Weighing in with SAR Converters
By eeProductCenter Senior Technical Editor, Paul O'Shea
In the world of analog-to-digital converters (A/D converters) there are several designs used to convert analog signals to digital, including successive approximation register (SAR), sigma-delta, pipeline, flash, and others. The SAR A/D converter, however, is one of the most popular architectures because of its excellent resolution, low power, and its small package size.
How does the SAR converter achieve these superior features? Certainly, its design and operation are important. Texas Instruments uses the example of a balance scale to describe the operation of the SAR converter. For example, with a balance scale you take an unknown weight and place it on one side of the balance point, and then you take known weights and place them on the other side until the two sides are perfectly balanced. The unknown weight then can be measured by totaling up the kept, known weights. Extending that idea to a SAR converter, the input signal would be analogous to the unknown weight, and it would be sampled and held. This voltage then is compared to successive known voltages, and the results are output by the converter. Unlike the weighing scale, conversion occurs very quickly through the use of a capacitive charge redistribution technique.
The capacitive charge technique isn't the only the only SAR architecture; there is another older design that uses a resistive ladder technique. However, the older resistive type SAR A/D converter isn't used as much as the more popular charge distribution design. The resistive ladder technique was abandoned because it burns up too much power and it takes up too much space compared to the capacitive or charge distribution SAR converter design. Additionally, the resistive design doesn't have a sample and hold amplifier so it needs a separate sample and hold amplifier circuit. By comparison, today's capacitive SAR A/D converter uses a sample capacitor that is charged to the voltage of the input signal. Due to the converter's input capacitance, input impedance, and external circuitry, a settling time is required for the sample capacitor's voltage to match the measured input voltage. However, the settling time can be minimized with the source impedance of the external circuitry, but you must ensure that the input signal is accurately acquired within the converter's acquisition time.
“Compared to other A/D converters,” says Wayne Talley, Product Marketing Manager for Precision Nyquist Converters at Analog Devices, “the SAR A/D converter fits between the sigma-delta and the pipeline architectures. It's not as accurate as many sigma-delta type converters and not as fast as the pipeline architecture. However, it gives a nicer blend of both worlds,” he said.
Some SAR A/D converters provide up to 18-bits of accuracy and sampling rates to 4 mega samples per second (MSPS). For example, the AD7686 from Analog Devices provides a competitive 16-bits resolution and 550 kSPS but also offers 16-bits of no missing code, which means all the output code will get to the A/D converter's output. Just before this article went to press, ADI introduced the even more impressive AD7621 SAR converter, which pushes the envelope with a multitude of leading specs including 3 MSPS, 16 bits of no missing code, and a signal to noise plus distortion spec of 90 dB.
If more speed is what you need then you should look at the ADS7881 from TI. It offers 4 MSPS with 12-bit no missing code and 1 LSB linearity. The company says it's the only device available to address high-speed data acquisition by providing an input signal frequency as high as 1.8 MHz.
If you compare the SAR architecture to some similar speed sigma-delta converters you will find that the integral non-linearity (INL), which is a measure of the deviation of each individual code, is typically not as good as the SAR converter. Take a look again at TI's ADS7881; it also has an impressive INL error spec of only 1 LSB to go along with its speed, something you would be hard pressed to find in other architectures.
So, why would a designer choose a sigma-delta instead of a SAR architecture? According to Mr. Talley it's the 24-bit no missing code resolution and the 16-million code of dynamic range. However, no one needs that level of dynamic range. Designers only use a small portion of that dynamic range. Unlike the SAR converter, the 24-bit no missing code sigma-delta converter can directly interface to a thermocouple without having to use a programmable gain amplifier for boosting the signal. However, designers really only need to toggle about 200 mV, and if the temperature output is 200 mV then the SAR converter can provide all the resolution they need.
The combination of resolution, low power, and small size are certainly impressive features of SAR converters. For example the MAX1066 from Maxim offers the low power of 3.2 mA at the highest conversion rate but only 10 micro amps at slow data rates, which is just right for industrial process control and medical applications. If you have a portable application then real estate is important and the less the better. For small size you might want to look at the AD7680 from Analog Devices. It's available in a SOT-23 package which takes up only 3 mm x 3 mm compared to the more typical 3 mm x 5 mm of the MSOP-8 package. Another possibility is the ADCS7476 from National Semiconductor, which also is available in the 6-lead SOT-23 package and offers a serial interface that's compatible with several standards such as SPI, QSPI, Microwire and many common DSP serial interfaces.
The way a converter handles signals is another important characteristic. The SAR A/D converter samples the input signal and holds the sampled value until conversion is complete, which means it does not make any assumptions about the nature of the input signal, and therefore, the signal does not need to be continuous. The ability to handle non-continuous signals makes the SAR architecture ideal especially for applications where a multiplexer is used prior to the converter; the converter is used to make a measurement every few seconds, or where a fast measurement is required. Application examples would include data acquisition and portable medical products. The LTC 1865L is a good fit for a portable design because it draws only 450 micro amps from a 3-V supply and it's available in 10-pin MSOP and 8-pin SO packages. Its auto shutdown feature also helps reduce the supply current to 3 micro amps at low sampling rates.
TI says that a more troublesome design constraint of the SAR converter is the dynamic load that the converter's input presents to the driving circuitry. The op amp driver of the converter input must be able to handle this dynamic load and settle to the desired accuracy within the required acquisition time. The SAR converter's reference input circuitry presents a similar load to the reference voltage. The reference voltage is supposed to be a very stable DC voltage, but the dynamic load that the converter's reference input presents, makes achieving this goal somewhat difficult. That means that a buffer circuitry is required for the reference voltage. The op amp used has similar requirements as those used for driving the input of the A/D converter. Some SAR converter designs use an internal reference buffer. TI says that buffering these inputs using op amps with low, wideband output impedance is the best way to preserve accuracy with these converters.
Another important but often overlooked aspect of the SAR A/D converter compared to other architectures is its ease-of-use. Many designers are not comfortable with sigma-delta designs because with typical sigma-delta converters you have to be aware of decimation ratios and the digital filtering used. Most designers just want something easy to use. They don't want to worry about these criteria. A SAR converter works without this concern for filtering. There's no need to do averaging unless you want to filter out noise. It's not required to filter noise as it is on the sigma-delta design.
The SAR converter may not be the fastest sampling converter but it has made significant improvements in the past five years that make it a good choice for real-time applications such as industrial control, motor control, power management, as well as the portable and battery-powered instruments, test equipment and data or signal acquisition environments.
Analog Devices, Inc.
Tel: 1/800-ANALOGD (262-5643)
Tel: (321) 724-7000
Linear Technology Corp.
Maxim Integrated Products, Inc.
Tel: 408-737-7600 or 800-998-8800
National Semiconductor Corp.
Texas Instruments Inc.
Tel: 800-477-8924, ext. 4500