Introduction to Technical Specifications of Digital Multimeters
1. Number of display digits and display characteristics
The display digits of a digital multimeter are usually 31/2 to 81/2 digits. There are two principles for determining the display digits of a digital instrument:
One is that the number of digits that can display all numbers from 0 to 9 is an integer;
The second is that the numerical value of the fractional digit is represented by the high digit in the * large display value as the numerator. At full scale, the value is 2000, indicating that the instrument has 3 integer digits. The numerator of the decimal digit is 1, and the denominator is 2, so it is called 31/2 digits, pronounced as "three and a half digits". The high digit can only display 0 or 1 (0 is usually not displayed).
The high bit * of a 32/3 digit (pronounced as "three and two-thirds digits") digital multimeter can only display 0-2 digits, so the * large display value is ± 2999. In the same situation, it is 50% higher than the limit of a 31/2 digit digital multimeter, especially valuable for measuring 380V AC voltage.
For example, when measuring power grid voltage with a digital multimeter, the highest digit of a regular 31/2 digit digital multimeter can only be 0 or 1. To measure 220V or 380V power grid voltage, only three digits can be displayed, and the resolution of this range is only 1V.
In contrast, using a 33/4-bit digital multimeter to measure grid voltage, the high bit can display 0-3, which can be displayed in four digits with a resolution of 0.1V, which is the same as a 41/2-bit digital multimeter.
Universal digital multimeters generally belong to handheld multimeters with a 31/2 digit display. 41/2, 51/2 digit (below 6 digits) digital multimeters are divided into two types: handheld and desktop. Most desktop digital multimeters with 61/2 digits or more belong to the category.
The digital multimeter adopts advanced digital display technology, with clear and intuitive display and accurate reading. It not only ensures the objectivity of readings, but also conforms 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.
2. Accuracy
The accuracy of a digital multimeter is the combination of systematic and random errors in measurement results. It represents the degree of consistency between the measured value and the true value, and also reflects the magnitude of 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 equation (2.2.1), RDG represents the reading value (i.e. display value), FS represents the full scale value, the previous item in parentheses represents the comprehensive error of the A/D converter and functional converter (such as voltage divider, splitter, true RMS converter), and the latter item is the error caused by digital processing.
In equation (2.2.2), n is the change in quantization error reflected in the last digit. If the error of n words is converted into a percentage of full scale, it becomes equation (2.2.1). Equation (2.2.3) is quite unique, and some manufacturers use this expression. One of the last two represents errors introduced by other environments or functions.
The accuracy of a digital multimeter is much better than that of an analog pointer multimeter. Taking the accuracy index of the basic range for measuring DC voltage as an example, it can reach ± 0.5% for 3 and a half bits, and 0.03% for 4 and a half bits.
For example, OI857 and OI859CF multimeters. The accuracy of a multimeter is a very important indicator, which reflects the quality and process capability of the multimeter. A multimeter with poor accuracy is difficult to express the true value, which can easily lead to misjudgment in measurement.
3. Resolution (resolution)
The voltage value corresponding to the last one word on the low voltage range of a digital multimeter is called resolution, which reflects the sensitivity of the instrument.
The resolution of digital instruments increases with the number of displayed digits. The high resolution indicators that a digital multimeter with different digits can achieve are different, such as a 31/2 digit multimeter with 100 μ V.
The resolution index of a digital multimeter can also be displayed using resolution. Resolution refers to the percentage of * small digits (excluding zero) and * large digits that the instrument can display.
For example, a typical 31/2 digit multimeter can display a resolution of 1/1999 ≈ 0.05%, with a small number of 1 and a large number of 1999.
It should be pointed out that resolution and accuracy belong to two different concepts. The former characterizes the "sensitivity" of the instrument, that is, the ability to "recognize" small voltages; The latter reflects the "accuracy" of the measurement, that is, the degree of consistency between the measurement results and the true value.
The two are not necessarily related, so they cannot be confused, let alone mistakenly assume that resolution (or resolution) is similar to accuracy, which depends on the comprehensive error and quantization error of the internal A/D converter and functional converter of the instrument.
From a measurement perspective, resolution is the "virtual" indicator (independent of measurement error), while accuracy is the "real" indicator (which determines the size of measurement error). Therefore, increasing the number of display digits arbitrarily to improve the resolution of the instrument is not feasible.
4. Measurement range
In a multifunctional digital multimeter, different functions have corresponding maximum and minimum values that can be measured. For example, with a 41/2 digit multimeter, the testing range for DC voltage range is 0.01mV to 1000V.
5. Measurement rate
The number of times a digital multimeter measures the amount of electricity being measured per second is called the measurement rate, and its unit is "times/s. It mainly depends on the conversion rate of the A/D converter.
Some handheld digital multimeters use measurement cycles to indicate the speed of measurement. The time required to complete a measurement process is called the measurement cycle.
There is a contradiction between measurement rate and accuracy indicators, usually the higher the accuracy, the lower the measurement rate, and it is difficult to balance the two. To solve this contradiction, different display digits or measurement speed conversion switches can be set on the same multimeter:
Add a fast measurement range for A/D converters with faster measurement rates; By reducing the number of display digits, the measurement rate can be significantly increased. This method is currently commonly used and can meet the needs of different users for measurement rate.
6. Input impedance
When measuring voltage, the instrument should have a high input impedance, so that the current drawn from the measured circuit during the measurement process is minimal and does not affect the working state of the measured circuit or signal source, which can reduce measurement errors.
For example, the input resistance of a 31/2-bit handheld digital multimeter in the DC voltage range is generally 10 μ Ω. The AC voltage range is influenced by the input capacitance, and its input impedance is generally lower than the DC voltage range.
When measuring current, the instrument should have a very low input impedance, which can minimize the impact of the instrument on the measured circuit as much as possible after being connected to the measured circuit. However, when using the current range of a multimeter, due to the small input impedance, it is easier to burn the instrument. Please be careful when using it.
