programable analog threshold network: was RE: [sdiy] new project in the works

[Tim Ressel]

Yo,

Why not use a 74HC148 priority encoder?? The
comparators can be made to put out TTL levels, and the
switches can be of the DGxxx variety and thus use TTL
inputs.

--TR


--- Czech Martin <Martin.Czech at micronas.com> wrote:
> Hi Michael,
> here is the proposed circuit in a (very rough)
> schematic.
> 
> caveat: this is only a circuit idea. Depending on
> the
> actual hardware, some precautions have to be made
> for a working circuit. This is obvious for those
> experienced in the art.
> 
> http://www.geocities.com/SoHo/Museum/4459/test.gif
> 
> On the left side you can see the input in.
> A bias level can be added (or control voltage).
> This goes into a discrete flash ADC with 5
> comparators.
> You can see the reference ladder, which is driven by
> Vref+ and
> Vref-. The reference voltages are realized via pots,
> could be fixed resistors as well. Pots and Vref+,
> Vref-
> give complete control on where the input is
> quantized.
> Note that uneven quantization is possible and that
> any
> pot dial will give a unique characteristic (well, in
> theory,
> you must not dial the pot to the extremes, otherwise
> the steps will overlap. Additional resistors can
> take care
> of that. Hysteresis is left away, perhaps the
> comparators
> should have some feedback from output to +).
> 
> Note: by variation of Vref+ and/or Vref- the signal
> moves
> relative to the steps, or vice versa. This has
> exactly the same
> effect then using a costly multiplier on the input,
> if the reference voltages behave in the right way.
> 
> I guess this circuit has less problems in terms of
> this scaling.
> Of course you can vary individual thresholds, or
> vary only Vref-...
> etc.
> 
> 
> However, next comes a thermometer to 1 out of 5
> priority decoder.
> The box below shows the operation for a 1 out of 3,
> you see how it works.
> Again, this is a basic schematic, the actual ICs you
> use may need
> alterations or modifications.
> 
> Once we have the 1 out of N code, we can simply
> steer CMOS switches,
> or other switching means (npn transistors can do
> similar things,
> look at the usual tri to saw converters).
> Of course, the generic CMOS switches can not deal
> with +-13V comparator
> voltage, so some extra level shifting circuitry will
> be needed, depending
> on the actual ICs.
> 
> The selected pot voltage will go to a summing
> amplifier, which is
> bandwidth limited and output protected.
> The lowpass characteristic may help to overcome
> glitch problems.
> 
> Glitches: as you can see, the priority decoder will
> perhaps spit out
> intermediate "false" results, which can lead to
> trmporarily
> wrong switching action. But I think this circuit has
> little problems.
> 
> This circuit will quantize an incomming signal (with
> arbitrary step
> size and shape). The individual steps will in turn
> lead to
> completely independent output steps (only limited by
> switch characteristics).
> This goes very much further then any PWM or
> comparator scheme I have seen
> so far (but perhaps this is already existing
> somewhere).
> 
> 
> This circuit can be used in the audio range for
> spectra generation.
> Some people say that these "Manhattan" functions do
> not sound -good.
> I've got only little experience so far, but got some
> interesting
> results (simulation and wave player).
> 
> Note that the same circuit can serve as sequencer.
> The input then must be a low freq. saw or tri wave.
> Trigger generation, step count limiting etc. is
> straight
> forward.
> 
> In a certain way such a sequencer would be more
> suitable for an
> analog system then the usuall counter/decoder types.
> Of course, we have some drift/hold problem.
> 
> m.c.
> 
> 
> 
> 
> 


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