Differing Structures of Successive Approximation Register (SAR) and Delta-Sigma Analog-to-Digital Converters (ADCs) Explained

Differing Structures of Successive Approximation Register (SAR) and Delta-Sigma Analog-to-Digital Converters (ADCs) Explained

In the realm of electronic product design, the choice of an Analog-to-Digital Converter (ADC) plays a crucial role. Two common types of ADCs, the Successive-Approximation Register (SAR) and the Delta-Sigma (ΔΣ) converter, each have their unique strengths and weaknesses.

SAR ADCs and ΔΣ ADCs: A Comparative Analysis

Resolution

Delta-Sigma ADCs typically provide higher resolution due to oversampling and noise shaping techniques, making them ideal for precision measurements and audio applications. SAR ADCs offer good resolution but generally lower than Delta-Sigma converters, balancing accuracy with speed.

Sampling Rate

SAR ADCs have much faster sampling rates suitable for applications requiring quick conversions because they use a successive approximation binary search method. Delta-Sigma ADCs have slower effective sampling rates as they rely on oversampling and digital filtering, making them better for signals that don't require very high speed.

Application Suitability

• SAR ADCs are suited for general-purpose, industrial, and consumer electronics where medium-to-high speed and moderate-to-high resolution are needed, such as multifunction I/O devices.
• Delta-Sigma ADCs excel in applications demanding high precision and noise immunity such as audio processing, vibration analysis, strain gauges, pressure sensors, and other low-frequency sensing applications requiring high resolution and excellent anti-aliasing filtering.

How They Work

SAR ADC

The design of a successive-approximation ADC includes a successive-approximation register (SAR) and a sample-and-hold (S/H) amplifier. The S/H amplifier contains a switch and a capacitor that continuously charge/discharge to the input voltage, and sample the input voltage at the start of a conversion. The SAR operates based on a special algorithm used by the SAR, with unique control logic setting or resetting the individual flip-flops in the SAR one bit at a time according to the comparator's output state.

ΔΣ ADC

The main circuit of a ΔΣ converter is the modulator, which produces a serial pulse train from the D flip flop (FF) whose density is proportional to the input voltage level. The serial output of the ΔΣ modulator is further processed in a DSP low-pass filter and a circuit called a decimator, which eliminates most of the high-frequency quantizing noise generated by the sampling process and reduces the number of output words generated by a factor called the decimation ratio.

In summary, the key differences between SAR and Delta-Sigma ADCs concern resolution, sampling rate, and application suitability. SAR ADCs are chosen when speed and moderate resolution are key, while Delta-Sigma ADCs are preferred when ultra-high resolution and noise performance are critical and lower speed can be tolerated.

| Feature | SAR ADC | Delta-Sigma ADC | |-------------------|--------------------------------------|-----------------------------------------| | Resolution | Moderate to high | Very high | | Sampling Rate | High (fast conversions) | Lower (due to oversampling/filtering) | | Typical Applications | Industrial control, general data acquisition, consumer electronics | Precision audio, vibration, strain, pressure measurements |

• The comparison between SAR ADCs and Delta-Sigma ADCs reveals that SAR ADCs, being equipped with faster sampling rates, are suitable for applications that necessitate quick conversions, such as multifunction I/O devices in general-purpose, industrial, and consumer electronics.
• On the other hand, technology like Delta-Sigma ADCs, known for their higher resolution and noise immunity, excels in applications requiring high precision, such as audio processing, vibration analysis, and other low-frequency sensing applications where exceptional anti-aliasing filtering is essential.

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