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Chapter 2 Hardware Overv iew of the NI 78x x Multifunction Reconfigur able I/O User Manual 2-6 ni.com eight AI channels of the NI 783 x /784 x /785 x . For all input modes, the AI<0.. n >+ signals are connected to …

Chapter 2 Hardware Overview of the NI 78xx

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.

Chapter 2 Hardware Overview of the NI 78xx

Connection of AI signals to the NI 783x/784x/785x depends on the input

mode of the AI channels you are using and the type of input signal source.

With different input modes, you can use the instrumentation amplifier in

different ways. Figure 2-4 shows a diagram of the device instrumentation

amplifier.

Figure 2-4. NI 783x/784x/785x Instrumentation Amplifier

The instrumentation amplifier applies common-mode voltage rejection

and presents high input impedance to the AI signals connected to the

NI 783x/784x/785x. Input multiplexers on the device route signals to the

positive and negative inputs of the instrumentation amplifier. The

instrumentation amplifier converts two input signals to a signal that is the

difference between the two input signals. The amplifier output voltage is

referenced to the device ground. The device ADC measures this output

voltage when it performs A/D conversions.

You must reference all signals to ground either at the source device or at the

NI 783x/784x/785x. If you have a floating source, reference the signal to

ground by using RSE input mode or the DIFF input mode with bias

resistors. Refer to the Differential Connections for Nonreferenced or

Floating Signal Sources section of this chapter for more information about

these input modes. If you have a grounded source, do not reference the

signal to AIGND. You can avoid this reference by using DIFF or NRSE

input modes.

–

= [V

in+

– V

in–

]

in+

in–

Instrumentation

Amplifier

Measured

Voltage