Alt-BEAM Archive
Message #06010
To: alt-beam@egroups.com
From: Donald McCorvey dlm3@netaxs.com
Date: Thu, 09 Sep 1999 22:46:44 -0400
Subject: [alt-beam] Re: ground?
Bumper314@aol.com wrote:
>
> hello...i was wondering if anyone knows the difference between digital
> ground, auido ground, and ground? thanks
Not something they usually teach in electronics courses (though they
should!), but it goes like this:
Everything in an electronic circuit is referenced to a single point
called "ground" or "common". For systems physically connected to the
Earth, that ground is literally the Earth itself. For systems isolated
from the Earth, it's usually the largest conductive mass in the system
such as an aircraft fuselage or a car's body/chassis.
The trouble is that ideally, everything is actually connected to that
same point. If you have any distance between a current flow and your
reference point, there will be a voltage across it (standard Ohm's law
stuff) which can cause any number of unpleasant things to happen, not
the least of which being ground currents - circulating currents in the
ground wiring which show up in your analog system as noise.
Analog circuits (especially audio) are considered 'low' speed - i.e.,
their frequency content has finite bandwidth and is usually a continuous
signal. Digital circuits, on the other hand, are generally considered
"high" speed because they include frequent switching transients on the
order of several (sometimes hundred) megahertz. Both are subject to
ground noise, but in different ways.
An analog circuit is subject to two major types of ground problems -
noise, induced by improperly terminated transmission lines (wires)
picking up ambient electromagnetic noise (most frequently 60Hz power
line noise) or conducted from a noise source elsewhere in the system;
and ground level drift - where the local ground reference to an analog
circuit rises to a value near or greater than the power rail of the
circuit. This can lead to saturation in amplifiers as the effective DC
bias causes the signal to exceed the voltage of the power rails.
Digital circuits are, of course, themselves analog circuits (albeit
specialized for high-speed switching) and are subject to both of those
problems. The effects, however, are usually seen in the form of glitches
(aperiodic switching transients), jitter (false triggering on noise
edges) or saturation (ground level above the transition level for the gate).
Both types of circuits can interfere with the other. As a result, in
complex mixed-signal circuits, grounds are separated into digital and
analog ground. The noise from the digital circuits is localized to the
digital part of the circuit. Likewise the noise from the analog circuits
is localized. The two grounds are eventually connected together at a
single point so that both have an identical value (otherwise, you can't
communicate from one side of the system to the other). A common way to
do this is to create a ground plane (a copper layer) for each ground in
a printed circuit board, then tie the two grounds together (usually) at
the power supply.
The key here is that there is only ONE point where the grounds actually
come together. It's not accidental, and there are NO other connections
between the two ground types. The result is that you minimize the ground
currents and significantly reduce any interference from one side to the
other or pickup from outside EM radiation.
Naturally, there's more to it. In general, we set everything to chassis
or earth ground eventually. You can actually set the ground arbitrarily
to wherever it is most useful (I once built a 5V digital circuit that
was referenced to -135Vdc in an optically-isolated 270Vdc power
amplifier). Proper grounding is usually a matter of bookkeeping and
careful design, keeping the two (or more) grounds separated until they
can be connected together at a specific point.
Did that answer your question ?
Don McCorvey
Philadelphia, PA
6011 Thu, 9 Sep 1999 23:40:42 EDT [alt-beam] non-beam but electronics related beam@corp.sgi.com Bumper314@aol.com hello, i am wondering if anyone has worked with the mas3507d mp3 decoder or
and dac3550a digital to analgue converter?
steve
6012 Thu, 09 Sep 1999 23:29:33 -0400 [alt-beam] Re: motor driver: h-bridge or IC? beam@sgiblab.sgi.com SG Thanks guys!
I'll try the 4 mosfets
but i'll have to do some research on the "pinout"
not clear on which is base, collector, and emit.
or equivilant.... never worked with them before
and not sure of the specifics of the component in regard to
the schematic....
Thanks though!
-Sparky
At 05:48 PM 9/9/99 -0700, you wrote:
>I don't like the big voltage drop (~2V) on the L293 and L298 chips. They run
>hot and are really too inefficient to be used applications below 12V. On the
>other hand a mosfet h-bridge is perfect for 5V applications. The mosfets
>have high input resistance and low forward voltage drop depending on the
>"on" resistance. Only 4 components are needed for a full h-bridge and these
>are available for free if you are resourceful and willing to de-solder
>components. A dead 80M Conner and similar IDE harddrive typically has
>6-10 complementary IR mosfets often in 4 pin DIP packages. I have found some
>with part numbers IRFD123 (N-channel) and IRFD9020 (P-channel). The specs
>for those parts are approximately 0.2 ohm "on" resistance and max current of
>1.2A @30V. They can comfortably drive hobby servos with minimal losses.
>Attached is a schematic of the mosfet h-bridge ready to be attached to a
>Bicore or microcore.
>
>enjoy
>
> <>
>
> Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
> tel: (604)590-7493
> fax: (604)590-3411
>
>> -----Original Message-----
>> From: Dan Larson [SMTP:dlarson@citilink.com]
>> Sent: Thursday, September 09, 1999 2:54 PM
>> To: beam@sgiblab.sgi.com
>> Subject: Re: motor driver: h-bridge or IC?
>>
>> If you want a real h-bridge for battery powered
>> applications, get an L293D (SGS Thompson).
>> It is rated up to 600mA, *dual* hbridge,
>> and has back emf diodes built in.
>>
>> There is also the L298N which is good up to 3A.
>>
>> You can get them at Mouser http://www.mouser.com
>> for US $2.12 each (The L293)
>>
>> 74HCT245s might be fine for smaller current
>> motors, but anything bigger cries out for
>> a real h-bridge chip. Making your own h-bridges
>> from discrete transistors (I've done it) is
>> too much trouble.
>>
>>
>> Dan
>>
>> On Thu, 09 Sep 1999 14:47:39 -0400, SG wrote:
>>
>> >Hi all,
>> >I think one of my 6 transistor h-bridges burned
>> >out (i hope thats what the problem is anyway)
>> >and rather than solder some more transitors together, i thought i might
>> try
>> >two 74HCT245's stacked.
>> >Does anyone know if this will be enough to power standard servo's?
>> >Thanks!
>> >-Sparky
>> >
>> >
>>
>>
>>
>
>Attachment Converted: "c:\pipeplus\eudora\attach\FETBRIDGE.gif"
>
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