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Chapter 2 Hardware Overv iew of the NI 78x x Multifunction Reconfigurable I/O U ser Manual 2-16 ni.com Common-Mode Signal Re jection Considerations Figure 2-6 and Figure 2-9 show connectio ns for signal sources that are …

Chapter 2 Hardware Overview of the NI 78xx

Single-Ended Connections for Grounded Signal

Sources (NRSE Input Mode)

To measure a grounded signal source with a single-ended input mode, you

must configure the NI 783x/784x/785x in the NRSE input mode. Then

connect the signal to the positive input of the NI 783x/784x/785x

instrumentation amplifier and connect the signal local ground reference to

the negative input of the instrumentation amplifier. The ground point of the

signal should be connected to AISENSE. Any potential difference between

the NI 783x/784x/785x ground and the signal ground appears as a

common-mode signal at both the positive and negative inputs of the

instrumentation amplifier. The instrumentation amplifier rejects this

difference. If the input circuitry of a NI 783x/784x/785x is referenced to

ground in RSE input mode, this difference in ground potentials appears as

an error in the measured voltage.

Figure 2-9 shows how to connect a grounded signal source to a channel on

the NI 783x/784x/785x configured for NRSE input mode.

Figure 2-9. Single-Ended Input Connections for Ground-Referenced Signals

–

I/O Connector

AISENSE

AIGND

AI+

AI–

Instrumentation

Amplifier

Measured

Voltage

Ground-

Referenced

Signal

Source

Common-

Mode

Noise and

Ground

Potential

NRSE Input Mode Selected

Chapter 2 Hardware Overview of the NI 78xx

Multifunction Reconfigurable I/O User Manual 2-16 ni.com

Common-Mode Signal Rejection Considerations

Figure 2-6 and Figure 2-9 show connections for signal sources that

are already referenced to some ground point with respect to the

NI 783x/784x/785x. In these cases, the instrumentation amplifier can reject

any voltage caused by ground potential differences between the signal

source and the device. With differential input connections, the

instrumentation amplifier can reject common-mode noise pickup in the

leads connecting the signal sources to the device. The instrumentation

amplifier can reject common-mode signals when V+

and V–

(input signals) are both within their specified input ranges. Refer to

the Multifunction Reconfigurable I/O Specifications, available at

ni.com/

docs

, for more information about input ranges.

Analog Output

The bipolar output range of the NI 783x/784x/785x AO channels is fixed at

±10 V. Some applications require that the AO channels power on to known

voltage levels. To set the power-on levels, you can configure the

NI 783x/784x/785x to load and run a VI when the system powers on. The

VI can set the AO channels to the desired voltage levels. The VI interprets

data written to the DAC in two’s complement format. Table 2-3 shows the

ideal AO voltage generated for a given input code.

Note If your VI does not set the output value for an AO channel, then the AO channel

voltage output will be undefined.

Table 2-3. Ideal Output Voltage and Input Code Mapping

Output Description AO Voltage

Input 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

Negative full-scale range –10.000000 8000

Any output voltage —

AO Voltage

10.0 V

------------------------------- 32,768×

Chapter 2 Hardware Overview of the NI 78xx

Connecting Analog Output Signals

The AO signals are AO <0..n> and AOGND.

AO <0..n> are the AO channels. AOGND is the ground reference signal for

the AO channels.

Figure 2-10 shows how to make AO connections to the NI 783x/784x/785x.

Figure 2-10. Analog Output Connections

Digital I/O

You can configure the NI 78xx DIO lines individually for either input or

output. When the system powers on, the DIO lines are at high impedance.

To set another power-on state, you can configure the NI 78xx to load a VI

when the system powers on. The VI can then set the DIO lines to any

power-on state.

Connecting Digital I/O Signals

The DIO signals on the NI 78xx RDIO connectors are DGND and

DIO<0..39>. The DIO signals on the NI 783x/784x/785x RMIO connector

are DGND and DIO<0..15>. The DIO<0..n> signals make up the DIO port

and DGND is the ground reference signal for the DIO port. The NI 781x

has four RDIO connectors for a total of 160 DIO lines. The NI 7830 has one

RMIO and one RDIO connector for a total of 56 DIO lines. The

Load

VOUT 0

–

AOGND0

NI 783x/784x/785x

AO0

Channel 0