Understanding SAR ADCs: their architecture and comparison with other ADCs - AN1080

Successive-approximation-register (SAR) analog-to-digital converters (ADCs) represent the majority of the ADC market for mediumto high-resolution ADCs. SAR ADCs provide up to 5Msps sampling rates with resolutions from 8 to 18 bits. The SAR architecture allows for high-performance, low-power ADCs to be packaged in small form factors for today's demanding applications. This paper will explain how the SAR ADC operates by using a binary search algorithm to converge on the input signal. It also explains the heart of the SAR ADC, the capacitive DAC, and the high-speed comparator. Finally, the article will contrast the SAR architecture with pipeline, flash, and sigma-delta ADCs. Introduction Successive-approximation-register (SAR) analog-to-digital converters (ADCs) are frequently the architecture of choice for mediumto-high-resolution applications with sample rates under 5 megasamples per second (Msps). Resolution for SAR ADCs most commonly ranges from 8 to 16 bits, and they provide low power consumption as well as a small form factor. This combination of features makes these ADCs ideal for a wide variety of applications, such as portable/battery-powered instruments, pen digitizers, industrial controls, and data/signal acquisition. As the name implies, the SAR ADC basically implements a binary search algorithm. Therefore, while the internal circuitry may be running at several megahertz (MHz), the ADC sample rate is a fraction of that number due to the successive-approximation algorithm. SAR ADC architecture Although there are many variations for implementing a SAR ADC, the basic architecture is quite simple (see Figure 1). The analog input voltage (VIN) is held on a track/hold. To implement the binary search algorithm, the N-bit register is first set to midscale (that is, 100... .00, where the MSB is set to 1). This forces the DAC output (VDAC) to be VREF/2, where VREF is the reference voltage provided to the ADC. A comparison is then performed to determine if VIN is less than, or greater than, VDAC. If VIN is greater than VDAC, the comparator output is a logic high, or 1, and the MSB of the N-bit register remains at 1. Conversely, if VIN is less than VDAC, the comparator output is a logic low and the MSB of the register is cleared to logic 0. The SAR control logic then moves to the next bit down, forces that bit high, and does another comparison. The sequence continues all the way down to the LSB. Once this is done, the conversion is complete and the N-bit digital word is available in the register.