> From PHXSYS@AOL.COM Fri Dec  4 08:46:43 1998
> Subject:      PIC14000 Slope ADC whats the difference?
>
> Hello,
>
> What is the difference btween a slope ADC (PIC14000) and the ADC used in a
> ADC0834 or LTC1298?
>
I will not make a comparison, but I will describe how the PIC14000 ADC
works -- I've not seem any other ADC like it.  To make it simple, I will
pretend that there are only 2 channels to this ADC; in fact, there are
more like 14 of them.

You have 2 voltage sources: a known voltage, and the signal you
are attempting to measure.  Each of these 2 voltage sources are,
in turn, connected to one input of a voltage comparitor.  The
other input of the voltage comparitor is connected to voltage ramp
generator.  So, suppose we start with the known voltage source:
time how long it takes the voltage ramp generator to reach the
same potential as the known voltage source; call that time T1.
Now, repeat the process with your signal you are trying to measure;
call that time T2.  If the voltage ramp generator is perfectly
linear, and start out at exactly zero volts, and there is no
voltage offset error in the voltage comparitor, and there is no
delay in the voltage comparitor in signaling when the inputs are
crossing each other, then the ratio

        V(signal)      T2
        _________  =  ____
        V(known)       T1


Now, of course, all these assumptions are not met, so there are
a number of additional steps that can be taken to account for
voltage offsets, propagation delays, temperature effects on the
reference, and so on.  The end result is that after a bunch of
floating point calculations (let the reader understand what this
implies), as well as some averaging of the samples, you get a
pretty good reading to 12 bits accuracy (imho).

Dave Reinagel
cisco Systems, Inc.


Opinions expressed here are mine and are not those
of cisco Systems, Inc. or anyone else.

from the
at least 12 bits of accuracy.
is