ADLINK PXI-2006 - Manual (Page 81)

Calibration 71
5 Calibration
This chapter introduces the calibration process to minimize AD
meas-urement errors and DA output errors.
5.1 Loading Calibration Constants
The DAQ/PXI-20XX is factory calibrated before shipment by writ-
ing the associated calibration constants of TrimDACs to the on-
board EEPROM. TrimDACs are devices containing multiple DACs
within a single package. TrimDACs do not have memory capabil-
ity. That means the calibration constants do not retain their values
after the system power is turned off. Loading calibration constants
is the process of loading the values of TrimDACs stored in the on-
board EEPROM. ADLINK provides software to make it easy to
read the calibration constants automatically when neces-sary.
There is a dedicated space for calibration constants In the
EEPROM. In addition to the default bank of factory calibration con-
stants, there are three extra user-modifiable banks. This means
users can load the TrimDACs values either from the original fac-
tory calibration or from a calibration that is subsequently per-
formed.
Because of the fact that errors in measurements and outputs will
vary with time and temperature, it is recommended re-calibratation
when the card is installed in the users environment. The auto-cali-
bration function used to minimize errors will be introduced in the
next sub-section.
5.2 Auto-calibration
By using the auto-calibration feature of the DAQ/PXI-20XX, the
calibration software can measure and correct almost all the cali-
bration errors without any external signal connections, reference
voltages, or measurement de-vices.
The DAQ/PXI-20XX has an on-board calibration reference to
ensure the accuracy of auto-calibration. The reference voltage is
measured at the factory and adjusted through a digital potentiome-
ter by using an ul-tra-precision calibrator. The impedance of the
digital potentiometer is memorized after this adjustment. It is not