User`s manual

18 Operation Theory
4.1 A/D Conversion
When using an A/D converter, users should know the properties of
the signal to be measured. In addition, users should setup the A/D
configura-tions, including scan channels, input range, and polari-
ties.
The A/D acquisition is initiated by a trigger signal. The data acqui-
sition will start once the trigger signal matches the trigger condi-
tions. Converted data are queued into the FIFO buffer, and then
transferred to the host PC's memory for further processing.
Two acquisition modes: Software Polling and Programmable Scan
are described in the following sections, including the timing, trigger
modes, trigger sources, and transfer methods.
DAQ/PXI-2500 series AD Data Format
The data format of the acquired 14-bit A/D data is 2’s Complement
coding. Table 4-1 and 4-2 lists the valid input ranges and the ideal
transfer characteristics.
Magnitude Bipolar Input Range Digital code
FSR ±10V ±5V ±2.5V ±1.25V
LSB 1120.78uV 610.39uV 305.19uV 152.60uV
FSR-1LSB 9.998779V 4.999389V 2.499694V 1.249847V 1FFF
Midscale + LSB 1120.78uV 610.39uV 305.19uV 152.60uV 0001
Midscale 0V 0V 0V 0V 0000
Midscale - LSB -1120.78uV -610.39uV -305.19uV -152.60uV 3FFF
-FSR -10V -5V -2.5V -1.25V 2000
Table 4-1: Bipolar Input Range and Converted Digital Codes
Magnitude Unipolar Input Range Digital code
FSR 0V ~ 10V 0 ~ +5V 0 ~ +2.5V 0 ~ +1.25V
LSB 610.39uV 305.19uV 152.60uV 76.3uV
FSR - LSB 4.999389V 2.499694V 1.249847V 1.249923V 1FFF
Midscale + LSB 5.000611V 2.500306V 1.250153V 0.625076V 0001
Midscale 5V 2.5V 1.25V 0.625V 0000
Table 4-2: Unipolar Input Range and Converted Digital Codes