Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!uflorida!gatech!emcard!stiatl!john From: john@stiatl.UUCP (John DeArmond) Newsgroups: sci.electronics Subject: Re: A Challenging PWM Design Problem Message-ID: <2009@stiatl.UUCP> Date: 6 Dec 88 18:59:04 GMT References: <1663@maccs.McMaster.CA> Reply-To: john@stiatl.UUCP (John DeArmond) Distribution: na Organization: Sales Technologies Inc., Atlanta, GA Lines: 59 In article <1663@maccs.McMaster.CA> darel@sparkles.UUCP (Darel Mesher) writes: > >I have an interesting Pulse Width Modulation design problem; > >Given the following PWM waveforms: > > Minimum pulse width: > > <----------------- 18 mS -------------------> > .--. .--. >____| |________________________________________| |_____ > > --> <-- > 1.0mS > > > Maximum pulse width: > <----------------- 18 mS -------------------> > .----. .----. >____| |______________________________________| |___ > > --> <-- > 1.9mS > > The pulse width varies linearly with a transducer variance of >0 - 100%. My question: is there a simple solution for the conversion >of this PWM signal into a corresponding analog signal, ie. a simple >circuit for the following black box? > This is an easy one. The average DC value of the waveform increases with increasing width. All that's necessary is a suitable integrator. The following is the minimum required: |\ ----|/--------------\/\/\/\ ------------------- (diode) | | | | ------ \ ------ / | \ | | ----------- (gnd) For more precision, replace the diode with a precision rectified (op amp with a diode in the feedback loop). For even more precision, bufffer the output with an op-amp and use a FET switch driven by the low level (0 volts) of the input signal to gate the input signal to a capacitor connected to the reference leg of the op-amp. This is known as baseline compensation and will compensate for any DC bias in the input signal. In any event, you should be able to do it with less than 10 components or so. John De Armond | "I can't drive 85!" Sales Technologies, Inc. Atlanta, GA | Sammy Hagar driving ...!gatech!stiatl!john | thru Atlanta!