[sdiy] dirty/clean ground again

jhaible jhaible at debitel.net
Sat Aug 17 18:43:26 CEST 2002


First let me admit that I have *no* complete understanding of decoupling.

But as much as this, that

(a) it serves at least two different purposes (preventing oscillations and
      improoving SNR)
(b) it's normally one supply pin that needs decoupling more badly than
      the other
(c) you might need much less decoupling capacitors than you think.
      and
(d) you might sometimes need them where you least expected it.


a) SNR - an opamp has normally excellent power supply rejection
    (noise from the supply pins not showing at the output), but only
     for low frequencies. At high frequencies, you need a filter in the
     supply trace, i.e a capacitor or even an RC circuit, if noise is
     an issue.
     Oscillation - this is related. If a signal can go from the supply pin
     to the output, and if the output of this, or another opamp in the
circuit,
     has an effect on the supply rails, you can build loops of positive
     feedback that way. The latter part involves load _currents_ - but
     keep in mind that stray capacitance can be enough "load" for
     frequencies in the MHz range, where the oscillations occur.
     Depending on the specific opamp you're using, internal currents
     might be enough to for musch a loop, if the power supply rails
     don't form a "shunt" (decoupling ...) to keep the loop gain low.

b) Most opamps have a voltage gain stage that forms an integrator.
    This often means that feeding noise into one of the power
    supply pins is feeding it in *after* the integrator. So the negative
    feedback loop around the opamp, thru the input stage, and thru
    the integrating voltage gain stage has a "disadvantage" against
    HF that comes in from this power pin. The other side is usually
    much less sensitive. So if you only want to use a single cap, it
    can sometimes be better to go from -vee to GND rather than
    from +vdd to -vee.
    I'm not able to deduce this in all detail. But let this be a hint:
    Sometimes, when you use a single supply (-vee = GND), and
    two resistors to form a "midpoint" on the positive opamp
    input, it can be necessary to add a cap across the resistor from
    the midpoint to GND (= -vee). (You also need this for SNR,
    if you are referencing your signal to GND elsewhere in the
    circuit, but I have *also* seen this being called for because
    of stability.)

c) Opamp manufacturers have a tendency to demand a decoupling
    with which the opamp is stable under all circumstances. Normally
     you are not building circuits for all circumstances, but for a single,
     dedicated application. I have sucessfully built boards of
     160cm x 233cm size with nothing more than one pair of aluminium
     electrolytics and no other decoupling caps at all. But ...

d) ... sometimes I have fought desperately to get a single stage stable,
    even though all decoupling from the textbook had been applied.

If you are building for fun (DIY, of "synth-DIY"), my advice would be
to keep space for many caps on the board, but only solder in the ones you
really need. If you're building commercially, it's a different story,
because
you don't know how an opamp from a different batch will behave.

JH.





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