Analog Input Circuitry - National Instruments DAQ Series User Manual

Instrumentation
Amplifier
Input
Mux1 Dual
8
(8-Channel
Single-
8
Ended)
Scanning
Counter

Analog Input Circuitry

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Buffer
Input
Mux2 Sampling
(4-Channel)
A/D Timing
EXTCONV*
Figure 4-3. Analog Input and DAQ Circuitry Block Diagram
The AI circuitry consists of an input multiplexer, a software-selectable gain
stage, and a 12-bit sampling ADC. The 12-bit output is sign-extended to
16 bits, then stored in a FIFO memory that is 512 words deep.
The input multiplexer stage is made up of two CMOS AI multiplexers. In
single-ended mode, the input multiplexers switch between 16 AI channels
(channels 0 through 15). In DIFF input mode, one of the input multiplexers
switches between eight differential pairs (channels 0 and 8, 1 and 9, and so
on). With the input multiplexer stage, input overvoltage protection of
± 30 V is available, powered on or off.
The DAQCard-700 uses a 12-bit successive-approximation ADC.
Software-selectable gains of 1, 0.5, and 0.25 for the input signal combined
with the ADC fixed input range of ± 10 V yield three useful AI signal
ranges. These ranges are ±10 V, ± 5 V, and ± 2.5 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 512 words deep. This FIFO
12
512-Word
12-Bit
FIFO,
A/D
Sign
Data
ADC
Extension
A/D RD
OUT0
MSM82C54
CLK0
4-5
Chapter 4 Theory of Operation
16
PCMCIA
Data
I/O Channel
Interface
Interrupt
Interface
1 MHz
DAQCard-700 User Manual