Automatic zero circuitry for indicating devices

Claim 1

Automatic zero circuitry for eliminating errors due to DC amplifier offset voltage comprising a DC-excited Wheatstone bridge arrangement including strain gage elements in the arms thereof and producing low-level DC output signals as the result of unbalance thereof, measuring and indicating means including a dual-slope integrator, a DC amplifier for amplifying said output signals and applying related DC voltage signals to said dualslope integrator, a capacitor for storing voltage equal to the offset voltage of said amplifier, and switch means for periodically interrupting input to said amplifier while simultaneously connecting said capacitor for storage of said voltage during first periods and for periodically connecting the said low-level DC output signals to said amplifier while simultaneously applying to said dual-slope integrator the voltage across said capacitor in opposing relation to said DC voltage signals from said amplifier during second periods alternating with said first periods, said switch means comprising a first field-effect-transistor switch connected in series with said input to said amplifier, a second field-effect-transistor switch connected across said input to said amplifier, and a third fieldeffect-transistor switch connected between said capacitor and ground potential, said dual-slope integrator including means which, during said second periods, integrates up the voltage signals from said amplifier with the voltage across said capacitor subtracted therefrom, and which integrates down with respect to a reference input and passes through a dead time period during periods including said first periods, said first switch being off and said second and third switches being on during said first periods anD said first switch being on and said second and third switches being off during said second periods, whereby error due to said DC amplifier offset voltage is eliminated from measurements by said dual-slope integrator.