Decoupling Capacitors Revisited

Discussion in 'General Electronics Chat' started by ramancini8, Jul 18, 2012.

  1. ramancini8

    Thread Starter Member

    Jul 18, 2012
    442
    118
    Decoupling capacitor function is different for digital and analog circuits.

    Digital circuits draw large currents when they are switched (all types), and if these currents must pass through the power distribution system they induce voltage spikes throughout the PCB causing noise and unreliable operation. Placing a decoupling capacitor (.1UF capacitors are small and inexpensive) across the power pins of each digital IC provides a local return path for the current spike thus preventing noise.

    Analog circuits like op amps seldom generate noise because the currents are much lower, but they are very suceptable to noise generated by other circuits on the PCB. Placing a small resistor (20 to 51 ohms) in series with each power lead with a .1UF capacitor from the IC pin to ground creates a filter that shunts the noise to ground before it can reach the IC power pins. Check power supply rejection for op amps and you will see that it is quite low and it decreases with increasing frequency.

    Decoupling is required to obtain a reliable low noise system.
     
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  2. #12

    Expert

    Nov 30, 2010
    16,247
    6,739
    You could also look at the sticky at the top of the Chat page labeled, "Decoupling or Bypass Capacitors, Why?"
     
  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    There is a delightful fairytale which is used in theoretical discussions,where DC power supplies are assumed to have zero impedance to earth for AC signals.

    Out in the real,big,bad,world of practical circuits,this is not the case,& the internal Z of the supply,plus any series reactances,resistance,etc, from the connecting leads,appears as part of the load of an active device.
    This means that signal voltages may be present on the power supply bus for many devices in a complex circuit.

    In analog circuits,this may lead to instability,noise,or oscillation.
    In digital circuits,it can cause mis-triggering,& yes,oscillation--gates can still operate as amplifiers.

    All the above suggests that a means of making the supply look like zero impedance,or close to it at the frequencies of interest,for each circuit is desirable.

    Decoupling capacitors appear as very low impedances to earth,so that the circuit operates as though the theoretical concept was true.

    At the same time,it is common to introduce additional reactance and/or resistance in the power supply line,so the impedance looking back into the supply is increased,reducing the coupling between circuits.
     
  4. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    Decoupling caps are absolutely critcal. When using larger IC's such as micro-processors be sure to read through the datasheet for the part and use the caps they recommend. They will often recommend more than one value in parallel. Don't be lazy or cheap with bypassing!

    Also, don't forget to pay attention to layout. PC board layout is often the source of noise and other issues in circuits. Make sure you have good power and ground paths. Keep high current and voltage switching circuits away from low level signals. And make sure to put those decoupling caps as close as possible to the IC they are decoupling. The cap will not do its job well if it has a long lead length.
     
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