Sometimes, an IC will have pins that your circuit doesn’t need to use, or which are marked as Unused or N.C. on the datasheet. Should these pins be left ‘Open Circuit’ in your design?
In general, unused inputs should not be left open circuit. The preferred methods of terminating unused outputs will depend on the unused pin’s functions.
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Digital Circuitry often has inputs that are used to configure its operations, and are permanently connected to Logic 0 or Logic 1. If an unused logic input has internal pull-up or pull-down resistors/currents, it is not necessary to connect this pin, but if the pin is likely to exposed to electrostatic or RF fields, it may be sensible to do so anyways by making an external connection to Vdd or Vss respectively. This reduces noise disruption to the device and will improve its performance.
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Unused analog signal inputs should usually be connected to a DC potential but may sometimes need to be grounded at AC only by a capacitor. Unused pins on CMOS switches and Muxes can pick up the signals from stray electrostatic fields and inject them into the devices. This will degrade the device’s performance, so try to avoid leaving unused pins open on these parts.
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Unused voltage outputs rarely need termination and can often be left floating, but please note that some amplifier outputs (this includes the output of buffered voltage references) may oscillate without capactive or, very occasionally, resistive loading. Such oscillation may disrupt the operation of other parts of the system and must be prevented by the use of an appropriate capacitor or resistor on the output pin.
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Unused current outputs often require a pull-up (or pull-down) connection to a supply or to ground to prevent incorrect operation of other parts of the circuit.
Most importantly, it is necessary to understand the unused pin’s function and finds out what potentials are present on it, what currents may flow to or from it, how sensitive it is to electrostatic or RF interference, and if it requires capacitive or resisitive loading of any sort. The IC’s datasheet can usually help you determine what to do with any pin you aren’t using in your design.