Analog Input Circuitry - National Instruments DAQCard-1200 User Manual

Chapter 4 Theory of Operation
ACH0
ACH1
ACH2
ACH3
ACH4
Mux
ACH5
ACH6
ACH7
AISENSE/
AIGND
EXTTRIG
EXTCONV*
OUTB1

Analog Input Circuitry

DAQCard-1200 User Manual
Pro-
Sample-
grammable
and-Hold
Gain Amp
Amp
GAIN0
GAIN1
GAIN2
6
Dither
Circuitry
Dither
External Trigger
External Convert
Output B1
The analog input circuitry consists of two analog input multiplexers, mux
counter/gain select circuitry, a software-programmable gain amplifier, a
12-bit ADC, and a 12-bit FIFO memory that is sign-extended to 16 bits.
The two input multiplexers route the input channels to the instrumentation
amplifier in either RSE, NRSE, or DIFF mode. The input multiplexers
provide input overvoltage protection of ±35 V powered on and ±25 V
powered off.
The mux counters control the input multiplexers. The DAQCard-1200
can perform either single-channel data acquisition or multiple-channel
(scanned) data acquisition. These two modes are software selectable.
For single-channel data acquisition, you select the channel and gain before
initiating data acquisition. These gain and multiplexer settings remain
constant during the entire data acquisition process. For multiple-channel
data acquisition, you select the highest numbered channel and gain before
data acquisition is initiated. Then the mux counter decrements from the
highest numbered channel to channel 0 and repeats the process. Thus you
A/D
Data
12-Bit
Sample
ADC
12
Gain Select/
Mux Counter
Dither Enable
DAQ
Counter/Timer
Timing
Signals
Figure 4-4. Analog Input Circuitry
4-6
Data
1k
12
A/D
FIFO
Data
CON
V
Data
8
8
WRT/RD
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