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Chapter 2 Hardware Overview of the NI 78x x © National Instruments Corporation 2-5 Mult ifunction Reconfigurable I/O User Manual Input Modes The NI 783 x /784 x /785 x inp ut mode is software configurable. The input chan…
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-4 ni.com
NI 7831/7833 Overview
The NI 7831/7833 each have eight independent, 16-bit AI channels; eight
independent, 16-bit AO channels; 96 bidirectional DIO lines that you can
configure individually for input or output; and a Virtex-II XC2V3000
FPGA.
NI 784x Overview
The NI 784x each have eight independent, 16-bit AI channels;
eight independent, 16-bit AO channels; and 96 bidirectional DIO lines that
you can configure individually for input or output. The PXI-7841 has a
Virtex-5 LX30 FPGA, and the PXI-7842 has a Virtex-5 LX50 FPGA.
NI 785x Overview
The NI 785x each have eight independent, 16-bit AI channels;
eight independent, 16-bit AO channels; and 96 bidirectional DIO lines that
you can configure individually for input or output. The PXI-7851 has a
Virtex-5 LX30 FPGA, the PXI-7852 has a Virtex-5 LX50 FPGA,
the PXI-7853 has a Virtex-5 LX85 FPGA, and the PXI-7854 has a Virtex-5
LX110 FPGA.
Analog Input (NI 783x /784x /785x Only)
You can sample NI 783x/784x/785x AI channels simultaneously or at
different rates. The input mode is software configurable, and the input
range is fixed at ±10 V. The converters return data in two’s complement
format. Table 2-1 shows the ideal output code returned for a given AI
voltage.
Table 2-1. Ideal Output Code and AI Voltage Mapping
Input Description AI Voltage
Output Code (Hex)
(Two’s Complement)
Full-scale range –1 LSB 9.999695 7FFF
Full-scale range –2 LSB 9.999390 7FFE
Midscale 0.000000 0000
Negative full-scale range +1 LSB –9.999695 8001
Chapter 2 Hardware Overview of the NI 78xx
© National Instruments Corporation 2-5 Multifunction Reconfigurable I/O User Manual
Input Modes
The NI 783x/784x/785x input mode is software configurable. The input
channels support three input modes—differential (DIFF), referenced single
ended (RSE), and nonreferenced single ended (NRSE). The selected input
mode applies to all the input channels. Table 2-2 describes the three input
modes.
Input Range
The NI 783x/784x/785x AI range is fixed at ±10 V.
Connecting Analog Input Signals
The AI signals for the NI 783x/784x/785x are AI<0..n>+, AI<0..n>–,
AIGND, and AISENSE. For the NI 7830, n=4. For the NI 7831/7833/
784x/785x, n=8. The AI<0..n>+ and AI<0..n>– signals are connected to the
Negative full-scale range –10.000000 8000
Any input voltage —
Table 2-2. Available Input Modes for the NI 783x/784x/785x
Input Mode Description
DIFF When the NI 783x/784x/785x is configured in DIFF input mode, each channel
uses two AI lines. The positive input pin connects to the positive terminal of the
onboard instrumentation amplifier. The negative input pin connects to the
negative input of the instrumentation amplifier.
RSE When the NI 783x/784x/785x is configured in RSE input mode, each channel
uses only its positive AI pin. This pin connects to the positive terminal of the
onboard instrumentation amplifier. The negative input of the instrumentation
amplifier connects internally to the AI ground (AIGND).
NRSE When the NI 783x/784x/785x is configured in NRSE input mode, each channel
uses only its positive AI pin. This pin connects to the positive terminal of the
onboard instrumentation amplifier. The negative input of the instrumentation
amplifier on each AI channel connects internally to the AISENSE input pin.
Table 2-1. Ideal Output Code and AI Voltage Mapping (Continued)
Input Description AI Voltage
Output Code (Hex)
(Two’s Complement)
Output Code
32,768
----------------------------------
10.0 V×
Chapter 2 Hardware Overview of the NI 78xx
Multifunction Reconfigurable I/O User Manual 2-6 ni.com
eight AI channels of the NI 783x/784x/785x. For all input modes, the
AI<0..n>+ signals are connected to the positive input of the
instrumentation amplifier on each channel. The signal connected to the
negative input of the instrumentation amplifier depends on how you
configure the input mode of the device.
In differential input mode, signals connected to AI<0..n>– are routed to the
negative input of the instrumentation amplifier for each channel. In RSE
input mode, the negative input of the instrumentation amplifier for each
channel is internally connected to AIGND. In NRSE input mode, the
AISENSE signal is connected internally to the negative input of the
instrumentation amplifier for each channel. In DIFF and RSE input modes,
AISENSE is not used.
Caution Exceeding the differential and common-mode input ranges distorts the
input signals. Exceeding the maximum input voltage rating can damage the
NI 783x/784x/785x and the computer. NI is not liable for any damage resulting from such
signal connections. The maximum input voltage ratings are listed in Table A-2, NI 78xx
I/O Signal Summary.
AIGND is a common AI signal that is routed directly to the ground tie point
on the NI 783x/784x/785x. You can use this signal for a general analog
ground tie point to the device if necessary.