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> I also looked into it deeper, and the 'best' value for the source
> resistor is about 14.5 rather than 16 to maximize the minimum delta

A test with 750, 1k5, 3k0, 6k0 and 12k0 1% metal oxide

Vdd =3D 5.02V, Vdd*2 =3D 9.47V @ 0000 (9.33V @ 1111)

Total R =3D 12k + 11.25k =3D 23.25k

Theoretical

0000 =3D (11.25/23.25) * 9.47 =3D 4.58V 933
0001 =3D (10.5/22.5) * 9.47 =3D 4.419V 900 33 difference
0010 =3D (9.75/21.75) * 9.47 =3D 4.24V
etc

Practical

Vref =3D 4.57V, rather than Vdd (5.02V), spreads them out a little

DIP    Van   Vdiff   ADC  ADCdiff      Vref =3D 4.57V

0000  4.57           931                1023
0001  4.41   0.16    898    33  3% FSD   986 diff =3D 37
0010  4.23   0.18    862    36           947        39
0011  4.04   0.19    823    39           904        43
0100  3.84   0.20    782    41
0101  3.62   0.22    737    45
0111  3.14   0.48    639    98
1000  2.86   0.28    582    57
1001  2.56   0.30    521    61
1010  2.24   0.32    456    65
1011  1.88   0.36    383    73
1100  1.48   0.40    301    82
1101  1.04   0.44    211    90
1110  0.55   0.454   112    99
1111  0.00   0.550   000   112  11% FSD

> You might want to compare the sensistitivity of the parallel and
> serial schemes to variations in the resistor value

12k 1% high =3D 12.12k
Others 1% low (11.25*0.99) =3D 11.1375
Total 23.2575k

0000 11.1375/23.2575 * 9.47 =3D 4.53 923
0001 10.395/22.275 * 9.47 =3D 4.419 895 28 difference (vs 33 above)

> resistor is about 14.5 rather than 16 to maximize the minimum delta

(14.5/16) * 12k =3D 10.875k
0.75 + 1.5 + 3 + 6 =3D 11.25k
Total 22.125k

0000 11.25/22.125  4.81  980
0001 10.5/21.375   4.65  947  33 difference
0010 9.75/20.625   4.48  912  35

Joe

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