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
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
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.
Table 1. Conversion frequency and resolution for delta-sigma and SAR ADCs.
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.
ranks these two architectures for throughput, resolution, latency, and power consumption.
Table 2. A ranking of characteristics between the SAR and delta-sigma ADCs.
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.
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.
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
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 firstname.lastname@example.org