Tying unused pins?

Discussion in 'General Electronics Chat' started by vijaybala85, Apr 28, 2010.

  1. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Hi,

    In using LM324 op amp and IC7404 hex inverter, Should we tie unused pins to the ground or Vcc? Or would it not matter for these specific IC's? Because I have them not tied right now and it seems to be working but someone asked me this question and I was wondering.

    V
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    CMOS device inputs must always have a path to +V or GND, or your results may be unpredictable.

    If you have unused opamps in a package, connect the output of each amplifier to it's inverting (-) input, and connect the noninverting (+) input to a reference voltage that is between +V and GND or -V. This makes the amplifier into a unity-gain voltage follower. However, your opamp must be unity-gain stable for this to work properly.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    For an unused opamp, I'd tie positive in to ground and negative in to output.

    The 7404 could have it's unused inputs pulled high via a resistor, that should minimise current consumption. For CMOS logic, spare inouts can be grounded or tied to any nearby output. The one thing you should not do is leave them floating.
     
  4. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Thanks guys! Tying the unused pins would not affect the pins that are being used right? Currently, I dont have them tied and they seem to work fine, but I understand I may have problems in the future.

    So, I guess I will tie the unused inputs of inverter to Vcc using a 1k resistor?

    Now, i am having a printed circuit board done. Would you think I could spare tying pins up on opamp using copper tracks and tie them on my own using jumpers later?

    V :)
     
  5. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    It's a common mistake to tie both op-amp input pins together, and then connect that node to ground or wherever. This could lead to oscillation as fluctuating input bias currents swing the output from max to min and back again, affecting power consumption and possibly interfering with other parts of the circuit. That said, I can't think of a single easy technique that would work for all op-amps, though there are awkward and inelegant circuits that would work. SgtWookie mentions the minimum gain criteria of some devices, some op-amps don't like the potentials of their input pins to be too different (e.g. the TLE2022 will latch up if the inputs are > 600 mV apart), and some op-amps protest if the any input voltage is outside the input common mode range, so tying to a rail could cause issues. If in doubt, read the relevant datasheet.

    Floating logic gate inputs can pick up all sorts of noise, including the states of their own outputs, and so can oscillate if left untied. This can destroy the device through overheating - CMOS are particularly vulnerable to this. Luckily dealing with unused logic inputs is trivial, just tie to the logic supply or logic 0V, with or without a resistor.
     
    PackratKing and kingdano like this.
  6. vijaybala85

    Thread Starter Member

    Jan 7, 2010
    92
    0
    Thanks a bunch for such a clear explanation... I will do as you said! Thanks all :)
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Generally I tie the inverting input of op amps to the output pin, they are normally right next to each other, then tie the positive input to ground

    With comparators I tie both inputs to ground.
     
  8. picbuck

    New Member

    Dec 13, 2010
    3
    0
    Coming in a year late here, but even so Maxim has this covered. Here's what they have to say.

    http://pdfserv.maxim-ic.com/en/an/AN1957.pdf
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Yeah, I'm going to add extra components just to tie unused parts of a chip down. Not.

    The one thing most designers try for is minimum components.
     
Loading...