PicoStrain: Strain Gauge / Load Cells
During the discharge of the capacitor the single resistors of the strain gage lie in series with the switching transistors of the PS021. The switch-on resistance (= Rdson) of these transistors as well as their temperature dependence and supply voltage dependence are added as a failure to the measurement result. The Rdson 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 PS021 intrinsic DSP calculates the correction Rdson correction value from these three data.
The comparator itself shows a significant propagation delay, which is added to the result. It introduces an additional, temperature dependent gain error. The absolute maximum of this error is in the range of 1% of the maximum measured value. PS021 is capable of measuring the comparator's propagation delay and therefore can compensate for the gain error.
This compensation takes 1 measurement (= 1 cycle time) per half-bridge. The measurement is done at the end of a normal measurement. For noise reduction the correction data can be averaged.
Using the PS021 gain compensation the gain drift over temperature is only about 1 ppm/K (0.0001 %/K) of full scale. Even for calibrated scales it is not necessary to do further compensation actions.