Multimeter#Digital multimeters .28DMM or DVOM.29

A digital multimeter (short DMM ) is a digital test and measurement device in electrical engineering, which for measuring electric currents, voltages, and, depending on the design of several other variables ( ohmic resistance, capacitance, inductance, temperature) and to test for example transistors is used.

It works with an electronic analog -to-digital converter (ADC, Eng. ADC) and displays the measured value with an LED or liquid crystal display to decimal to. Some devices also allow for data transfer to a computer via a serial interface. To obtain the benefits of a scale display, some of the DMM are additionally equipped in the display with a pointer and scale.

Until the introduction of digital devices Analog Multimeter were common, preferably based on a moving-coil mechanism.

Basically need a DMM to its function batteries or any other type of power supply ( battery or power supply).

• 3.1 Balance deviation for zero and span
• 3.2 quantization distortion
• 3.3 Nonlinearity
• 3.4 Influence Effects 3.4.1 temperature
• 3.4.2 Waveform

Operation

A digital multimeter can measure different electrical quantities. Are usual voltage, current (both AC and DC variable) and resistance. The change of the measured variables and ranges is mostly mechanical. High-quality DMM select the voltage measurement range itself, can protect against overload and over-voltages and measure change sizes as effective value.

ADC with dual-slope method

Heart of a DMM is the ADU. Most DMM work with a converter according to the dual-slope method, see section Analog -to-digital converter or digital measurement technology. In this integrated process, the arithmetic mean value of the voltage signal is measured by comparison with a built-in reference voltage. The period in which the voltage to be measured is integrated or averaged over which is, typically 100 ... 300 ms.

Characteristics of the dual-slope method:

• Primarily only DC voltage measuring,
• Cost,
• Long-term stability; Changes in capacitance, the input resistance and the clock frequency will fall out through the comparative method from the result;
• Well störunterdrückend regarding hum and noise voltages
• Slowly; adapted to the human ability to react to the reading.

Multimeter Functions

DC

The smallest measuring range is mostly up to 200 mV. Standard equipment loose on a measuring range in 2000 measured points, thus is the smallest measurement step 100 microvolts. Higher-quality devices can resolve to one or even several orders of magnitude finer.

Range switching is done by a switchable voltage divider in front of the ADC. The division is made in increments of a whole order of magnitude (unlike analog multimeters with typical two measuring ranges per decade ). The highest measurement range may however be used only to eg 700 V. The input resistance is typically in all measuring ranges at

AC

AC voltage is to be measured, a rectifier is necessary as absolute value. DMM to use a circuit as a precision rectifier, so that the distortions occurring in battery-free analog multimeters near zero omitted. It is the arithmetic mean of the rectified AC voltage ( the rectified value ) is measured. When an AC voltage is applied, the display of the rms value is expected. In the majority of measurement tasks one has to do with sinusoidal alternating quantities. For this, the rectified value formed by the form factor of 1.11 ( = π / √ 8) is displayed enlarged. In this case, the form factor is defined as the ratio of rms to rectified value; the concrete value of 1.11 is only valid for sinusoidal waveform. Thus, the effective value is displayed for sinusoidal voltages. In a different time course of this display is faulty, it differs partially from the catastrophic RMS.

Digital multimeter, the voltage curve of an arbitrary measure the actual RMS (English true RMS), are equipped with a device (integrated circuit or software in a microcontroller ), the calculated analog or digital the rms value. Analog -working modules for RMS detection are available as an integrated circuit, see RMS. In higher-quality multimeters their implantation has become common.

Fast converter required that can price reasons (yet) prevail - to rms digital literacy - depending on the required sampling rate.

As a result of incomplete smoothing while rectifying or RMS detection is reliable readings arise only at a Aufintegrationsdauer which covers a whole ( or very large ) number of cycles of the alternating voltage. For effective noise suppression at mains frequency integration over 100 ms ( 5 cycles at 50 Hz or 6 periods at 60 Hz) or an integer multiple is common.

Partial show digital multimeter in the AC range also for mixed voltages, ie at voltages contain a DC component and an AC component, only the AC component; which is the effective value of the alternating component in effective value education. Occasionally a multimeter to RMS measurement to measure the rms value of the total voltage without the DC component is removed before:

In some cases you can choose between the two options "AC" or " AC DC". This is not the case, one must experimentally or determined by studying the manual, if mixed voltage or AC component is measured.

Amperage

To measure current, the voltage is measured via a built-in measuring resistance - depending on the setting as a DC or AC voltage. It is given by

Example: In the current measurement range is 200 uA ≥ 200 mV / 200 uA = 1 k.

Most DMM are therefore inferior to other current measurement methods that do with much less voltage drop.

Multiple ranges can be realized with a parallel to the voltmeter connected in series connection of several different measuring resistors, wherein a step switch which one or more switches in the circuit, without the connection when switching to interrupt (English make before break ).

For the measurement of large currents from about 10 A, the electromagnetic field surrounding the current conductor is detected rather than the voltage drop at the measuring resistor. There are clamp-on probes with measuring ranges exist until about 1000 A. Advantages of the current probe, the fact that one does not have to unpick the conductor for measurement, and in the galvanic isolation.

For the measurement of alternating current the same applies as for AC voltage.

Resistance

Resistance measuring a DMM contains an electronically stabilized constant current source supplying a load independent of the direct current. When connecting the resistor to be measured at the input terminals of the current through the DUT, and the resulting voltage is measured, preferably in the smallest voltage range. The power source is then switched to the measuring range switching.

Example: = 10.00 uA obtained with the smallest voltage range 200 mV a resistance measuring range 20 k.

The relationship between the measured variable and display is a proportionality, and right to get accurate readings. The margin of error arises from the margin of error for the DC voltage measurement and the margin of error for the adjustment of the current. The quality of the measured value is significantly higher than with analog multimeters with a display range 0 ... ∞, where the result is very inaccurate if only by gross readability.

Resolution

If a meter shows five decimal places in his display, as in a measurement range of 200 mV, it has a resolution of 0.01 mV. It may be 20 000 different values ​​... 000.00 199.99 Show mV. The leading body is not fully developed. Colloquially, the device is referred to as 4 ½ digits. The same is true for a 250 mV range with 25 000 steps. In how many steps one dissolves "half" position is always clear only if one knows the range or the number of steps.

Measuring variations in the Digital Multimeter

Balance deviation for zero and span

The characteristic of an ADC ( with extremely fine grading) is. A straight line through the origin and represents the desired proportionality between the display and measure The zero point must be adjusted by horizontal displacement. The sensitivity must be adjusted by twisting ( changing the slope of the curve), see also under the heading of instruments deviation. Either is possible only within certain limits of error.

Quantization distortion

Characterized in that the measured variable is displayed only gradually creates a quantization distortion.

Nonlinearity

This measurement error is significantly smaller than the typical matching variations that occur. A distinction is made between

The limits of zero-point, quantization and linearity deviations are constant over the entire measuring range, the limit of sensitivity deviation is proportional to the measured value. In summary, you get this as an error limit of the instrument consists of two summands,

Influence Effects

The limits mentioned so far are valid for the intrinsic error when operating under specified conditions. If deviations from these reference conditions, such influence effects may increase the measurement error of the instrument. The problem is the same as the case of analog instruments; for explanation of terms, see below accuracy class.

Temperature

Digital multimeters are usually adjusted according to DIN 43751 at a temperatures of 20, 23 or 25 ° C. When changing the temperature of the meter, the electrical properties of its components change. By exerting effects the gauges deviation may be larger. The influence of temperature on the measurement value is given by means of a parameter.

Waveform

The curve form of the measured parameter can be described by different parameters. One of these variables is the crest factor (English Crest Factor), defined as the ratio of peak to rms value. For DC and sinusoidal AC voltage. If the peak value is much larger than the rms value ( So ), as for example in pulses, it comes to incorrect measurements.

At the same richtwert -forming instruments for alternating quantities, the sine form is mandatory, otherwise significant deviations can occur. When rms DMM forming the waveform is typically up of little impact if the fundamental frequency is not too high (50 to part 400 Hz).

Calculation of the margin of error

Example ( operational limit at reference conditions):

Example (expanding the limits of error by influence):