I don't know where to ask this, so I'll try here.
Are analog to digital converters fundamentally, in their core inner
design, basically tiny systems which operate like 555 timers, with a
series of resistors and capacitors designed to sample ranges, essentially
counting ticks per fixed units of time, resulting in the digital data
necessary to perform an indexed lookup from the inner sampler
that's in range, to produce an output bit patfern? With then
some tail logic to prevent jitter beyond an expected operating
range / frequency?
Rick C. Hodgin
You can see there are many methods - none of which fits very well with
the description you wrote, as far as I can see.
Successive approximation ADC's are the most common for general purpose
ADCs. Flash (direct conversion) ADC's are used for very high speed, and
Sigma-Delta is the usual method for high resolution (such as audio ADC's).
Single or Dual-slope integrating ADCs (very slow) are more like the
555 timer approach (if I understand the OP correctly). These are
used where speed of conversion is not important, but high resolution
and noise immunity are. You might find them in a digital multimeter.
I used one in a blood chemistry analyzer. Single-slope is less common.
Dual slope works by integrating up on a reference input (constant up
slope), then down using the input signal (variable down slope. The
time to integrate down to the low threshold is measured to give the
ADC output. The up integration time is fixed (there is no high
threshold). This allows the output value to be independant of the
integrating capacitor value as long as the cap is large enough to
prevent saturation during ramp up.
Oops! I got that backwards. Up slope is fixed time but rate is
based on the input signal. Down slope is fixed rate set by a reference
and time is measured to get ADC value. The way I originally described
it would end up with a 1/x factor in the conversion value. In either
case, the circuitry is quite simple since the control logic is mostly
a big counter. The timing is generally provided externally, as
is the integrating capacitor, which can be quite large and generally
wants to be polypropylene or similar dielectric to reduce effects
of surface charge storage (rebound).
Last time I used one of these was back in the early 1980's.
The MicroChip part in the link has an additional "auto-zero phase"
that removes any offset. See section 3.1.1 in the data sheet.
Another advantage for DC measurement is that the conversion is taken
on the integral of the input voltage over some fixed time period.
This makes it easy to filter out AC noise including power line noise.
The the blood chemistry analyzer we used a multiple of the line
period (50 or 60 Hz) as the integration time. A sampling time of
0.1 second would remove line-frequency noise from 50 or 60 Hz power.
This is useful in a handheld multimeter, where the leads are long
and can pick up noise easily.