| 摘要 |
<p>To measure a voltage source with a capacitive inner impedance (ZG), the source is connected to a charge amplifier (7) of which the input capacity comprises an oscillating capacity (C(t)). The output of the charge amplifier is fed back in series to the input of the measuring circuit through a controller. The frequency response of said transmission section (7, PID) is so designed that the closed controller circuit has a good sequential behaviour at least up to frequencies corresponding to the energization frequency of the oscillating capacity (C(t)). Due to the oscillation of the capacity, there is provided at the output (A) an amplitude modulated alternating signal with the voltage to said capacity. This signal is filtered (13), amplified (15) and amplitude demodulated (17). The demodulated signal is filtered and supplied through a high ohmic resistance (R) as a current to the current summation node of the charge amplifier (7). By superposing on one hand the current passing through the oscillating capacity and on the other hand a current proportional to the charge on the oscillating capacity, a high amplification is obtained before the charge amplifier, thereby enabling to control the perturbating magnitudes (Iz) at its input. The influences of the remaining perturbating magnitudes are attenuated by introducing at the charge amplifier input a current having the excitation frequency (fmod). </p> |
