclosures such as the DBK10 or
DaqBook/216. It does fit, however, in the DBK41 enclosure, and in “drawer-type” products, such as the
DaqBook/260.
Installation of the DBK82 is possible in DBK41 connectors CN3, CN5, CN7, and CN9. The connector
labels are visible near the upper edge of the DBK41’s printed circuit board, as indicated in the following
figure.
DBK41’s Printed Circuit Board
DBK82 cards can be connected to CN3, CN5, CN7, and CN9.
DBK Option Cards and Modules 989494 DBK81, DBK82, and DBK83 pg. 3