Analog Devices AN528 Application Notes

AN-528

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APPLICATION NOTE

Designing a 5B Series System

Design Considerations

FEATURES
Module Pins Fit Widely Available Sockets
Single Threaded Insert for Module Hold-Down
Cold Junction Compensation Sensors Available—Model

AC1361

Input Modules Have Internal Series Output Switches

OVERVIEW

The 5B Series was designed to facilitate integration by a
system designer into a circuit board or backplane. Only a
single 3.0 mm threaded insert is required for module
hold-down. Module pins are accommodated by widely
available sockets, and a temperature sensor for thermo­couple cold junction compensation is available as a one­piece precalibrated unit.

The 5B Series was also designed to minimize system in­terface space and cost. Each input module has an inter­nal series output switch controlled by a TTL-compatible
enable signal eliminating the need for external multi­plexers so all modules can be served by a single input
bus. Each output module has a track-and-hold that al­lows a single digital-to-analog converter to serve numer­ous channels. Alternatively, the module enable lines can
be grounded, the 5B Series input modules can be used
with a conventional mux and the output modules with
one DAC per channel.

Ease of system application of these modules is en­hanced by the fact that modules have enable and signal
input pin assignments that do not coincide with the en­able and output pins of the input modules. This means
that in a single mix-and-match backplane environment
the reading of inputs and the writing and refreshing of
outputs are completely independent and can occur si­multaneously. For example, the input system may dwell
for a long time on a single channel to collect thousands
of samples without having to interrupt the process to do
an output refresh or set a new output value. Similarly, a
“dumb” refresh circuit can be built that can maintain
outputs without even knowing which channels have out­put modules; it can refresh all channels, and those that
are inputs will ignore the operation.

BASIC DESIGN GUIDELINES

Modules may be mounted in any position and will nor­mally be placed next to the screw terminals connecting
to the associated field wiring. The temperature sensor is
only used by thermocouple modules, but it is normally
installed in all channel locations to enable “mix-and­match” applications. This sensor must be physically
close to the terminals where the thermocouple wire con­nects to copper. Because the low power dissipation of
the 5B Series minimizes temperature gradients on the
backplane, no special precautions are needed to obtain
accurate temperature sensing. Provision must also be
made on each channel for the 5B32’s current conversion
resistor.

The width of the modules is intended to permit installa­tion on 0.6" centers where required, but consideration
must be given in each application to the required dis­tance between backplane conductors where large
interchannel voltages exist or where code requirements
apply. The nature of the screw terminals used for field
wiring will also be a factor in determining practical
interchannel spacing.

The

5B Series User’s Manual

cussion of system design issues. This manual includes
the design of backplanes that take full advantage of the
5B Series’ capabilities by maintaining 1500 V rms isola­tion between each channel (channel-to-channel isola­tion) as well as 1500 V rms isolation between each input
to output (input-to-output isolation).

includes an extensive dis-

E3278–5–3/98

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