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The operation of digital multimeters is relatively straightforward - using today’s IC technology they are easy to develop, build and use.
Although they are very different to that much older analogue meters, their basic function is essentially the same, although using digital technology they are able to incorporate many more measurements.
Like any test instrument, the way the digital multimeter, DMM works has an impact on the best way to use it, and understanding its operation can help make the most of it, and understand why some measurements may not work out the way that may be expected.
... the most common approach is for DMMs to use the successive approximation approach ....
How a DMM works - fundamentals
When looking at how a DMM works, one of the key processes that occurs within for any measurement that takes place is that of voltage measurement. In fact, all other measurements are derived from this basic measurement.
One of the key processes involved in this is that of the analogue to digital conversion.
There are many forms of analogue to digital converter, ADC. However the one that is most widely used in digital multimeters, DMMs is known as the successive approximation register or SAR. Some SAR ADCs may only have resolution levels of 12 bits, but those used in test equipment including DMMs generally have 16 bits or possibly more dependent upon the application. Typically for DMMs resolution levels of 16 bits are generally used, with speeds of 100k samples per second. These levels of speed are more than adequate for most DMM applications, where high levels of speed are not normally required. Typically for most bench or general test instruments, measurements only need to be taken at a maximum rate of a few a second, possible ten a second.
As the name implies, the successive approximation register ADC operates by successively homing in on the value of the incoming voltage.
The first stage of the process is for the sample and hold circuit to sample the voltage at the input of the DMM and then to hold it steady.
With a steady input voltage the register starts at half its full scale value. This would typically require the most significant bit, MSB set to "1" and all the remaining ones set to "0". Assuming that the input voltage could be anywhere in the range, the mid-range means that the ADC is set in the middle of the range and this provides a faster settling time. As it only has to move a maximum of the full scale rather than possibly 100%.
To see how it works take the simple example of an 4-bit SAR. Its output will start at 1000. If the voltage is less than half the maximum capability the comparator output will be low and that will force the register to a level of 0100. If the voltage is above this, the register will move to 0110, and so forth until it homes in on the nearest value.
It can be seen that SAR converters, need one approximating cycle for each output bit, i.e. an n-bit ADC will require n cycles.
Although the analogue to digital converter forms the key element within the instrument, in order to fully understand how a digital multimeter works, it is necessary to look at some of the other functions around the ADC.
Although the ADC will take very many samples the overall digital multimeter will not display or return every sample taken. Instead the samples are buffered and averaged to achieve high accuracy and resolution. This will overcome the effects of small variations such as noise, etc., noise created by the analogue first stages of the DMM being an important factor that needs to be overcome to achieve the highest accuracy.
One of the key areas of understanding how a digital multimeter works is related to the measurement time. Apart from the basic measurement there are a number of other functions that are required and these all take time. Accordingly the measurement time of a digital multimeter, DMM, may not always appear straightforward.
The overall measurement time for a DMM is made up from several phases where different activities occur:
- Switch time: The switch time is the time required for the instrument to settle after the input has been switched. This includes the time to settle after a measurement type has been changed, e.g. from voltage to resistance, etc. It also includes time to settle after the range has been changed. If auto-ranging is included the meter will need to settle if a range change is required.
- Settling time: Once the value to be measured has been applied to the input, a certain time will be required for it to settle. This will overcome any input capacitance levels when high impedance tests are made, or generally for the circuit and instrument to settle.
- Signal measurement time: This is the basic time required to make the measurement itself. For AC measurements, the frequency of operation must be taken into account because the minimum signal measurement time is based on the minimum frequency required of the measurement. For example, for a minimum frequency of 50 Hz, an aperture of four time the period is required, i.e. 80 ms for a 50Hz signal, or 67ms for a 60Hz signal, etc.
- Auto-zero time: When autorange is selected, or range changes are made, it is necessary to zero the meter to ensure accuracy. Once the correct range is selected, the auto-zero is performance for that range.
- ADC calibration time: In some DMMs a calibration is periodically performed. This must be accounted for, especially where measurements are taken under automatic or computer control.
The concept of the way the digital multimeter works is relatively straightforward, but it can be understood that measuring varying waveforms or intermittent voltages can give unusual results. Also it is important to select the right accuracy setting for the time that the measurement can be taken. Understanding how the digital multimeter works enables more informed decisions like these and others to be made when using a DMM.
More Test Topics:
Analogue Multimeter Digital Multimeter Oscilloscope Signal generators Spectrum analyzer Frequency counter LCR meter / bridge Dip meter, GDO Logic analyzer Power meter (RF & microwave) RF signal generator Logic probe Time domain reflectometer, TDR LabVIEW PXI GPIB / IEEE 488 Boundary scan / JTAG
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