Already covered in various articles in this site is the necessity of using True RMS multimeters when working with alternating current so as to avoid problems of errors in measurements. However, even with appropriate multimeters misuse multimeter can cause problems that can be avoided if we know how they manifest.
Errors Common Mode
These errors can occur if the negative terminal of the multimeter is with an AC voltage above ground voltage. This can occur by the appearance of common-mode voltages as shown in Figure 1.
What happens is that many multimeters can perform at its negative terminal capacitance charging circuit thus changing the measures. Thus, we read different voltages when the meter has its inverted probes.
The error associated with the common mode will be greater the greater the measured voltage and the higher their frequency. The reader must be aware of the fact, making the measurement in both directions when suspicious result.
True RMS
As already explained as the "true RMS" is related to the heating voltage potential, or with the power actually represented by a signal having a particular waveform as shown in Figure 2.
Note that this measure is different from the average value that is given by some types of multimeters that do not take into account the peaks.
Power is proportional to the square of the RMS value measurement, regardless of the waveform. In Figure 3 we have the means and what happens to the calibration error to a multimeter to work only with sinusoidal signals.
Note that for sinusoidal, triangular and square signals, AC and AC + DC value are the same since there is no DC offset, but not symmetrical waves as pulse trains, these tensions are rejected by a multimeter True RMS.
Crest Factor Errors
A misconception that there is that "since an AC multimeter is true RMS, its accuracy for sinusoidal waveforms applies to any waveform." Actually, the waveform of the measured signal can dramatically affect the accuracy of measurement.
We define the "crest factor" as the relation between the peak value and the RMS value of a waveform as shown in Figure 4.
For a pulse train, for example, the crest factor is approximately equal to the square root of the inverse of the duty cycle. In general, the higher the crest factor, the higher the energy contained in the higher frequency harmonics.
All multimeters have errors that are dependent on the crest factor. To a multimeter, the equation above allows to calculate the total error, taking into account the crest factor error.
E = Error of the pass band
Fc = Crest factor
F = Fundamental frequency signal
BW = Passing track
Error by AC Load
The multimeters represent a load resistor in parallel with a capacitance which is connected to the circuit being measured, as shown in Figure 5.
The presence of capacitance causes the impedance presented by the meter depends on the frequency of the signal being measured. The higher the frequency the lower the impedance and therefore the greater the load that the multimeter will represent. This means that the higher the frequency of the measured signal, the greater the error introduced by the load represented by the instrument.
Special care should be taken when measuring higher frequencies voltages are measured. The use of low capacitance of test leads is crucial in this case.
Errors in Small Signal Intensity Measure
When very low voltages below 100 mV are measured, one should beware of errors that occur due to induction noise.
An exposed probe cord functions as an antenna can capture intense noise generated by sources close which overlap as tension, as shown in Figure 7.
For the measurement of low voltages should be careful with these spurious signals, if possible using shielded cables to the probes.
An important point is the signal that can be introduced into the measurement circuit by the power supply. An important caution when performing measurements with a multimeter powered by the power grid is that it is connected to the same outlet where the appliance is being analyzed. This helps to minimize any links feedback to the ground, capable of generating noise affecting the measurements.
An interesting feature that can be useful in these measures is the connection of a capacitor in parallel with the multimeter's input to reduce its impedance to noise ratio. The correct value of the capacitor used must be obtained experimentally.
Errors due to temperature
The features of a multimeter vary with temperature. Even keeping the equipment in a unique setting, with the operating temperature of the components rises, causing changes in their behavior. Offset currents can appear at critical points of the circuit, affecting the measures.
Of course, variations that occur are very low as specified for most multimeters, but they must be taken into account in certain operating conditions of the equipment.
Conclusion
Do not just take the best multimeter for your work. The introduction of errors in measurement also depends on the operator and the conditions under which the measurement is performed.
The professional who uses a digital multimeter should be aware of this fact and find out to what extent the way he is using the multimeter is not taking it to misleading results. Be aware and take into account that if the product is good, the operator is not foolproof and can be important external factors influencing the measure.
