AID converter for display. This method responds to the power dissipated in the resistors; therefore, it responds to the true rms value of the input signal including all of its harmonics.
In the log/antilog analog method, Fig. 5.19, the output of the range amplifier is a representation of the input voltage, ACVmcas. This voltage is applied to the input of the absolute-value amplifier. Positive and negative input cycles are converted to a pulsating dc at point 1. (Diamonds in the figure represent test points.) A voltage proportion al to the square of the input current is supplied to point 2 by the squaring amplifier. A current proportional to the voltage at point 2 is supplied to point 3.The output voltage of the integrator, V0 has a value proportional to the average level of the pulsating dc
current supplied to point 3 by Q3. Since the input current at point I is a direct function of the voltage being measured, and the output voltage is a direct function of the aver age square of the current at point 1, the digitized output V0 represents the true rms value of the input voltage.
Digital sampling, shown in Fig. 5.20, samples the input waveform during short intervals of time relative to the period of the input waveform. A fast A/D converter quickly digitizes each sample. Its value is squared by the microprocessor, and the result is stored in memory. When a sufficient number of samples have been taken, the microprocessor takes the square root of the average value of the squares. Mathematically the rms value of the samples in the sampling technique is expressed as