[sdiy] Re: Daffy CMOS ASCIImatic

[Happy Harry]

Hi Scott...

If you check the linearity curves of voltage vs resistance...
you will see where the distortion comes from. They look mostly
like a resistance...but the resistance varies a little with
voltage. If the load impedance is high you might never notice the
variation of on resistance...

The Prophet V used these in almost all the signal switching
and it worked very well. I would not run my stereo through
them... but otherwise they are OK.

If you tried to modulate the 'on' resistance (make it work in a linear
range) you would likely get horrible distortion with voltage swings.
Operating the switch into the summing junction of an opamp could help
a lot... you might almost think it was a current rather than a voltage...

but that would kill the floating resistor concept.

H^) harry


>From: Scott Gravenhorst <music.maker at gte.net>
>Reply-To: music.maker at gte.net
>To: synth-diy at dropmix.xs4all.nl
>Subject: [sdiy] Re: Daffy CMOS ASCIImatic
>Date: Mon, 29 Apr 2002 08:22:15
>
>This is good info.  And it brings me to a question.  Considering
>the 4016 analog switch, the spec claims that a signal of 5v p-p
>is usable with Vdd-Vss of 10v, for 0.4% "distortion".
>They seem to omit a noise spec altogether for the 4016.  But to
>my question regarding:
>
>"FETs as linear VC resistors need a rather small signal level."
>
>Why do they recommend using a signal as large as half the rail
>to rail span?
>
>Is this because in the 4016 it is expected that the pass through
>transistors are either full on or full off?
>
>So it appears that the _4016_ (not my morphodite 4007 thing) can
>work with a fairly large input signal...  This spec is why I
>thought this might work.  But I really don't know what happens
>distortion/noise wise when the control signal is continuous and
>not 2 state digital.
>
>I get the feeling that I should toss this onto a breadboard and
>just try it.  The problem is that I really have little in the
>way of measuring performance such as noise unless it is so large
>that I can see it on my scope as being a significant proportion
>of the output signal.
>
>Thanks again, you people are wonderful.
>
>
>jhaible at debitel.net wrote:
> >Just a thought:=20
> >FETs as linear VC resistors need a rather small signal level.
> >OTOH linear amplifiers built from 4000 series CMOS chips
> >are not exactly low-noise, so they benefit from rather high signal
> >levels.=20
> >Both are good ideas, but I would not necessarily use them *together*.
> >
> >I have often thought how nice it would be to have a "low noise" version
> >of a 4007 or a 4069. Of course it would be possible to do similar things
> >with discreete transistors, but that's expensive, and the elegance of the
> >creative use of a low cost 4007 would be lost ...
> >(I think there used to be a - rather expensive - CA3xxx part with a 
>CD4007
> >configuration, but I never checked this.)
> >
> >I have not thought this thru entirely, mind you. So I cannot quantify
> >"noisy" at this point. Maybe part of it just comes from the high resistor
> >values that are typical for linear 4000-series applications. (If you're
> >using small feedback resistors, they are loading the amp outputs, which 
>are
> >just two current sources in open-loop case, more or less.)
> >
> >JH.
> >
> >
> >
> >At 19:07 28.04.02, Scott Gravenhorst wrote:
> >>Or a dual supply, +/- 7.5 Volts  and then the load goes to=20
> >>ground ?
> >
> >Right, something like that.
> >
> >>I thought about this, couldn't figure why...  I assume you
> >>mean to use the 2nd linear inverter like a cv preamp to this?
> >
> >The output of the "switch" would be run directly into the=20
> >inverting input of that invertor.
> >
> >>I don't get this.  Which input and why?  CV in?  I've seen=20
> >>this in single transistor units (drain to gate) but it's=20
> >>coupled with a resistor.  Is this a linearity improvement?
> >
> >If one uses such a configuration, the output of the two MOSFETs is=20
> >held at a constant potential. This should improve linearity.
> >
> >>Also, I looked at this again, and I think that if only one inverter
> >>is used, this can be made out of one 4007.  Powering it will have no
> >>effect on protection diodes turning on.  Only an analog signal out
> >>side of the rail-rail boundarys would cause problems.  Also I think
> >>the other pair of transistors could make another VC resistor that
> >>reponds reverse of the first.  VC pot?
> >
> >That would be interesting.
> >
> >Cheers,
> > Ren=E9
>
>=========================================================
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>
>-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>-- Linux Rex         | RedWebMail by RedStarWare
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>
>




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