ADC Basics (Part 1): Does your ADC work in the real world?

In contrast, the SAR-ADC acquires a snap-shot of the analog signal. After acquiring this sample, the SAR converter uses an internal iterative process to finally determine the equivalent digital output value. The SAR-ADC’s output resolutions typically range from eight to 18 bits.

SAR converters are used for moderate speed conversions while providing medium-to-high resolutions. SAR converters are the back bone of general purpose application circuits that need to change analog signals to digital. The SAR converter resolution generally is lower than the delta-sigma converter. However, the SAR converter has a zero-cycle latency (or single cycle settling) while operating at higher speeds. SAR converters are used in many data acquisition applications like control loops, power monitoring, and low-to-medium frequency analysis. We will learn more about this converter and its inner workings in another future article.

SAR converters perform with zero-cycle latency and high DC / AC accuracy. These types of converters live well in low-power applications because they power-down automatically when not converting an analog signal. Today, the fastest sample rate of a SAR converter is approximately 5 MHz. However, this converter fills the gap in speed between the delta-sigma converter and the higher-speed converters, such as the pipeline.

Which converter is best for your application?
As you get ready to select your ADC for your application, Table 1 may be useful. This table compares the SAR and delta-sigma converter families in terms of conversion frequency and converter resolution.

The maximum conversion rate of SAR converters on the market today is around 5 Msps. Their resolution can go as high as 18 bits. However, the majority of SAR ADCs in applications across industry are 8- to 12-bit converters. The conversion rate of a delta-sigma converter is generally below 625 ksps. With this speed it is possible to have a converter that produces up to 24 bits of data. The resolution for delta-sigma devices, that convert a less than 10 Msps, is lower that its 24-bit cousin.  

Table 2 ranks these two architectures for throughput, resolution, latency, and power consumption.


Table 2 shows that the SAR converter is a better work horse in terms of speed (throughput), low-latency, multiplexing, and power consumption. The distinct advantage of the delta-sigma converter over the SAR converter is high resolution.

Conclusion

We initiated this series of articles with a general overview of the frequency and resolution ranges of sensors, then compared that to the SAR and delta-sigma converters. As you make temperature, pressure, or optical measurements, keep in mind that the converter architectures of choice are the SAR and delta-sigma. In next month’s article, I will dig into the details for what a delta-sigma converter is and how it is successful in generating very high resolution.

References

1. Baker, B. “Temperature Sensing Technologies,” Application note (DS00679A), Microchip Technology, 1998
2. “Understanding data converters,” Application Report (SLAA013), Texas Instruments, 1995.
3. Baker, B. “A Baker's Dozen: Real analog solutions for digital designers.” Burlington, MA: Elsevier/Newnes, 2005.

About the Author

Bonnie Baker is a Senior Applications Engineer with the WEBENCH team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. In addition to her fascination with circuit design, Bonnie has a drive to share her knowledge and experience. She has written hundreds of articles, design and application notes, conference papers, and authored a book: “A Baker’s Dozen: Real Analog Solutions for Digital Designers.” Bonnie can be reached at ti_bonniebaker@list.ti.com.