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Chapter 4
Theory of Operation
© National Instruments Corporation
4-3
DAQCard-500 User Manual
When you first insert the card, the system examines information stored in the DAQCard-500
Card Information Structure. This data is used to configure the card appropriately for the system
in which it is used. When the system has assigned the card to a section of memory, it updates the
PCMCIA control registers and initializes the card.
Analog Input and Data Acquisition Circuitry
The DAQCard-500 has 8 channels of analog input with 12-bit A/D conversion. Using the timing
circuitry, the DAQCard-500 can also automatically time multiple A/D conversions. Figure 4-3
shows a block diagram of the analog input and data acquisition circuitry.
1 MHz
A/D
Data
EXTCONV*
16
Data
A/D RD
DAVAIL
PCMCIA
I/O Channel
OUT0
Buffer
12-Bit
Sampling
ADC
PCMCIA
I/O
Channel
Interface
I/O Connector
Scanning Counter
3
A/D Timing
MSM82C54
Interrupt
Interface
1 MHz
Input
Mux
8-Channel
Single-
Ended
12
8
16-Word
FIFO,
Sign
Extension
Figure 4-3. Analog Input and Data Acquisition Circuitry Block Diagram
Analog Input Circuitry
The analog input circuitry consists of an input multiplexer, a buffer, and a 12-bit sampling ADC.
The 12-bit output is sign-extended to 16 bits, then stored in a 16-word-deep FIFO memory.
The input multiplexer is an 8-channel CMOS analog input multiplexer. The input multiplexer
selects one of 8 analog input channels (channels 0 through 7). With the input multiplexer stage,
input overvoltage protection of ± 25 V is available, powered on or off.
The DAQCard-500 uses a 12-bit successive-approximation ADC. The ADC has a fixed input
range of ±5 V.
When an A/D conversion is complete, the ADC clocks the result into the A/D FIFO. The A/D
FIFO is 16 bits wide and 16 words deep. This FIFO serves as a buffer to the ADC and has two