Measurement method for an electrical signal, series-parallel negative- feedback measuring circuit, as well as use of the method of the measuring circuit for measuring voltage sources with very-high-ohmage internal impedances

摘要

For measuring voltage sources with capactive internal impedance (Z.sub. G) the voltage source is connected to a charge amplifier (7) whose input capacitance comprises a vibrating capacitor (C(t)). The output of charge amplifier (7) is connected in series with negative feedback via a regulator to the input of the measuring circuit. The frequency response of this transmission line (7, PID) is designed so that a closed regulating circuit has a good follower behavior, at least up to signal frequencies corresponding to the exciting frequency of vibrating capacitor (C(t). The vibration of the capacitor causes an amplitude- modulated alternating signal with the voltage across said capacitor to appear at output (A). This signal is filtered out (13), amplified (15), and amplitude-demodulated (17). The demodulated signal is filtered, and supplied through a high-ohmage resistance (R) as a current to the current summation node of charge amplifier (7). By heterodyning on the one hand the current through the vibrating capacitor and on the other hand a current proportional to the charge on vibrating capacitor C(t), high amplification on the input side of charge amplifier (7) is produced, whereby disturbance factors (I.sub.Z) are smoothed out at its input. Residual disturbance factor influences are cancelled out when a current of the exciting frequency (f.sub.mod) is passed through in the vicinity of the charge amplifier input.