True RMS converter

When measuring the value of an alternating current signal it is often necessary to convert the signal into a direct current signal of equivalent value which is known as the root mean square (RMS) value. Most low-cost instrumentation and signal converters carry out this conversion by filtering the signal into an average rectified value and applying a correction factor.

The value of the correction factor applied is only correct if the input signal is sinusoidal. The true RMS value is actually proportional to the square root of the average of the square of the curve, and not to the average of the absolute value of the curve. For any given waveform, the ratio of these two averages is constant and, as most measurements are made on what are (nominally) sine waves, the correction factor assumes this waveform; but any distortion or offsets will lead to errors. Although in most cases this produces adequate results, a correct conversion or the measurement of non-sinusoidal values requires a more complex and costly converter, known as a true RMS converter.

The RMS value of an alternating current is also known as its heating value, as it is a voltage which is equivalent to the direct current value that would be required to get the same heating effect. For example, if we applied 120 V AC RMS to a resistive heating element it would heat up by exactly the same amount as if we had applied 120V DC.

This principle was exploited in early thermal converters. The AC signal would be applied to a small heating element that was matched with a thermistor, which could be used in a DC measuring circuit.

The technique is not very precise but it will measure any waveform at any frequency (except for extremely low frequencies). A big drawback is that it is low-impedance: that is, the power used to heat the thermistor comes from the circuit being measured. If the circuit being measured can support the heating current, then it is possible to make a post-measurement calculation to correct the effect, as the impedance of the heating element is known. If the signal is small then a pre-amplifier is necessary, and the measuring capabilities of the instrument will be limited by this pre-amplifier. In radio frequency (RF) work, the low impedance is not necessarily a drawback since 50 ohm driving and terminating impedances are widely used.

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Wikipedia