Inexpensive analog isolation using a digital isolator

Isolation is needed now more than ever with applications like LED lighting, brushless motors, power monitoring, and many others that are a combination of direct offline power electronics with isolated control electronics. These applications typically use a very inexpensive analog voltage control for speed or intensity adjustments, whether through a potentiometer or with an analog interface like 0 to 10V.

An isolated operational amplifier (op amp) is too expensive for most of these high-volume applications. Digital isolation is more cost-effective, but typically requires an analog-to-digital converter (ADC) and multiple digital isolators. This article shows a cost-optimized solution for moving analog across the isolation barrier.

A digital isolator takes a digital input and isolates it, but cannot pass an analog signal. The output is either a 1 or a 0. The simplified digital isolator is just a logic level shifter that adds isolation. Typically a digital isolator is used in an application like this to first convert the analog signal to digital, and then send the digital signal using these isolators.

This method adds cost in the ADC and requires multiple digital isolators for the clock, data, chip select, and any other I/O required to control the ADC, which also adds cost. A single digital isolator can be used, if we can convert the analog data to logic level through a pulse-width-modulated (PWM) signal. The PWM represents a linear voltage by the percentage of time the waveform is in on versus off mode. Duty cycle is the percentage of on time to a given period of time.

The PWM is a cross between analog and digital because you are not sending true digital data like a typical parallel or serial interface, but the signal is either high or low with nothing in between. The PWM deals with timing instead of voltage so any common-mode voltage shift across the isolation barrier does not affect the signal. The PWM signal will be very robust to noise since it is either a 0 or a 1 instead of a small analog voltage.

A class-D amplifier is a very inexpensive way to convert analog to PWM. A class-D amplifier block diagram is shown in Figure 1. The class-D amplifier takes an analog voltage and outputs a pulse-width modulated signal. We choose the TPA2006D1 for this design because it is very inexpensive, small, and has very few external components. The class-D amplifier outputs 50 percent duty cycle for IN+ = IN–. The duty cycle is less than 50 percent for IN+ < IN– down to zero percent and greater than 50 percent for IN+ > IN– up to 100 percent.

Figure 1: Class-D amplifier block diagram.