what to do with unused open drain output
Unused Pins
QUESTION:
If grounding is ane of the most common causes of analog problems, what is the next most important?
Answer:
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 earth. I did not get just for the beer, although Bavarian beer is extremely good, but to meet a group of German friends and (ex-)colleagues. Most of the states were, or had been, analog applications specialists and the conversation over the beer and brass bands covered the sillier issues we had seen during our careers. A surprising number involved wrong treatment of unused IC terminals, and then although the topic was covered in RAQs 3two, 46three, 704, and 1195, I think information technology needs to be revisited—simply similar the Oktoberfest.
One problem arises from the lack of standardization on data sheets, fifty-fifty from the aforementioned manufacturer. It is mutual for an IC to take one or more terminals that are marked "NC" on the pinout diagram. This can mean ii different things, and very ofttimes the data sheet does non explicate which. The more common meaning is that there is no connectedness from the IC chip to that pin. In this case it is best to ground it, merely in most cases information technology will exercise no harm if another runway running in that part of the PCB is continued to it instead.
But it is by no means unusual for NC to mean that the terminal carries an internal connectedness that is used during industry for calibration or test but must not exist connected to anything during normal use. If a last is labeled "IC" (internal connexion) this is about invariably the case. Making a connection to such pins may cause incorrect functioning or fifty-fifty destroy the device. If at that place is no indication on the data sheet which meaning of NC applies, such pins should be left unconnected—or the manufacturer's production engineers consulted for description.
In general, unused inputs should not be left open circuit:
- Digital circuitry often has inputs that are used to configure its operation and are permanently connected to Logic 0 or Logic ane. Although such inputs never change their logical land, they are non unused and must be connected as the device functioning requires. Real unused logic inputs are ones whose logic state does not touch 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, information technology may be sensible to do so past making an external connectedness to VDD or VSS, respectively7.
- Unused analog signal inputs should usually exist continued to a dc potential merely may sometimes demand to be grounded at ac only by a capacitor—RTFDSviii. In many cases this dc potential may be a ability supply or basis, but for reasons that are discussed in more than item in RAQ 46three, amplifier inputs frequently demand different treatment. RAQ 3two explains why unused analog signal connections to analog switches and multiplexers must never exist left floating but should always exist connected to ground or another convenient potential.
Unused voltage outputs rarely need termination and tin 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 organisation and must exist prevented by the use of an appropriate capacitor or resistor.
Unused electric current outputs often require a pull-up (or pull-downwardly) connection to a supply or to ground to forestall incorrect operation of other parts of the excursion. Kirchhoff's police says, in effect, that a electric current e'er has to go somewhere—it cannot merely disappear! This pull-up/down will more often than not exist a short circuit only may sometimes need to exist a resistor and so that power dissipation takes place in the resistor and not in the IC scrap.
The real message from all the above advice is that even if a particular final of an IC is unused in an application it is however necessary to empathise its function, what potentials are (or should be) present on it, what currents may (or must) period to/from it, how sensitive it is to electrostatic or RF influences, and if it requires capacitive or resistive loading of whatsoever sort. In other words, as we accept so frequently said before, "Read the Friendly Data Sheet"8 —and do what information technology tells you.
ane Oktoberfest, Munich.
2 Bryant, James. "Isolating the Key Detail (or Lunching with a Mermaid and Pickled Herring." Rarely Asked Questions, Issue iii.
three Bryant, James. "What Shall We Do with an Unused Op Amp?" Rarely Asked Questions, Issue 46.
4 Bryant, James. "Do Something with that Unused Pivot!" Rarely Asked Questions, Issue seventy.
5 Castro, Gustavo. "The Ingenious Admirer and the Mysterious Paddle." Rarely Asked Questions, Issue 119.
six Very occasionally digital circuits have "tristate" inputs where the iii states are Logic 0, Logic 1, and open excursion. 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 Information technology is permissible to pull up a terminal with internal pull-downwardly, or vice versa, and may sometimes exist necessary, but this will (very slightly) increment the system's current consumption.
8 Bryant, James. "Caveat Emptor." Rarely Asked Questions, Issue 4.
Bryant, James. "Some Thoughts on the Connexion of Unused Pins."
Bryant, James. "How to Brand Sense of a Data Sheet."
Author
cornwellwitir1949.blogspot.com
Source: https://www.analog.com/en/analog-dialogue/raqs/raq-issue-123.html
0 Response to "what to do with unused open drain output"
Post a Comment