User`s manual

28 Operation Theory
under heavy loading. Detailed function setup will be explained
in Section 4.2.2.
Note: When using waveform generation mode, all the four DACs in
the same D/A group must be configured for the same mode.
However, any one of the DAC can be disabled. If users need
to use the software update mode, they can use another D/A
group on the PXI/DAQ-2502.
Setting up the DACs
Before using the DACs, users should setup the reference
source and its polarity. Each DAC has its own reference and
polarity settings. For ex-ample; the internal voltage reference of
D/A Group A is tied to internal +10V, however, users can still
connect external reference thru AOEXTREF (pin 5 on CN2), for
example to a +3.3V voltage source. Therefore, each DAC in D/
A Group A has two reference options: 10V or 3.3V. However,
DA update timing, trigger Source, and trigger/stop mode are all
the same throughout that D/A Group.
DAQ/PXI-2500 SERIES provides the capability to fine tune the
voltage reference from the external source. The external refer-
ence is fed thru an on board calibrated circuit, with programma-
ble offset. Users can utilize this capability to generate precise
D/A outputs.
CAUTION: The range of external voltage reference should be within
±10V.
Utilizing Multiplying Characteristic of DACs
The D/A reference selection let users fully utilize the multiplying
charac-teristics of the DACs. Digital codes sent to the D/A con-
verters will be multiplied by the reference to generate output.
Magnitude
Bipolar Unipolar
Output Output Digital Code
FSR – LSB +Vref * (2046 / 2048) Vref * (4095 / 4096) 0FFF
Midscale + LSB +Vref * (1 / 2048) Vref * (2049 4096) 0801
Midscale 0 Vref * (2048 / 4096) 0800
Midscale – LSB -Vref * ( 1 / 2048) Vref * (2047 / 4096) 07FF
Table 4-5: D/A Output Versus Digital Codes