有效值计算 - 模拟电路如何高效有效值直流转换

图3：隐式(Implicit)有效值计算

　　这种有效值电路可以采用AD734模拟乘法器构建，若使用专用的有效值直流电路则更加简单。图4显示典型单芯片有效值直流转换器AD536A的简单示意图。

　　图4：AD536A单芯片有效值直流转换器

　　该图分为四个主要部分：绝对值电路(有源整流器)、平方器/除法器、电流镜，以及缓冲放大器。输入电压VIN可以为交流或直流，由绝对值电路A1、A2转换为单极性电流I1。I1驱动一象限平方器/除法器的一路输入，传递函数为：I4 = I12/I3。平方器/除法器的输出电流I4驱动电流镜，通过由R1和外接电容CAV构成的低通滤波器。如果R1CAV时间常数远远大于输入信号的最长周期，则I4可以有效取平均值。电流镜返回的电流I3等于AVG[I4]，回到平方器/除法器，以完成隐式有效值计算。因此：

　　I4 = AVG [I12/I4] = I1rms 公式 1

　　电流镜还会产生输出电流Iout，相当于2I4。Iout可以直接使用，或通过R2转换为电压，然后由A4缓冲，产生低阻抗电压输出。传递函数变为：

　　Vout = 2R2•Irms = VIN rms 公式2

　　The dB output is derived from the emitter of Q3, since the voltage at this point is proportional to–logVIN. Emitter follower, Q5, buffers and level shifts this voltage, so that the dB output voltageis zero when the externally supplied emitter current (IREF) to Q5 approximates I3. However, thegain of the dB circuit has a TC of approximately 3300 ppm/°C and must be temperaturecompensated.

　　There are a number of commercially available rms-to-dc converters in monolithic form whichmake use of these principles. The AD536A is a true rms-to-dc converter with a bandwidth ofapproximately 450 kHz for Vrms > 100 mV rms, and 2 MHz bandwidth for Vrms > 1 V rms. TheAD636 is designed to provide 1 MHz bandwidth for low-level signals up to 200 mV rms. TheAD637 has a 600 kHz bandwidth for 100 mV rms signals, and an 800 MHz bandwidth for 1 Vrms signals. Low cost, general purpose rms-to-dc converters such as the AD736 and AD737(power-down option) are also available.