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From: Issac Asimov
Subject: Re: sine to square wave converter?
Date: Thu, 05 Sep 2002 00:51:52 +0200
References: <3D753EF0.firstname.lastname@example.org> <3D758CEA.FCE7FEB9@webaccess.net> <3D75A7C8.email@example.com>
NNTP-Posting-Date: Wed, 4 Sep 2002 22:46:16 +0000 (UTC)
X-Mailer: Mozilla 4.78 [en] (Win98; U)
This one should be OK ..
> do you all think that the circuit found here would do a better job of
> what I want(taking a .01V sine wave and converting it to a 5V square
> wave with same frequency):
> http://www.discovercircuits.com/PDF-FILES/zerocross1.pdf ?
> any suggestions to make to this circuit to make is work better or more
> Chuck Simmons wrote:
> > 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.
> > Chuck
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