Not All ADCs Are Created Equal

Nearly every electronic product on the market contains an analog-to-digital converter (ADC). With all the wiz-bang features that some of these devices sport, the importance of ADCs can be overlooked. But one of the first decisions a designer must make is the type of ADC best suited for the application. The real challenge becomes sorting through the various types of ADCs and choosing the most appropriate converter. This decision is critical because it often determines the overall performance of the product and the success of the design.

To make the best decision, the design engineer must make tradeoffs, juggling resolution, sampling rate, accuracy and power dissipation to achieve the optimum balance to meet the application’s requirements. This calls for a solid understanding of the strengths and weaknesses of each type of ADC. To help with that, let’s look at the three main types of ADCs: The successive approximation register (SAR), sigma-delta and pipeline.

SAR Converters

The SAR converter is one of the oldest and most widely used ADC architectures on the market. Designers often pick it for medium-to-high-resolution applications, such as motor control, vibration analysis and system monitoring.

The architecture offers significant operational flexibility. Resolutions range from 6 to 18 bits, and the SAR typically operates between a few kilosamples per second to as high as 10 megasamples per second. While they are not as fast as pipeline converters, SAR ADCs are typically faster than sigma-delta converters. The high throughput rate of a SAR converter enables oversampling, which improves anti-aliasing and noise reduction.

Another strength of the SAR architecture is its ability to take a high-speed snapshot of an analog input signal. SAR converters sample one moment in time. This is useful when a designer must measure multiple signals simultaneously, which can be achieved by simultaneously sampling with multiple SAR ADCs.

This type of converter also brings key features to wearable, mobile and other low-power applications. SAR converters scale power dissipation directly with the sample rate. Because of their low power consumption, high resolution and small form factor, these ADCs can often be integrated with other larger functions. The architecture’s main limitation is its lower sampling rate.

Sigma-Delta ADCs

Sigma-delta converters entered the marketplace when digital signal processing became practical. While complex, this signal conversion architecture offers the greatest resolution of any ADC, complemented with noise mitigation.

The analog portion of the architecture is simple. The digital side, on the other hand, is more complex. Essentially, these converters consist of an oversampling modulator and a digital/decimation filter that together produce a high-resolution output. The converter’s low-pass filter eliminates most of the high-frequency noise generated by the sampling process.

Sigma-delta ADCs are ideal for converting analog signals over a broad spectrum of frequencies, ranging from DC to several megahertz. The downside of the design is that it is slower than other types of ADCs, so designers typically use sigma-delta converters only in applications with DC and audio frequencies.

Sigma-delta ADCs are well suited for low-noise, precision applications. When speed is not an issue, the oversampling of a sigma-delta converter provides very high precision. This makes them a good fit for communications systems, precision measurements and audio applications.

Pipeline ADCs

Pipeline ADCs deliver speed at the expense of power and latency. These converters use a parallel structure in which each stage processes one to a few bits of successive samples simultaneously. For optimum performance, these converters require accurate amplification in the ADC and interstage amplifiers. Pipeline ADCs offer speeds of 100 megasamples per second at 8- to 14-bit resolutions. With these sampling rates, the interface becomes critical. Parallel digital has long been the interface of choice, but new interfaces have begun to appear on the market.

With these resolutions and sampling rates, pipeline converters can serve a broad spectrum of applications. These range from RF and software-defined radios to CCD (charge-coupled device) imaging and ultrasonic medical imaging.

If you are looking for cut-and-dry guidelines for choosing an ADC, you’re going to be disappointed. The selection process is a balancing act in which the designer and the application determine the importance of speed, accuracy and power.