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Driving the Analog Inputs of a SAR A/D Converter
Bonnie C. Baker
p>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.

SAR, a/d converter, analog inputs, ADC
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