- AC power supply and DC power supply
- AC coupling and DC coupling
- AC output and DC output
The first AC/DC, in its most common sense, refers to the type of power source. For example, AC100V is an alternating current (AC) power source, while batteries such as dry cell batteries are direct current (DC) power sources. Many small electronic devices these days (including measuring instruments) are powered using AC (more accurately, AC/DC) adapters, which are AC/DC converters, so it is important to use the correct AC adapter that matches the specifications of the device.
The second term, AC/DC, refers to the type of connection method to the input amplifier, which is also familiar from oscilloscopes, and means alternating current coupling (AC) and direct current coupling (DC). Fig. 1 is a schematic block diagram of the input section. DC coupling means that the input is directly connected to the amplifier, while AC coupling is usually a first-order high-pass filter that cuts out the DC component of the signal.
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Fig. 1 Input coupling
For example, the typical characteristics of AC coupling in our FFT analyzer are shown in Fig. 2.
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Fig. 2 AC coupling characteristics of the FFT analyzer
Specifically, the cutoff frequency (fc) is 0.5Hz, and the signal is attenuated by approximately 3dB;
Therefore, if the input resistance (impedance) is 1MΩ, the capacitor C will be approximately 0.3μF.
Now, regarding the choice of input coupling, DC coupling is generally used when you want to faithfully observe time-domain waveforms, such as with an oscilloscope. However, AC coupling is more common when the main application is frequency analysis of sound or vibration, such as with an FFT analyzer.
When using DC coupling with an FFT analyzer;
- If you want to measure including the DC component
- When you want to accurately measure signals below 1 Hz (such as low-frequency vibrations)
These are some possible explanations.
One point to note when analyzing with AC coupling is that the power spectrum obtained from the FFT analysis includes a DC component due to the self-offset of the amplifier in the measuring instrument itself. This is about -60dB of the input range (about 1mV in the 1V range), so it is usually not a problem, but please be especially careful when determining the overall power (overall value) of very small signals.
The third classification, AC/DC, concerns the form of analog signal output from measuring instruments. AC output is the raw signal itself on the time axis, such as from a sensor, while DC output is a time signal (trend) that is a leveled-down version of the amplitude (magnitude) of the AC signal. As a concrete example, Fig. 3 shows a functional block diagram of a sound level meter. The method of converting an AC signal to a DC signal is the RMS calculation that we discussed in previous issues.
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Fig. 3 Functional block diagram of the sound level meter
DC output also calculates the instantaneous RMS value at that moment, so it is a function of time. According to JIS C 1509-1 Sound Level Meter (Noise Level Meter), the noise level is defined by the following equation ②.
Here;
This corresponds to ;.
Now, regarding one of the FAQs related to your question, is the input to the FFT analyzer the AC output or the DC output of the sound level meter?
The answer is AC output. It is common practice to use AC output as the input source for an FFT analyzer to perform frequency analysis on the acoustic time signal from a microphone.
References: JIS C1509-1 Electroacoustics – Sound level meters – Part 1
(Excerpt from the email newsletter issued on February 21, 2008)
