The effective value (rms) measures the amplitude of the AC signal. From a practical point of view, the rms value of the AC signal is equal to the amount of DC required to generate the same amount of heat from the same load. From a mathematical point of view, the voltage rms value is the value obtained by squaring the square, averaging, and then taking the square root. The averaging time must be long enough to filter at the lowest required operating frequency. We will use some examples to illustrate how an analog circuit can efficiently implement this function. The first method is called direct explicit, as shown in Figure 1. The input signal is first squared by an analog multiplier, then averaged using an appropriate filter, and then taken through the square root of the op amp with the second squarer in the feedback loop. The circuit has a limited dynamic range because the stages after the squarer must attempt to process signals with widely varying amplitudes. This limits this method so that the maximum dynamic range of the input is approximately 10:1 (20 dB). However, if you can use a multiplier such as the AD834 as a building block, you can achieve excellent bandwidth (above 100 MHz) and high precision (see Figure 2). Figure 1: Explicit RMS calculation Figure 2: Wideband rms measurement with the AD834 analog multiplier Figure 3 shows the circuit for calculating the rms value of the signal using the implicit method. Here, the output is fed back to the direct divider input of a multiplier such as the AD734. In this circuit, the output of the multiplier varies linearly with the effective value of the input (rather than squared). This implicit circuit significantly expands the dynamic range of the input compared to explicit circuits. If the input flows through the absolute value circuit first, the VIN2/VZ circuit can be driven by current and only requires one quadrant. The disadvantage of the implicit rms dc method is that its bandwidth is usually less than the explicit calculation. Instrument Controller,Pressure Sensor,Temperature And Humidity Sensor,Control Pressure Sensor Xiaogan Yueneng Electronic Technology Co., Ltd. , https://www.xgsensor.com
March 03, 2024