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Subject: Re: Return current path and magnitude??
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 20 Sep 2002 10:09:33 GMT
NNTP-Posting-Date: Fri, 20 Sep 2002 12:09:33 MET DST
"Håkan" schrieb im Newsbeitrag
> This I think is quite obvius:
> An op-amp is connected to V+ and GND (0-Volt), then current flows from V+
> GND. Making GND current return path. The current flowing in GND can be
> large. Therefore, it is important in critial op-amp applications to assure
> that this current in GND traces is separated from low current GND traces.
> Q: Now suppose, an op-amp connected to V+, V- and GND. Is GND still the
> current return path? Then this would imply that the op-amp draw current
> V+ to GND, which is as above. Wouldn't this also imply that current is
> from GND to V-, thus a negative current from V- to GND (or positive
> from GND to V- ;-) ). Thus resulting in an alternating current flowing
> through GND. Correct ???
No, the opamp is not connected to gnd, only resistors from input or output
nodes may be. If an opamp-input is connected to gnd there is only a very
tiny current running into.
The resistors might carry DC or AC and will inject this into the gnd
connection. So it is needed to analize the amount and type of current in
each resistor and arrange the proper connections. If a balanced design is
used, the gnd currents cancel.
> If it is so, how much current can be drawn from GND to V-? I have never
> considered GND to be current source but rather a "black hole" that can
> "swollow" what ever current comming from real current sources, until
> the fuse.
That depends on your power supply. You might have a virtual gnd connection
driven by a resistor pair or buffered by an opamp.
If there is a balanced supply you are right, it will swallow anything
provided it comes from its +/- rails. There is no inherent difference
between a neg. and pos. rail.
electronic hardware designer
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