During the discharge of the capacitor the single resistors of the strain gage lie in series with the switching transistors of the PSØ21. The switch-on resistance (= R dson ) of these transistors as well as their temperature dependence and supply voltage dependence are added as a failure to the measurement result. The R dson itself is in the range of 3 - 5 Ohm. The variation over temperature and voltage is in the range of 1 Ohm. It can be easily seen that this is bigger than the initial measurement signal of a 350 Ohm strain gage! The picostrain measurement principle is enhanced only with the patented compensation method for the Rdson.
For compensation the switch is split into two transistors. For each SG resistor the capacitor is discharged 3 times, through transistor 1, through transistor 2 and finally through both transistors in parallel. The PSØ21 intrinsic DSP calculates the correction R dson correction value from these three data.
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