XT2640 Operating Manual
13 July 2016
Page 70 of 187
13
OPTIMIZING
LOW
‐
LEVEL
DC
PERFORMANCE
If you are only using measurements configured as AC (i.e. not DC or AC+DC) or the AC levels are significantly higher than any
expected DC levels (e.g. more than 30:1 higher) then low level DC performance is not required and this section can be ignored.
13.1
INTERNAL
DC
ZERO
ADJUSTMENT
The XT2640 automatically checks the internal DC zeroes if the environment has significantly changed and then applies any changes
as needed. The automatic tracking of environmental changes can be disabled by selecting OFF for the AUTOZERO setting in the
PREFERENCES screen, in which case the user should occasionally perform this Internal DC Zero operation manually.
This is achieved by pressing the SYS CONFIG button from any main screen and then pressing the INTERNAL button in the DC ZERO
area. The XT2640 will then perform an Internal DC Zero operation and save the adjustments recorded. A screen will show the
progress of the operation and will wait for you to press the RETURN button on that screen when the action has been successfully
completed.
Recommendations:
If AUTOZERO has been set to OFF then it is recommended to perform this Internal DC Zero operation at least daily, or
whenever the environment has changed by more than ±2C from that in which it was last performed.
If AUTOZERO has been set to ON but the XT2640 is in a significantly different environment that in which it was calibrated
then there will be a few minutes after turning on the XT2640 while the DC zeroes are tracking that change in environment.
To reduce this small shift during the first few minutes of operation you may wish to perform an Internal DC Zero operation
in the normally used environment, otherwise there is no need to perform this operation.
Note:
This operation takes a few seconds and does not require that signals be removed from the XT2640 terminals. Note that
this operation only affects channels which are configured for use.
While performing the Internal DC Zero you should not send any configuration interface commands to the XT2640.
13.2
EXTERNAL
DC
ZERO
ADJUSTMENT
If you are using external current transducers or shunts which may have a significant DC offset in their output you can perform an
External DC Zero operation to set those offsets to the presently measured values. This external DC zero operation can also be used to
account for any external DC offsets (leakage currents or thermally induced voltages) or to adjust any remaining offsets in the XT2640
caused by adverse environments (e.g. nearby heat sources and/or unusual orientation of the XT2640).
This differs from the Internal DC Zero described above because it corrects for any externally applied DC offset, so you must ensure
that there are no DC signals present on the terminals of the XT2640 which are not wished to be adjusted for.
This is achieved by pressing the SYS CONFIG button from any main screen and then pressing the EXTERNAL button in the DC ZERO
area. You are then shown a screen which requests that you select whether to adjust the DC offset for all channels (ALL), or only
those configured for scaling (ONLY SCALED), or to abort the action (NO). After you select the desired choice the XT2640 will
immediately use the latest obtained DC measurements and apply them as DC offsets. If no channels were affected by this operation
then a message is displayed for a short time. If the measured DC values were too large to be adjusted then a message is displayed for
a short time and no adjustments are made.
Recommendation:
If it is desired to use this external DC zero ability when AUTOZERO is set to OFF then it is recommended to first perform an
internal DC zero operation and ensure that valid measurement results are available by returning to any of the
measurement results screens and checking that the readings being indicated are those expected before returning to the SYS
CONFIG screen and performing the external DC zero operation.
Note:
This takes very little time and has an immediate effect. Channels which are not configured are not affected by this action.
13.3
USING
AN
X
CURRENT
OPTION
INPUT
AT
LOW
LEVELS
X option current inputs have a resolution of 1µV or lower when used with an external current shunt. To achieve this level of
performance the user must be aware of thermally induced DC voltages which can occur both within the shunt itself and in the wiring
between the shunt and the terminals of the XT2640. The user can reduce these effects by using high quality wiring and connectors,
and balancing the two sense wires by using the same gages and lengths, using the same connector types, and routing the two wires
together. In severe circumstances this can also be affected by air currents passing over the shunt or the wiring, so you may need to
take precautions to prevent unwanted airflow over them.
Recommendations:
Twisting together the two sense wires from the shunt to the XT2640 terminals both reduces these thermal voltage affects
and also reduces the inductive pick‐up of AC current in these wires, so is highly recommended. You should also note that
the use of thinner gage wiring (e.g. 26awg or thinner) will produce the best thermal voltage performance as thinner wiring
does not allow significant heat transfer through it.
The use of coaxial cable is not recommended as often the two conductors are not of the same material.
Even when all precautions are taken, there will typically be several micro‐volts of thermally induced offset voltages. The
regular use of the External DC Zero capability described above is highly recommended when using shunts.
