Envelope Filter

One of the ways to discern the overall amplitude of an audio signal is to use something called an envelope detector. This type of circuit is often used in portable radios to demodulate incoming signals that ride on a carrier frequency such as in AM implementations.

The sample circuit shows that the negative portion of the signal, with respect to virtual ground, is chopped off and fed into a resistor-capacitor circuit. This RC circuit provides the means necessary for the device to average the current amplitude value with past amplitude values for the analog to digital converter on the other end. By doing so, the voltage across the capacitor models that of the overall amplitude and not the individual amplitudes of the frequencies that make up the signal itself. The response of this circuit is governed by the values of the resistor, capacitor, and impedance of the operational amplifier with negative feedback.

One thing to note is that although sampling is done only on the upper half of the waveform, the overall response of the circuit to the audio signal will be representative of both the upper and lower halves since the they are similar to one another in amplitude over the long term. A more immediate concern comes in the form of the diode's forward voltage drop on the signal itself that is due to to the internal built-in voltage of the PN junction. Because of these concerns, there are a couple of methods that fully rectifies both halves of the signal and avoids the forward voltage drop of the diode as shown here.

Analog to Digital Converter Range

It was not until a few weeks after having constructed the final circuit that I came up with the idea of how to get rid of the offset afforded by the virtual ground. The micro controller used in this project did not have a V- reference built in for the ADC and, as such, I decided to only sample within the upper half of the ADC's range. Since this method essentially wastes the lower half range of the ADC, an alternative is presented here to allow full utilization. Note that although adding this block will allow for full utilization of the ADC, it won't be needed in our case because the precision of the ADC used more than makes up for it.

The following circuit uses all the theory presented in this project and thus should be quite familiar. The input to this circuit should be from the output of the initial amplifier stage while the output should be connected to the envelope filter.

The above circuit simply removes the DC component of the signal, the virtual ground reference, and amplifies the upper half of the AC portion. Since the incoming signal to be amplified no longer has a "negative" voltage, the reference point can now be the Vss of the circuit. The gain is set at two because the signal coming out of this circuit essentially has about two times more headroom than the signal coming into the circuit due to the removal of the offset when set at Vdd/2.