Summary: Using a very cheap op-map ($US0.16 in true volume) and a few
relatively cheap parts and an (internal to uP) comparator an Analog
to Digital conveter with 9+ bits accuracy is achieved. Better
accuracy will be achieveable with longer conversion times. The
technique is slow but cheap to implement.


Must be time to stop playing with this.
Results keep getting better.
Many things that people say don't seem to be true!

Latest trial uses 2 transistors to act as current sources to drive
the integrating capacitor.

Here's some shocking ASCII art (I hope)(I can see it - YMMV)

          ______________________  Vcc
                    |    |
                    R    R
                    R    R Re-hi
                    |    |
                    |    E
PIC Output --.-RRR--.---B     Q1 - PNP
             |           C
             |           |              Rin
             |           -------------RRRR--- <-- Vinput
             |           |          |
             | Rbi-lo    C          |
             .-RRR--.---B   Q2     ---  Cint
                    |    E  NPN    ---
                    |    |          |
                    R    R  Re-lo   |
            Rbg-lo  R    R          |
                    |    |          |
 ------------------------------------- Gnd


Q1 and Q2 act as current sources.
PIC output swings fully high/low
When eg Hi Q1 is turned off.
Q1 is turned on by Rbi-lo and Rbg-lo divider.
V at Q2 base is Rbg/(Rbi+Rbg) * Vcc (PIC out)
so V at emitter is 0.6v less or so and Current through Q2 is this
voltage across Re-lo.
This current is sunk out of Cint.
When PIC output is low Q2 conducts and sources current into Cint.
Vinput will sink/source current into Cint.
The voltage on Cint is sensed by a comparator and the PIC drives the
output pin hi/lo as required to keep capacitor at switching point.

I'm actually using a Z8 :-) at present but was using a PIC and will
do so again later.
Vin is driven by a LM324 in my case and must be a low impedance
point.

With Cint =  1 uF
Rin = 4K7
R Re-lo = 2K7
R bg-lo = 5K6
Rbi-lo= 18k and othe rvalues mirrored

I am getting 9 to 10 bits of accuracy (11 in places) with 2048 counts
across an input voltage range of about 0.8 to 2.8 volts.