Project Analog - February 2008 - (Page 12) fig. 1 a blocK diagram of a 12-bit cmos sar a/d convErtEr. this convErtEr usEs a chargE distribution across a caPacitivE array. the digital-to-convert (DAC) and 12bit SAR are analog. The remaining portions of the circuit are digital. Therefore, most of the power and current needed for this converter is used for the internal analog circuitry. There is very little digital current coming from the device with the exception of the small amount of switching that occurs in the DAC and at the digital interface. These types of converters can have several pins for the ground and power connections. The pin names are often misleading in that the analog and digital connections can be differentiated with the pin label. These labels are not meant to describe the system connections to the PCB, but rather they identify how the digital and analog currents come off the chip. Knowing this information and understanding that the primary real estate consumed on the chip is analog, it makes sense to connect the power and ground pins on the same planes, e.g., analog planes. For instance, the pinout for a representative sample of 10-bit fig. 2 thE sar convErtEr, rEgardlEss of rEsolution, usually has at lEast two ground connEcts: agnd and dgnd. thE convErtErs illustratEd hErE arE thE mcP3201 and mcP3008 from microchiP. and 12-bit converters are shown in Fig. 2. With these devices, the ground is usually directed off the chip with two pins: AGND and DGND. The power is taken for a single pin. When implementing the PCB layout using these chips, the AGND and DGND should be connected to the analog ground plane. The analog and digital power pins should also be connected to the analog power plane or at least connected to the analog power train with proper bypass capacitors as close to each pin as possible. The only reason that these devices would have only one ground pin and one positive supply pin, as with the MCP3201, is due to package pin limitations. However, separate grounds enhance the probability of getting good and repeatable accuracy from the converter. With all of the converters, the power supply strategy should be to connect all grounds, positive supply and negative supply pins to the analog plane. In addition, the ‘COM’ pin or ‘IN’ pin associated with the input signal should be connected as close to the signal ground as possible. Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training 12 · ProjeCT ANALog · feb 08 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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