Project Analog - February 2008 - (Page 11) Layout Techniques for High Accuracy and resolution ADCs Contributed by Microchip Technology www.microchip.com Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news I nitially, analog-to-digital (A/D) converters rose from an analog paradigm where a large percentage of the physical silicon was analog. As the progression of new design topologies evolves, this paradigm shifted to where slower speed A/D converters were predominately digital. Even with this on-chip shift from analog to digital, the PCB layout practices have not changed. Now as always, when the layout designer is working with mixed signal circuits, key layout knowledge is still needed in order to implement an effective layout. Sar cOnverter layOut SAR A/D converters can be found with 8-bit, 10-bit, 12-bit, 16-bit and sometimes 18-bit resolution. Originally, the process and architecture for these converters 11 · ProjeCT ANALog · feb 08 was bipolar with R-2R ladders. Recently these devices have migrated to a CMOS process with a capacitive charge distribution topology. The system layout strategy for these converters has not changed with this migration. The basic approach to layout is consistent except for higher resolution devices. These devices require more attention to the prevention digital feedback from the serial or parallel output interface of the converter. The SAR converter is predominately analog in terms of circuitry and the amount of real estate dedicated to the different domains on the chip. In Fig. 1, a block diagram of a 12-bit CMOS SAR converter is shown. Within this block diagram the Sample/Hold, comparator, most of Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training http://www.microchip.com http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mixedsignal http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423
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