How to do low level AC measurements accurately?

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İslam İsgandarov

Dear Indula Munasinghe! You have raised a very topical issue. It is impossible to measure small alternating voltages by conventional methods and means. We know that there are special factory voltmeters. Ie it is necessary to resort to such principles used in them. Creating such a portable small-sized voltmeter is even more difficult. This is already a case study provided.

Indula Munasinghe

Thank you for your answer!

@ İslam İsgandarov What are the possible ways that are known for measuring those low level currents? If I use a sensitive instrumentation OPAMP to build the required circuitry for an ammeter what kind of difficulty that I would face?

P. K. Dubey

The AC low level measuring devices work on entirely different concept. You can not use DC measuring devices. However, it also depends on the signal frequency. If it is extremely small say less than a Hz then probably the DC instrument may function reasonably good. It actually depends on the parameter being measured. If you can send more details about your objective the better solution may be suggested.

I greet you dear colleagues! Dear P. K. Dubey right. In fact, if there is a more specific goal, then in accordance with this, you can give a proposal.

P. K. Dubey My intent is to measure the electrode current of a capacitively coupled plasma which is usually powered by a radio frequency power source. Since the voltage waveform that ionizes the gas to create plasma is sinusoidal the current waveform is also sinusoidal (At higher frequencies which we can call it an AC plasma). I want to measure this current accurately to see if it can be used to determine the parameters of the plasma such as plasma density.

I think that for starters, you can analyze the option of using a conventional resistive shunt. With conventional multimeters, high-frequency current and voltage cannot be measured. If the voltage on the resistive shunt is sufficient (approximately more than 2V), then using a radio frequency rectifier (detector), converting this alternating voltage to direct, then can be measured using a multimeter. If the voltage is small, then first you will need to amplify (using a transistor amplifier or an operational amplifier). Everything will depend on what effect the resistive shunt will have on the operation of capacitive plasma. If there is no strong influence, then it is easier. The greater the resistance, naturally the greater will be the voltage on it.

Absolutely agreed with the suggestions given by Islam Iskandarov. If you have low signal rf voltmeter then as suggested you can a small resistor in series with source and measure the RF voltage and calculate the amount of current as the resistor is known. Otherwise if rf voltmeter is not available use DC conversion approach and measure the voltage. Obviously the resistor should be selected in such a way that it generate voltage that should drive the DC convertor.