Current measurements

Test measurement using unit #1

S1 S2     R   Measured resistance
0  0 510.00   512.2
0  1   1.79   4.1
1  0  30.99   33.3
1  1   1.70   4.0

multimeter: 88.3 mA
V_IN : 4.044
V_OUT: 3.703
delta: 0.340
I = dV / R:  0.340 / 4.0 (1,1 setting) = 0.085 A


ADC values are off. Unit has bad soldering of 0R resisistor.
Measurements are made with extra 3.3v wire hooked in to VDDA.

V_IN:  33346
V_OUT: 38196

VREF measured by multimeter to be 3.356V

Actual voltage = (ADC_value / UINT16_MAX) * VREF * 2.0 (because of voltage divider)

In [66]: (38196/65536) * 3.356 * 2.0
Out[66]: 3.91191943359375

In [67]: (33346/65536) * 3.356 * 2.0
Out[67]: 3.4151970214843748

# delta voltage from ADC values
In [68]: 3.91191943359375 - 3.4151970214843748
Out[68]: 0.4967224121093752

# delta voltage from multimeter
In [69]: 4043-3703
Out[69]: 340

Diff is way too large. Why?

# Measured with multimeter on the voltage divider
In [74]: 1.991 * 2 - 1.831 * 2
Out[74]: 0.320

There is quite a lot of error but it's still within a reasonable range.
320 vs 340, instead of 496 - wtf. ADC impedance messing things up?