Hi Reinhard,
Thankyou for you reply!
The measuring current are:
- Measuring current for Ohmmeter (Fluke 117) : 0,290 mA
- Measuring current of the measuring device: 1.8 mA
Voltage Drop on AD8400: 203,7 mV, calculated resistence =113,16 ohm. Measured with Ohmmeter: 112,5.
Difference: 0,66 ohm , means 1.7°C. Maybe i've to consider that FLUKE 117 in the range of 0-600 ohm have 0.9% of Accuracy.
The consideration i've made about 2 potentiometer in parallel are:
1 RDAC of 1 KOhm, 256 step = 3.9 ohm/ step
Using Two RDAC's i can play with Two variable resistor, and finding the right combination to achieve the 0.5 ° Precision.
I've made a matrix using MS. Excell , rappresenting the possible results in ohm. (attached).
Maybe my consideration is wrong, i'm open to new solution