(2) dirty/clean ground again [sdiy]

media.nai at rcn.com media.nai at rcn.com
Mon Aug 19 19:17:41 CEST 2002


>Firstly, the situation.  Moving *any* current about a circuit, be it on a
>PCB trace or down a wire, produces a voltage differential across the
>conducting medium (unless you happen to be using superconductors with
>zero impedance :-).  Copper has a finite, if small, resistivity, and its
>that which generates the voltage difference.

Around here we call that Ohm's Law :)

>Now, look at op-amp circuits.  They draw (or dump) current through their
>supply pins *only*.

Depending on the circuit, they can sink or source significant output
current, and off course the inputs wouldn't be inputs if they didn't...

>I personally don't like decoupling op-amp rails to ground, because
>basically all op-amp current flows through the supply rails, nowhere near
>ground.  Indeed, placing two caps across an opamp, via ground, actually
>halves the rail-rail decoupling capacitance:
>
>Now, any signal flowing from +V to -V sees C1 and C2 in series, which now
>looks like half of C1 (or C2, assuming they're the same value).  So
>although you've put two expensive 100n monolithic ceramics next to your
>op-amp, it actually looks like 50n across the rails.  Mmm....

Well, monolithic caps are hardly expensive, and if "seeing" half the value
were a problem, doubling the values would solve it.  Nor do I see what sort
of signal would flow from one rail to the other.

>Unless its for safety (in which case you have a separate circuit) ground
>should only be used for reference, not carrying current.  That's what the
>supply rails are for.

Since this is synth diy, that's not very realistic. Let's look at something
as basic as an envelope generator.  It could have a single-sided timer
chip, a single-sided mux chip, voltage dividers for the pots, discrete
semiconductors, and the timing cap, all connected to ground.

>Of course, we also want to reduce the amount of crap on the supply rails,
>because op-amps only have so much PSRR, and we don't want to screw
>ourselves.  Hence we put hefty local decoupling (10-100uF) to keep the
>supply noise down.

While that's a good value for a filter cap local to an entire module or
PCB, a cap that large would not provide the high frequency decoupling
recommended for the specified performance of many op-amps.

>That resistor/capacitor trick does two things: it filters out any crap
>coming in from other op-amps, and also reduces any local crap getting out
>and affecting anyone else.
>
>> I'm under the impression that using "extra" decoupling caps can't hurt.
>> For a one-time project, the additional cost seems negligible.
>
>From a cost perspective, it could.  From a circuit perspective, it
>shouldn't normally, unless you have so much capacitance that the voltage
>regulator is unable to cope with the capacitive load.

That seems unlikely.

>There's also the issue that placing them further away from their
>respective device reduces their effectiveness.

That's a separate, although valid issue.  Considering lead inductance it's
another argument for using two caps instead of one between both rails.

>Ideally, a multilayer board with a single, massive ground plane is heaven,
>and then it doesn't matter so much where the decoupling caps are located
>as there's such a low impedance from any supply pin to a decoupling cap.

According to some, a board trace longer than 2mm is too long.  A small cap
is useless if it is too far from the supply pins.

>> So how can we solve this??  Can ground be regulated??  I've never seen a
>> 7800 :)  However, a voltage  can be regulated, and if that regulated
>> voltage is held on one side of a cap, wouldn't that locally "re-zero" the
>> ground on the other side??  Perhaps this goes back to JH's point b)
>> above, by stabilizing the local ground of a single-sided amp, you
>> stabilize the amp.
>
>But, see above.  The local ground used by an op-amp is only there for
>reference purposes, not for carrying any current.  For instance, in a
>differential amp, both op-amp inputs are connected to signals.  And
>indeed, there is no need to use ground at all if that's what you need.

By "single-sided" I meant one of the supply pins is connected to ground.






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