UM-0085-B09
DT80 Range User Manual
Page 373
RG
Avoiding Ground Loops
For each type of ground loop, the basic strategy is to break the ground loop.
Isolation
The design of the DT80 helps eliminate ground loops between the sensors and the DT80/ computer, because the DT80
provides over 100V of
isolation
between the
analog section
(which connects to analog sensors) and the
digital
section
(which connects to digital devices and computers).
The ground point for the analog section is the internal
analog ground
, which is electrically isolated from the "system"
ground (
DGND
terminal). Furthermore, each analog channel is electrically isolated (to 100V) from other channels.
See
for more details.
Connecting Sensors
The following points should be considered:
•
There should normally not be any external connection made between
AGND
/
EXT#
and
DGND
.
•
The sensor's "return" wire should be grounded at one end only (the DT80 end) of the sensor cable. Normally
the return wire would be connected to the -, # or
AGND
/
EXT#
terminal on the DT80.
•
There should be no connection to ground at the sensor itself, unless that connection is isolated from the sensor
return wire. For example, if the sensor has its own power supply then the power supply should be isolated from
ground (e.g. by using a transformer-isolated mains supply)
•
Use an independent input configuration in preference to a shared configuration.
Noise Pickup
There are two main ways in which noise can be introduced into signal wiring: by capacitive coupling and by magnetic
induction. There are different counter-measures for each.
Shield signal wiring to minimize capacitive noise pick-up. Signal wiring that is close to line voltage cable should always
be shielded.
Shields should be connected to system ground (
DGND
) at the DT80 end only.
Magnetic induction of noise from current-carrying cables or from electrical machines (especially motors and
transformers) is a greater problem. Shielded cable is not an effective counter-measure. The only practical measures are
to
•
avoid magnetic fields
•
use close-twisted conductors for the signal wiring.
Shielding in steel pipe can be effective, but is generally not economic or convenient.
Noise Rejection
The DT80 is designed to reject mains noise. For best noise rejection, set the DT80’s parameter 11 to your local mains
frequency, 50Hz or 60Hz — see
P11
To force the DT80 to load this parameter setting every time it restarts use the following command
PROFILE PARAMETERS P11=60
'for 60Hz line frequency
Self-Heating of Sensors
Sensors that need excitation power to be read are heated by power dissipation. This issue can be particularly acute with
temperature sensors and some sensitive bridges. If self-heating is a problem, consider:
•
selecting 200µA excitation (
I
channel option) in preference to 2.5mA excitation (the trade-off is a reduction in
the range of resistances that can be measured). Series 4 models provides an additional ~2uA internal precise
current source as an option.
•
reducing measurement time, e.g. setting
P11=200
will reduce the time spent exciting the sensor from the
default 20ms to 5ms (the trade-off is a reduction in mains noise rejection).
