A True-RMS or TRMS is a type of converter which converts RMS value to equivalent DC value. Here in this tutorial, we will learn about true RMS to DC converter, how it works and how measurement methods can affect displayed results.

The IC AD736 has few functional subsections like the input amplifier, full-wave rectifier (FWR), RMS core, output amplifier, and bias section. The Input amplifier is constructed with MOSFETs, so it is responsible for the high impedance of this IC.

After the input amplifier, there is a precision full-wave rectifier which is responsible for driving the RMS core. The essential RMS operations of squaring, averaging, and square rooting are performed in the core with the help of an external averaging capacitor CAV. Please note that without CAV, the rectified input signal travels through the core unprocessed.

Finally, an output amplifier buffers the output from the RMS core and allows optional low-pass filtering to be performed via the external capacitor CF, which is connected across the feedback path of the amplifier.

True RMS to DC Measurements Methods

There are mainly three methods available which DVM’s use to measure AC, they are-

True-RMS Measurement 

Average Rectified Measurement

True-RMS AC+DC Measurement 

True-RMS Measurement 

True-RMS is a pretty common and popular method to measure dynamic signals of all shapes and sizes. In a True-RMS multimeter, the multimeter calculates the RMS value of the input signal and shows the result. This is why it's a very accurate compare to an average rectified measurement method.

Average Rectified Measurement

In an average rectified DVM, it takes the average or the mean value of the input signal and multiplies it by 1.11 and displays the RMS value. So, we can say that it's an average rectified RMS display multimeter.

True-RMS AC+DC Measurement

To overcome the loopholes in a True-RMS multimeter, there exists the True-RMS AC+DC measurement method. If you were to measure a PWM signal with a True-RMS multimeter,

Value of Resistor R1: 50.75K

Value of Resistor R1: 220R

Input Voltage on the Arduino board

Now put these values in an online voltage divider calculator and calculate, we will get the output voltage of 0.02141V OR 21.41mV.

This is the peak voltage of the input PWM signal and to find the RMS voltage, we need to simply divide it by √2 so the calculation becomes

VRMS = Vm/√2 = 0.02141/√2 = 0.01514V or 15.14mV