Selection principles of voltage measuring instruments

　　The voltage in the electronic circuit has the characteristics of wide frequency range, wide voltage range, various waveforms, coexistence of AC and DC, and even noise interference. In addition, the measurement purposes are different (engineering measurement or precision measurement). Therefore, when formulating the measurement plan, the measurement instrument should be reasonably selected according to the measurement purpose and requirements to achieve the expected measurement results. The principles for selecting voltage measuring instruments are:

　　1. Select according to the frequency range of the voltage to be measured

　　The frequency of voltage in electronic circuits varies from zero to hundreds of megahertz. To measure DC voltage, a high-sensitivity multimeter or DC millivoltmeter can be used. When the operating frequency is in the range of 20Hz to 1MHz, a transistor millivoltmeter is usually selected. When the operating frequency is in the range of 5 to 300MHz, an ultra-high frequency transistor millivoltmeter can be used.

　　2. Select according to the range of the measured voltage value

　　In electronic circuits, voltage measurements of microvolts or millivolts are often encountered, so a voltmeter with higher sensitivity should be used for measurement. For engineering measurements, the DA-16 transistor millivoltmeter can meet the requirements. Although multi-digit digital voltmeters have a wider voltage measurement range, they are more expensive.

　　3. Select according to the equivalent resistance of the circuit being tested

　　Since the input resistance of the measuring instrument is the additional load of the circuit under test, considering the impact of the instrument access on the circuit under test, the measuring instrument is required to have a higher input resistance.

　　4. Select according to the waveform of the voltage being measured

　　In electronic circuits, except for sinusoidal voltages which can be measured with transistor voltmeters, non-sinusoidal voltages (including pulse voltages) measured with transistor voltmeters often result in large waveform errors. Therefore, non-sinusoidal voltage values ​​are usually measured with an oscilloscope.

　　5. Select according to the measurement accuracy requirements of the circuit under test

　　In engineering measurement, we must consider both accuracy requirements and economic costs. Therefore, we should try to choose low-priced instruments while meeting accuracy requirements.

　　6. Notes

　　(1) The DC component of the measured AC voltage cannot exceed the allowable value of the instrument.

　　(2) When measuring AC power directly, the casing may be energized. You must be very careful to avoid danger.

　　(3) After use, turn off the power, short-circuit the input terminal, and set the range selector switch to the maximum position.

　　(4) If the instrument is found to be inaccurate, it must be calibrated.