What is the principle of measurement using a digital multimeter?

As the name suggests, the working principle of a digital multimeter is mainly based on digital circuits to detect and analyze signals, and then provide LCD display through an analog-to-digital converter.

Although the digital multimeter is a more complex electronic measuring tool, if you operate it correctly according to the instructions, pay attention to the general precautions, and maintain a good use environment, the service life and accuracy of the instrument will be greatly improved and stabilized. The digital multimeter uses a liquid crystal display as a reading device, which has the characteristics of high measurement accuracy and safe and reliable use. It has a wide variety of models and is very easy to measure.

Digital multimeters generally use 7106 A/D conversion integrated circuits, and some use micro-power 7126 or its improved 7136 integrated circuits. During the measurement process, it can automatically complete the three-stage cycle of automatic zero adjustment, signal integration, and reverse integration. Its count value N is proportional to the measured voltage V, which can be expressed by the mathematical formula:

Number of screen displays N = signal integration time T1/(clock oscillation period TC×reference voltage V0)×measurement voltage V

In the circuit inside the meter, the signal integration time is taken as 1000 times the oscillation period. The clock oscillation period is determined by the clock oscillation RC component value connected to the 38, 39, and 40 pins of the 7106 integrated circuit, and is generally adjusted to a frequency of about 40kHz. The reference voltage is determined by the voltage divider resistor connected to the 35 and 36 pins, and is generally adjusted to 100.00 mV. Calculated in this way:

N=1000×V/100.00=10V

Obviously, if the decimal point is set to the tenth place, the measurement result can be read directly. The maximum display value of the most basic range of a 3 1/2 digit digital multimeter is 199.99 mV.

Digital square meters measure different DC voltages by resistor division, measure different DC currents by resistor shunting, and measure different AC voltages by first dividing the voltage through resistors and then linearly rectifying them through an op amp.

Digital multimeters use the ratio method to measure resistance. As shown in the figure, the reference resistor is connected in series with the resistor to be measured, and then connected between the +V of the integrated circuit and the COM analog ground to obtain the reading. When R0=Rx, the displayed value is 1000. Taking the 200Ω range as an example, the reference resistor is 100Ω, the displayed value is 10Rx, and the decimal point is set to the tenth place to read the result directly. It can be seen that different resistance measurement ranges are achieved by changing the resistance value of the reference resistor.