Amplifier current detection method commonly used for film capacitor

The film capacitor is a capacitor in which a metal foil is used as an electrode and a plastic film such as polyethylene, polypropylene, polystyrene or polycarbonate is stacked from both ends and wound into a cylindrical shape. The film capacitor is subjected to a series of strict tests before leaving the factory. Current detection is one of them. The following describes the current detection method of the amplifier.

The amplifier current measurement of the film capacitor is basically performed by converting the current signal into a voltage signal in a certain manner. Depending on the type of conversion, the measurement methods using amplifiers can be roughly divided into two categories. One type is the voltage difference between the input current and the known resistance, also known as the direct current type current conversion method. The other type is to charge the known capacitance in the amplifier by the input current and then to detect the output voltage of the amplifier. This is also the current common amplification method: high input impedance method and integral method.

The high input impedance method of the film capacitor requires high performance of the op amp. The op amp will absorb a certain current from the signal source during operation. When the amplifying circuit is connected to the signal source, this resistor becomes the load of the signal source, and naturally the current is taken from the signal source. The magnitude of the current indicates the degree of influence of the amplifying circuit on the signal source. Therefore, the larger the input resistance, the smaller the current demanded by the amplifier circuit from the signal source, and the closer the input voltage of the amplifier circuit is to the signal source voltage, that is, the smaller the voltage on the internal resistance of the signal source, the smaller the signal voltage loss.

The sensitivity and resolution of the amplifier used for current measurement are susceptible to drift and noise. By using a carrier-amplified linear component, the effect of drift on the circuit can be reduced, while the noise needs to be processed by the integral method. The effect of the measurement results, and the longer the integration time, the smaller the influence of noise. However, its shortcomings are also obvious, that is, the response time of the circuit is extended, which reduces the test efficiency.