SNAP Analog Input Modules

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gain.

Isolation

All SNAP analog input modules are isolated from all other

modules and from the SNAP I/O processor. The modules in

this data sheet do not have channel-to-channel isolation,

however. (If you need isolated analog modules, see Opto 22

form #1182.)

Transformer isolation prevents ground loop currents from

flowing between field devices and causing noise that

produces erroneous readings. Ground loop currents are

caused when two grounded field devices share a connection,

and the ground potential at each device is different.

Isolation also protects sensitive control electronics from

industrial field signals.

IMPORTANT: Since these analog input modules provide

multiple single-ended input channels with a common

reference, the channels are not isolated from each other. (See

Opto 22 form #1182 for isolated modules.)

Bipolar and Unipolar Input Modules

Most SNAP analog input modules are considered to be

bipolar, which means the range extends equal amounts above

and below zero. An example of this is the SNAP-AIV module,

which has a range of -10 to +10 VDC.

Some modules are considered unipolar, which means the

range starts or ends at zero. For example, the SNAP-AIVRMS

module has a range of 0 to 250 VAC because AC current

cannot be negative.

Nominal Range and Over-range Limits

All SNAP analog input modules have a nominal range for the

field signal and most support a 10% over-range limit. The

nominal range is the normal range of the field signal for the

module or point configuration. The over-range limit is the

maximum valid field signal the module or point configuration

can read outside of the nominal range. For example, the over-

range limits for the SNAP-AIV are -11 and +11 VDC, and for the

SNAP-AIVRMS, the over-range limit is 275 VAC.

Some modules or point configurations do not support field

signals outside of the nominal range. For example, points

configured as temperature inputs (thermocouple, RTD, ICTD)

do not support over-range readings.

When the field signal is outside of the over-range limits of the

module, the brain will not be able to determine if the value is

too high or too low, so it will return an "out of range" value of -

32768.0

Over-range limits only apply to input modules. Output

modules are limited to their nominal ranges.