[sdiy] VCO waveform logic selection

[Dave Manley]

Hi Harry,

I was referring to diode related comments and schematic at Rene's site 
at these links:

http://www.uni-bonn.de/~uzs159/switch.html
http://www.uni-bonn.de/~uzs159/switching.png

I took a quick glance at the Prophet 5 V3 Technical Manual and do see 
the method you're describing.  The VSS on the 4016's is connected to a 
signal called "SWITCH REF" which is a 1K to -15, clamped with a diode to 
one drop below ground.  On the voice pages with the 4016 this is labeled 
"-.6V".  Unless you're zoomed in, on the scans I have it is difficult to 
see the decimal point.  In any case, an interesting technique.

-Dave



harrybissell at wowway.com wrote:
> No that's not it...
> 
> Normally a 4000 series CMOS logic chip works on a single supply.
> 
> The Pro V circuit takes advantage of two things...
> 
> 1)  The voltage divider action when the switch is on.
> 
> With the 100K in series (effectively) with the input, the input
> protection diodes can easily uhhh... protect the input against a
> negative swing when the input is off
> 
> 2) Biasing the Vss supply (normally ground) one diode drop negative
> allows a swing of -0.7V at the input. Fortunately this negative bias
> is so small that the normal logic level signals can be used. The input
> control might be 0-15V and the chip input would trip at 1/2 the difference
> between Vdd and Vss... so the loss of that -0.7 volts has no effect except to
> change the 'noise immunity' a tiny bit.
> 
> The Pro V used the 4016 analog switches in many places, which have only two
> power supply pins (the 4051 (52,53) have three pins to allow the logic levels
> to be ground referenced
> 
> H^) harry
> 
> 
> 
> On Wed, 12 Sep 2007 09:06:31 -0700, Dave Manley wrote
>>>> Are the protection diodes built into the device inadequate for this 
>>>> purpose?
>>> Those diodes delimit the amplitude to +-0.6V around GND, while the input 
>>> protectors do clamp against Vcc and GND. Delimiting the swing at the 
>>> open switch does reduce the risk of capacitive crosstalk.
>>>
>>> Cheers,
>>>   René
>> Ahhh, thank you...I see I completely missed the point.  The open 
>> switch is not connected to the virtual ground, and so it swings with 
>> the input signal.  The diode clamps to ground limit this swing.
>>
>> The follow-up question then is with a typical +/-5V swing on the 
>> unconnected input how much of that signal appears in the output?
>>
>> -Dave
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> 
> Harry Bissell & Nora Abdullah 4eva
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> 
>