How to choose a digital multimeter

Digital multimeter is the most commonly used digital instrument at present. Its main features are high accuracy, strong resolution, complete test function, fast measurement speed, intuitive display, strong filtering ability, low power consumption, and easy to carry. Since the 1990s, digital multimeters have been rapidly popularized and widely used in China. They have become an essential instrument for modern electronic measurement and maintenance work, and are gradually replacing traditional analog (i.e. pointer) multimeters.
Digital multimeters are also called digital multimeters (DMM). There are many types and models. Every electronics worker hopes to have an ideal digital multimeter. There are many principles for choosing a digital multimeter, and sometimes it even varies from person to person. But for handheld (pocket) digital multimeters, they should generally have the following characteristics: clear display, high accuracy, strong resolution, wide test range, complete test functions, strong anti-interference ability, relatively complete protection circuit, beautiful and generous appearance, simple and flexible operation, good reliability, low power consumption, easy to carry, moderate price, etc. The prices of handheld multimeters on the market vary greatly. If we are not satisfied with the selection, we will choose a multimeter that cannot meet our requirements. Now, the technical staff of Beijing Ocean Xingye Technology Co., Ltd. will tell you how to choose a digital multimeter based on many years of market experience.

2. Main indicators of digital multimeters, display digits and display characteristics
The display digits of digital multimeters are usually 3 1/2 digits to 8 1/2 digits. There are two principles for determining the display digits of digital instruments: first, the number of digits that can display all numbers from 0 to 9 is an integer; second, the value of the fractional digit is the numerator of the highest digit in the maximum display value. When the full scale is used, the count value is 2000, which indicates that the instrument has 3 integer digits, and the numerator of the fractional digit is 1 and the denominator is 2, so it is called 3 1/2 digits, read as "three and a half digits", and its highest digit can only display 0 or 1 (0 is usually not displayed). The highest digit of a 3 2/3 digit (read as "three and two-thirds digit") digital multimeter can only display digits from 0 to 2, so the maximum display value is ±2999. Under the same circumstances, it is 50% higher than the limit of a 3 1/2 digit digital multimeter, especially when measuring 380V AC voltage, which is very valuable.
For example, when using a digital multimeter to measure the grid voltage, the highest digit of an ordinary 3 1/2-digit digital multimeter can only be 0 or 1. If you want to measure the 220V or 380V grid voltage, you can only use three-digit display, and the resolution of this file is only 1V. In contrast, when using a 3 3/4-digit digital multimeter to measure the grid voltage, the highest digit can display 0 to 3, so that it can be displayed in four digits with a resolution of 0.1V, which is the same as the resolution of a 4 1/2-digit digital multimeter. 
Popular digital multimeters are generally handheld multimeters with 3 1/2-digit display. 4 1/2 and 5 1/2-digit (less than 6-digit) digital multimeters are divided into handheld and desktop types. Most of the digital multimeters above 6 1/2 digits are desktop digital multimeters.
Digital multimeters use advanced digital display technology, with clear and intuitive display and accurate reading. It can not only ensure the objectivity of the reading, but also conform to people's reading habits, and can shorten the reading or recording time. These advantages are not possessed by traditional analog (i.e. pointer) multimeters.

Accuracy (precision)
The accuracy of a digital multimeter is a combination of the systematic error and random error in the measurement result. It indicates the degree of consistency between the measured value and the true value, and also reflects the size of the measurement error. Generally speaking, the higher the accuracy, the smaller the measurement error, and vice versa. 
There are three ways to express accuracy, as follows: 
Accuracy = ± (a% RDG + b% FS) (2.2.1) 
Accuracy = ± (a% RDG + n words) (2.2.2) 
Accuracy = ± (a% RDG + b% FS + n words) (2.2.3) 
In formula (2.2.1), RDG is the reading value (i.e., the displayed value), FS represents the full scale value, the first item in the brackets represents the combined error of the A/D converter and the functional converter (such as the voltage divider, current divider, true RMS converter), and the second item is the error caused by digital processing. In formula (2.2.2), n is the change in the last digit reflected by the quantization error. If the error of n digits is converted into a percentage of the full scale, it becomes formula (2.2.1). Formula (2.2.3) is quite special. Some manufacturers use this expression. One of the last two items represents the error introduced by other environments or functions. The 
accuracy of digital multimeters is much better than that of analog pointer multimeters. Taking the accuracy index of the basic range of measuring DC voltage as an example, 3.5 digits can reach ± ​​0.5%, 4.5 digits can reach 0.03%, etc. For example: OI857 and OI859CF multimeters. The accuracy of a multimeter is a very important indicator. It reflects the quality and process capability of the multimeter. A multimeter with poor accuracy is difficult to express the true value, which is easy to cause misjudgment in measurement.

Resolution (resolution)
The voltage value corresponding to the last digit of the digital multimeter on the lowest voltage range is called resolution, which reflects the sensitivity of the instrument. The resolution of a digital meter increases with the increase of the number of displayed digits. The highest resolution index that can be achieved by digital multimeters with different digits is different, for example: 100μV for a 3 1/2-digit multimeter. The 
resolution index of a digital multimeter can also be displayed by resolution. Resolution refers to the percentage of the smallest digit (except zero) that the instrument can display to the largest digit. For example, the smallest digit that a general 3 1/2-digit digital multimeter can display is 1, and the largest digit can be 1999, so the resolution is equal to 1/1999≈0.05%. 
It should be pointed out that resolution and accuracy are two different concepts. The former characterizes the "sensitivity" of the instrument, that is, the ability to "recognize" tiny voltages; the latter reflects the "accuracy" of the measurement, that is, the degree of consistency between the measurement result and the true value. There is no necessary connection between the two, so they cannot be confused, and resolution (or resolution) should not be mistaken for something similar to accuracy, which depends on the comprehensive error of the A/D converter and function converter inside the instrument and the quantization error. From a measurement perspective, resolution is a "virtual" indicator (irrelevant to the measurement error), and accuracy is a "real" indicator (it determines the size of the measurement error). Therefore, it is not possible to arbitrarily increase the number of display digits to improve the resolution of the instrument.

Measuring range 
In a multi-function digital multimeter, different functions have their corresponding maximum and minimum values ​​that can be measured. For example: For a 4 1/2-digit multimeter, the test range of the DC voltage range is 0.01mV to 1000V.

The measurement rate 
is the number of times a digital multimeter measures the measured quantity per second, and its unit is "times/s". It mainly depends on the conversion rate of the A/D converter. Some handheld digital multimeters use the measurement cycle to indicate the speed of measurement. The time required to complete a measurement process is called the measurement cycle.
There is a contradiction between the measurement rate and the accuracy index. Usually, the higher the accuracy, the lower the measurement rate, and it is difficult to take both into account. To solve this contradiction, different display digits can be set on the same multimeter or the measurement speed conversion switch can be set: add a fast measurement gear, which is used for A/D converters with faster measurement rates; by reducing the display digits to significantly increase the measurement rate, this method is currently commonly used and can meet the measurement rate needs of different users.

When measuring voltage 
, the meter should have a very high input impedance, so that during the measurement process, very little current is drawn from the measured circuit, which will not affect the working state of the measured circuit or signal source, and can reduce measurement errors. For example, the DC voltage range input resistance of a 3 1/2-digit handheld digital multimeter is generally 10μΩ. The AC voltage range is affected by the input capacitance, and its input impedance is generally lower than the DC voltage range.
When measuring current, the meter should have a very low input impedance, so that after connecting to the measured circuit, the impact of the meter on the measured circuit can be minimized. However, when using the multimeter current range, due to the small input impedance, it is easier to burn the meter, so please pay attention when using it.

3. Functions of Digital Multimeter

Classification of digital multimeters 
Digital multimeters can be classified into three types according to the range conversion method: manual range (MAN RANGZ), automatic range (AUTO RANGZ), automatic/manual range (AUTO/MAN RANGZ). 
According to the functions, uses and prices, digital multimeters can be roughly divided into 9 categories: low-end digital multimeters (also known as popular digital multimeters), mid-range digital multimeters, mid-/high-end digital multimeters, digital/analog hybrid meters, digital/analog dual display meters, and multi-oscilloscopes (integrating the functions of digital multimeters, digital storage oscilloscopes, etc.).

The test functions of digital multimeters 
Digital multimeters can not only measure DC voltage (DCV), AC voltage (ACV), DC current (DCA), AC current (ACA), resistance (Ω), diode forward voltage drop (VF), transistor emitter current amplification factor (hrg), but also capacitance (C), conductivity (ns), temperature (T), frequency (f), and add a buzzer gear (BZ) for checking the continuity of the circuit and a low-power method for measuring resistance (L0Ω). Some instruments also have inductance gear, signal gear, AC/DC automatic conversion function, and capacitance gear automatic range conversion function. 
Most digital multimeters have added the following novel and practical test functions: reading hold (HOLD), logic test (LOGIC), true effective value (TRMS), relative value measurement (RELΔ), automatic shutdown (AUTO OFF POWER), etc.

The anti-interference ability of digital multimeters. 
Simple digital multimeters generally adopt the principle of integral A/D conversion. As long as the forward integration time is selected to be exactly equal to the integer multiple of the cross-frame interference signal period, the cross-frame interference can be effectively suppressed. This is because the cross-frame interference signal is averaged out in the forward integration stage. The common frame rejection ratio (CMRR) of mid- and low-end digital multimeters can reach 86 to 120 dB.

For details on the fault capture function of a digital multimeter, please 
refer to the article "How to Use a Multimeter to Collect and Detect Intermittent Electrical Faults" on the website of Beijing Ocean Xingye Technology Co., Ltd.

4. Development trend of digital multimeter

Integrated 
handheld digital multimeters use a single-chip A/D converter, and the peripheral circuit is relatively simple, requiring only a small number of auxiliary chips and components. In recent years, single-chip digital multimeter dedicated chips have been continuously introduced, and a relatively complete automatic range digital multimeter can be constructed using a single IC, creating favorable conditions for simplifying design and reducing costs.

Low power consumption 
New digital multimeters generally use CMOS large-scale integrated circuit A/D converters, and the power consumption of the whole machine is very low.

The above is just the main principle of choosing a multimeter, and price and other human factors have not been considered. Users need to choose a multimeter based on actual needs and conditions. Welcome to consult the technicians of Beijing Ocean Xingye Technology Co., Ltd.