(3) dirty/clean ground again [sdiy]

media.nai at rcn.com media.nai at rcn.com
Tue Aug 20 22:00:44 CEST 2002


At 11:40 PM -0400 08/19/02, Glen wrote:
>
>What about the idea of using inductors in place of the resistors? Would
>>that be even better?

An inductor would prevent sudden changes in current which, based on what
Harry and Rene have said, is responible for the sudden changes in voltage
at the supply pins, but that current is what the op-amp wants in the first
place.  So it's sort of like throwing the baby out with the bathwater.

At 7:51 AM -0600 08/20/02, Ian Fritz wrote:
>
>I know that decoupling with a series resistor is a popular technique, and I
>have used it in some circumstances, but it has one major drawback that must
>be considered. In fact, some time ago I read an article that vehemently
>insisted that this technique should never be used! The reason is that the
>series resistance effective increases the output impedance of the power
>supply regulator to the value of the resistor. So instead of your carefully
>designed and constructed low-impedance power supply, you now have a 100Ohm
>(or whatever) power source.

That sounds like very good advice.

>I've had good luck decoupling with ferrite beads. Odd that no one has
>mentioned this possibility in this thread.

Using inductors, like ferrite beads, is good at reducing RF interrence that
might be on the power supply lines, but it should be followed by a large
enough capacitor to provide for sudden demands in current.

>In my last VCO project I put beads on the supply lines where they enter
>the board and also from that point to several different sections of the
>>circuit. This is the first VCO I have made where the switching transients
>>don't couple back into the expo converter section.

That might also be a good idea sub-circuits using noisy high-impedance
devices like logic, but I don't know if it would work for noisy devices
like LED's or timing caps that draw large amounts of current.  It might
just slow them down a bit while smoothing out their effect on the power
supply.  Otoh, I wouldn't be surprised if it created oscillations of its
own.

At 12:17 AM +0100 08/20/02, Neil Johnson wrote:
>
>> Nor do I see what sort of signal would flow from one rail to the other.
>
>Think of each rail as a low-impedance path to the PSU ground.  Sure, they
>have some DC on them, but at AC they look like a path to ground.  Except
>they only join ground back at the PSU, via the output decoupling caps.

So the signal would flow to ground, not the other rail.

>> >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.
>
>Bzzt!  Wrong---now you have two sets of lead inductance in series, so
>you've doubled the total inductance, upping the impedance even more.

You have obviously chosen making rude noises over thinking.  If you use
only one cap, the shortest possible trace length is the diagonal line
between the supply pins minus the width of the cap itself.  In practice,
the traces would actually be much longer since both the cap and the op-amp
would be mounted on the same side of the board.  If you use two caps, you
can place each one right next to each supply pin.






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