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UNUSED PINS

Editor’s note: In this latest edition of Analog Devices popular Rarely Asked Questions the esteemed James Bryant discusses another key factor in problems encountered in analog design.

Q : If grounding is one of the commonest causes of analog problems what is the next most important?

A : The misconnection of unused IC terminals.

I spent the weekend before writing this RAQ at the Oktoberfest in Munich1 , the original Oktoberfest and the largest in the World. I did not go just for the beer, although Bavarian beer is extremely good, but to meet a group of German friends and (ex-) colleagues. Most of us were, or had been, analog applications specialists and the conversation over the beer and brass bands covered the sillier problems we had seen during our careers. A surprising number involved incorrect treatment of unused IC terminals, so although the topic was covered in RAQs 31 , 461 , 701 , and 1195 , I think it needs to be revisited – just like the Oktoberfest.

One problem arises from the lack of standardization on data sheets, even from the same manufacturer. It is common for an IC to have one or more terminals which are marked “NC” on the pinout diagram. This can mean two different things, and very often the data sheet does not explain which. The more common meaning is that there is no connection from the IC chip to that pin. In this case it is best to ground it, but in most cases it will do no harm if another track running in that part of the PCB is connected to it instead.

But it is by no means unusual for “NC” to mean that the terminal carries an internal connection which is used during manufacture for calibration or test but must not be connected to anything during normal use. If a terminal is labelled “IC” (Internal Connection) this is almost invariably the case. Making a connection to such pins may cause incorrect operation or even destroy the device. If there is no indication on the data sheet which meaning of “NC” applies such pins should be left unconnected – or the manufacturer's product engineers consulted for clarification.

In general unused inputs should not be left open-circuit:

  • Digital circuitry often has inputs which are used to configure its operation and are permanently connected to logic 0 or logic 1. Although such inputs never change their logical state they are not “unused” and must be connected as the device operation requires. Real “unused” logic inputs are ones whose logic state does not affect the particular application required – these should be connected to fixed logic 0 or logic 1 potentials and not left floating6 . If an unused logic input has internal pull-up or pull-down resistors/currents it is not necessary to connect to the pin, but if the pin is likely to be exposed to electrostatic or RF fields it may be sensible to do so by making an external connection to Vdd or Vss respectively7.
  • 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 – RTFDS8 . In many cases this DC potential may be a power supply or ground, but for reasons which are discussed in more detail in RAQ 463 amplifier inputs often need different treatment. RAQ 32 explains why unused analog signal connections to analog switches and multiplexers must never be left floating but should always be connected to ground or some other convenient potential.

Unused voltage outputs rarely need termination and can usually be left floating, but some amplifier outputs (and this includes the outputs of buffered voltage references) may oscillate without capacitive, 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.

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. (Kirchoff's Law says, in effect, that a current always has to go somewhere – it cannot just disappear!) This pull-up/down will generally be a short circuit but may sometimes need to be a resistor so that power dissipation takes place in the resistor and not in the IC chip.

The real message from all the above advice is that even if a particular terminal of an IC is unused in an application it is still necessary to understand its function, what potentials are (or should be) present on it, what currents may (or must ) flow to/from it, how sensitive it is to electrostatic or RF influences, and if it requires capacitive or resistive loading of any sort. In other words, as we have so often said before, “Read the friendly Data Sheet8” – and do what it tells you.

References

  1. Welcome to Oktoberfest 2016
  2. There is a Fault in my CMOS Multiplexer….?
  3. In the RAQ on op-amps as comparators you mentioned an unused op-amp in a quad as being possible justification for using it as a comparator. What do you do if you’ve an unused op-amp and don’t need a comparator?
  4. What should I do with the unused terminals of analog ICs?
  5. The Ingenious Gentleman and the Mysterious Paddle
  6. Very occasionally digital circuits have “tri-state” inputs where the three states are logic 0, logic 1, and open-circuit. In such cases it may be necessary to leave an unused pin open-circuit, but if at all possible it is better not to RTFDS.
  7. It is permissible to pull up a terminal with internal pull-down, or vice versa, and may sometimes be necessary, but this will (very slightly) increase the system's current consumption.
  8. RTFDS – Read The Friendly Data Sheet. RAQ 4

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