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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: sine to square wave converter?
Date: Wed, 04 Sep 2002 04:32:57 GMT
NNTP-Posting-Date: Tue, 03 Sep 2002 21:32:57 PDT
Bob Wilson wrote:
> In article <3D753EF0.firstname.lastname@example.org>, email@example.com says...
> >hi, I was wondering if anyone knows of a circuit that will take an input
> >of a sine wave of approx .01V-.2V and convert it to a square wave with
> >same frequency and that is approx 5V?
> >currently I have a circuit found here:
> >http://home.att.net/~ralph.hartwell/wavshap2/wavshap2.htm but as others
> >have pointed out,
> >(quote from message I recieved)
> >At the specified input voltage of .6vpp, the absolute maximum (in a
> >negative sense) input input voltage of the LM324 of -0.3v has been
> >exceeded. An op-amp that only just meets spec without any safety factor
> >will be blown out in normal use. A minor concern, since all real op-amps
> >will have some spec margin, but why was this done for no reason? Another
> >1/2 cent on a bias resistor from the (+) input to +5v would have solved
> >this problem.
> >Mismatched input resistances give a designed-in uncontrolled offset
> >voltage. Input impedances should be equal to minimize offset voltage.
> >With a gain of 360 and an input voltage of up to 0.6vpp, this op-amp
> >will be heavily saturated. Op-amps don't like to be saturated, and have
> >a slow recovery time. This kind of design calls for a comparitor, not an
> >Four HCT gates to drive one 50 ohm output, but only one to drive the
> >other? Why? The HCT series buffers can source 35ma of current. It takes
> >two of them to drive a 50 ohm load with a 50 ohm source resistor as
> >shown on your schematic. Four is too many and one is too few.
> >The zener diode shunt regulator is more complicated, more expensive,
> >inefficient and noisier than the standard LM309 regulator design.
> >(end quote)
> >so can you suggest a cicuit that will do much the same thing or suggest
> >ways to improve the circuit found on this page:
> >http://home.att.net/~ralph.hartwell/wavshap2/wavshap2.htm or possibly
> >even give me a new schematic with improvements?
> >any help would be greatly appreciated,
> Why would anyone use an LM324 (which is an opamp) when what is required is
> an ordinary comparator?
> An ordinary LM339 (quad) of LM393 (dual) comparator will do exactly what you
> want. If you are worried about negative voltages, then clamp the input with
> a schottky. These comparators will tolerate a negative input voltage of up
> to 1 diode drop (silicon diode), so a schottky clamp will keep things legal.
> Just bias one input of the 339 at 0 Volts, and feed the signal to the other
> input via a (say) 10K resistor, and clamp this input to ground with a
> schottky. End of problem.
There are some slight improvements I have used. If the frequency range
of the AC is reasonable, I would AC couple and get off the rail. Granted
the LM339 works fine at the rail (I use them less than 100mv off the
rail for hall sensors) but getting off of the rail is nicer particularly
because it is easier to use positive feedback to set hysteresis. Another
idea I used in a BEMF detector was to do a simple minded DC restoration
at the comparator. This was done by feeding the signal through resistors
to both inputs but connecting a large capacitor to ground on the minus
input. The plus input, of course, has the positive feedback needed
because of the glitches in the raw BEMF from the high current PWM
driver. In this case, I used 1/2 of an LT1215 as a comparator and used
1/4 LM339 (left over from the hall sensor circuit) as a logic level
shifter to drive 3.3 volt CMOS with 5v tolerant inputs.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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