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Tue Nov 23 12:51:48 EST 2010

uC controlled analog circuits

The goal is to build something that can control an analog circuit
through a PIC uC.  The path I'd like to take is some SVF filter with
variable feedback, using a digital resistor as the control signal D/A
converter.

Rationale: 

  It doesn't seem to be so simple to get OTA ICs these days to build
  VCA, VCF circuits which have variable gain, so let's list some
  options:

  - transconductance-based VCA (transistor or OTA)
  - VCA circuits
  - digital resistors

The problem is resolution.  If the control envelopes are also
generated using analog circuitry that's not such a problem, but it
then does require a proper VCA.

Ok, let's try the digi pot.  I have DS1807[1] and DS1801[2].  Looks
like the DS1801 is more appropriate as I don't need the addressing.
It has a cool feature: zero crossing detection with a 50ms delay.

Questions:
  - Which PIC to use  (18F1320)
  - How fast does the 1801 switch?
  - Use a filter IC or opamps?
  - Max voltages on resistor terminals?

Switch frequency: the serial clock can go up to 10MHz (bit = 100ns),
there are 16 bits to transfer and the latch time is 200ns, so max rate
is 555kHz.  That should be enough to do any kind of interpolation
PWM/SD modulation.  The question with modulation is: how do glitches
behave?  For use as input current feedback resistors 

Using the PIC's HW shift register limits the debug interface.

[1] http://www.maxim-ic.com/datasheet/index.mvp/id/2782
[2] http://www.maxim-ic.com/datasheet/index.mvp/id/2777

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