Project Analog - February 2008 - (Page 13) mcP3221 sar adc The MCP3221 is a low power, tiny, 12-bit SAR Analog-to-Digital Converter(ADC) that is ideal for battery powered or portable data acquisition systems. Based on advanced CMOS architecture, this device draws only 180 µA with a 400 kHz I2C™ clock. With a 33 kHz I2C clock, the current drops to less than 30 µA. With only 1 µA standby current, the MCP3221 is currently the world’s lowest-power, 12-bit ADC in a small SOT-23A package. An on-chip conversion clock enables independent timing between the conversion clock, and the serial communication data rate. The device is also addressable, allowing up to eight devices on a single 2-wire bus. The MCP3221 runs on a single supply voltage that operates over a broad voltage range of 2.7V to 5.5V. Accuracy is superb, providing less than ±1 LSB of differential non-linearity (DNL) and less than ±2 LSB of integral non-linearity (INL). Click Here for more information on the MCP3221 is to try separating these pins into separate planes. Unfortunately, this tendency is misguided, particularly if you intend to solve critical noise problems with the 16-bit to 24-bit accuracy devices. With high-resolution DeltaSigma converters that have a 10 Hz data rate, the clock (internal or external) to the converter could be as high as 10 MHz or 20 MHz. This high frequency clock is used for switching the modulator and running the oversampling engine. With these circuits, the AGND and DGND pins are connected together on the same ground plane, as is the case with the SAR converter. Additionally, the analog and digital power pins are connected together, preferably on the same plane. The requirements on the analog and digital power planes are the same as with the high-resolution SAR converters. A ground plane is mandatory, which implies that a double-sided board is needed at minimum. On this doublesided board, the ground plane should cover at least 75% of the area if not more. The purpose of this ground plane layer is to reduce grounding resistance and inductance as well as provide a shield against electromagnetic interference (EMI) and radio-frequency interference (RFI). If circuit interconnect traces need to be put on the ground-plane side of the board, they should be as short as possible and perpendicular to the ground current return paths. cOncluSiOn You can get away without separating the analog and digital pins of low precision A/D converters, such as 6-, 8- or maybe even 10-bit converters. However, as the resolution/accuracy increases with your converter selection, the layout requirements also become more stringent. In both cases, with high-resolution SAR A/D converters and Delta-Sigma converters these devices need to be connected directly to the lower noise analog ground and power planes. 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 preciSiOn delta-Sigma layOut StrategieS The silicon area of the precision Delta-Sigma A/D converter is predominately digital. In the early days, when this type of converter was being produced, this shift in the paradigm prompted users to 13 · ProjeCT ANALog · feb 08 separate the digital noise from the analog noise by using the PCB planes. As with the SAR A/D Converter, these types of A/D converters can have multiple analogand digital ground and power pins. Once again, the common tendency of a digital or analog design engineer • http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010535 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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