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Theory of Operation Chapter 3
Lab-NB User Manual 3-8 © National Instruments Corporation
DAC0
DAC1
DAC Configuration
Register
Ref
Coding
Coding
Ref
DAC0 OUT
AGND
DAC1 OUT
EXTUPDATE*
5 V Internal
Reference
DAC0WR
Data
DAC1WR
Counter
A2
CNFGWR
TWOSDA0
TMRWGN1
TWOSDA1
TMRWGN0
TWOSDA0
TWOSDA1
12
I/O Connector
NuBus Interface
Figure 3-4. Analog Output Circuitry Block Diagram
Each analog output channel contains a 12-bit DAC. The DAC in each analog output channel
generates a voltage proportional to the input voltage reference (V
ref
) multiplied by the digital
code loaded into the DAC. Each DAC can be loaded with a 12-bit digital code by writing to the
DAC0 and DAC1 Registers on the Lab-NB board. The voltage output from the two DACs is
available at the Lab-NB I/O connector DAC0 OUT and DAC1 OUT pins.
The DAC voltages can be updated in any of three ways, depending on the setting of the
TMRWGN bit. If this bit is cleared, the DAC output voltage is updated as soon as the
corresponding DAC Data Register is written to. If the TMRWGN bit is set, the DAC output
voltage does not change until a falling edge is detected either from counter A2 or from
EXTUPDATE*.
Each DAC channel can be jumper-programmed for either a unipolar voltage output or a bipolar
voltage output range. A unipolar output gives an output voltage range of 0.0000 V to +9.9976 V.
A bipolar output gives an output voltage range of -5.0000 V
to +4.9976 V. For unipolar output,
0.0000 V output corresponds to a digital code word of 0. For bipolar output, -5.0000 V output
corresponds to a digital code word of F800 (hex). One LSB is the voltage increment