Method and circuit arrangement for conditioning direct current signals in electric measurand transmitters, particularly electromechanical precision and fine balances

Claim 1

A electrical circuit for suppressing the effect of interfering alternating signals of various frequencies superimposed upon a direct current signal in measuring apparatus, such as in an electromechanical precision or fine balance, the circuit comprising a signal line for carrying the direct current signal and the interfering alternating signals; coupling-out means for capacitively coupling-out of interfering alternating signals of frequencies lower than a predetermined limit frequency from said signal line; amplifying and inverting means for amplifying and inverting 180° in phase the coupled-out signals; coupling-back means for capacitively coupling-back into said signal line the amplified and inverted coupled-out signals; wherein said coupling-out means, said amplifying and inverting means, and said coupling-back means form an active low-pass filter and said coupling-out means and said coupling-back means each include a respective capacitor means directly connected to said signal line; variable resistive circuit means in circuit with said capacitor of said coupling-out means, said resistive circuit means, with respective said capacitor means of said coupling-out means and said coupling-back means, determining the time constant of said active low-pass filter; wherein said coupling-out means includes a pair of resistors in parallel with one another, one of which is a variable resistor for altering said time constant of said active low-pass filter, said variable resistor being a photoresistor; a light emitting diode positioned opposite said photoresistor; a circuit for supplying current to said light emitting diode; a control circuit coupled to said circuit for supplying current to said light emitting diode via a time delay circuit for turning on said light emitting diode after a predetermined delay upon receipt of a control signal; a switching circuit coupled to said control circuit and responsive to an output therefrom; said variable resistive circuit means being coupled to said switching circuit and responsive thereto for changing the resistance value of said resistive circuit means to provide a lower time constant for said active low-pass filter upon receipt of an output from said switching circuit.