I've just looked at your results and yes, they are similar in error
to mine.
It occurs to me that I've not been comparing apples with apples here.
It is important to me what the absolute value of the error is at any
given reading
eg (Reading-Actiual)/Actual * 100 %
So, my error percentages are per reading. without reference to other
readings.
But, A2Ds are normally specd as accuracy in bits with respect to full
scale. ie 1 5 volt convetre with an error of 50 millivolts would be
classed as an error of 1% whether the true value was 5 volts or 0.5
volts. However, at 0.5 volts thids apparent error of 1% would be an
actual error of 10%. Nothing startling here but I had got so tied up
in what I was trying to achieve for my purposes that I was reporting
in absolute terms without reflecting that others may be confused by
it.


>I've made some measurements of the 2-I/O line sigma delta A/D
converter
>and have been able to obtain fairly good results. I made
measurements over
>the range of about 0 to 1.4 volts and found that the maximum
linearity
>error was only 0.033% (assuming the data spans the full 4096 counts,
which
>it doesn't - I really need to measure over the full range to get an
>accurate estimate of the linearity). This corresponds to about 1.3
counts
>on a 12bit A/D converter. Here's the raw data I measured:
>
>-0.0098   803


snip

>1.3895    c62
>I also ran your (Russell's) data through the octave program and got
>similar results.  Which is to say, that the linearity is quite good!


Here are the full scale errors % that I get from your data - very
nice.
Two key questions are -

1.    How many counts in your loop? - whoops - you say that its 4096.
Mine are 2048 counts.

2.    What are you using as your "gate" input - standard PIC pin /
PIC Schmitt pin / external gate  or an internal/external comparator
?
    What PIC?


FS error % wrt 1.3895 volt.
-0.129718686
0.019030472
0.059385647
0.082006271
0.014629686
0.029538414
0.041547738
0.060901055
0.050731237
0.033806067
-0.047596284
-0.0354398
-0.02008959
-0.065360613
-0.013805496
-0.067156314
-0.07651675
0.029345728
-0.03468997
0.017380208
0.079988106
-0.008096016
-0.006992313
-0.063389696
-0.05516274
-0.039371049
-0.012894479
-0.098005616
0.015053696
0.08114964
0.121504816
0.01524466
0.102489623
-0.079447654





RM