Sensitive AC bridge for instrumentation - has square wave fed to resistance bridge and phase detector sensing balance

摘要

The sensitive, low resistance AC bridge for instrumentation purposes comprises a four-resistor network containing at least one variable arm. The usual DC source is replaced by a square-wave generator (2) operating at around 1 kHz, constant amplitude. The bridge diagonal is connected to the primary of a 1:25 transformer which raises the signal level to well above the noise level of the following amplifier (6) which is a simple AC amplifier. The secondary of the transformer is taken, via the amplifier, to a phase-sensitive detector (7) controlled from the AC generator. The detector's output signal is proportional to the bridge signal. Where a differential type of measurement is required, two such bridges may be used in conjunction with a differential transformer.