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Driving the Analog Inputs of a SAR A/D Converter

From Microchip Technology, Inc.
 

 
Driving any A/D Converter (ADC) can be challenging if all issues and trade-offs are not well understood from the beginning. With Successive Approximation Register (SAR) ADCs, the sampling speed and source impedance should be taken into consideration if the device is to be fully utilized. In this application note we will delve into the issues surrounding the SAr Converter's input and conversion nuances to insure that the converter is handled properly from the beginning of the design phase. We will also review the specifications available in most A/D Converter data sheets and identify the important specifications for driving your SAR. From this discussion, techniques will be explored which can be used to successfully drive the input of the SAR A/D Converter. Since most SAR applications require an active driving device at the converter's input, the final subject will be to explore the impact of an operational amplifier on the analog-to-digital conversion in terms of DC as well as ac responses. A typical system block diagram of the SAR converter application is shown in Figure 1. Some common SAR converter systems are Data Acquisition Systems, Transducers Sensing Circuits, Battery Monitoring applications and Data Logging. In all of these systems, DC specifications are important. Additionally, the required conversion rate is relatively fast (as compared to Delta-Sigma converters) and having a lower number of bits that are reliably converted is acceptable.
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Topics of Interest
Bonnie Baker In This Chapter The analog-to-digital converter (ADC) is always in the back seat of the station wagon, looking at the analog signal through the rear window. In a way, I am soft on... (Read More)
Many of Silicon Lab's devices feature an on-chip SAR analog-to-digital converter (ADC). These ADC's use a sample capacitor that is charged to the voltage of the input signal that is used by the SAR... (Read More)
The CMOS SAR Topology The CMOS SAR ADC is a sampling system that takes one sample for every conversion. The analog input signal to a SAR converter first sees a switch and a capacitive array, as... (Read More)
9.8 Analog-to-Digital Conversion: Sigma-Delta The operation of sigma-delta ADCs is entirely different from successive-approximation ADCs. Their distinctive characteristics are high precision and low... (Read More)
Successive-Approximations Register (SAR) architecture can be thought of as being at the other end of the spectrum from the flash architecture. While a flash converter uses many comparators to convert... (Read More)