Project Analog - February 2008 - (Page 18) mcP4725: 12-bit dac with EEProm in sot-23-6 CS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 do not track this closely in actual converters. fig. 1 thE data from this sErial adc clocKs msb out first and lsb last. CLK DOUT DD 11 10 D 9 D 8 D 7 D 6 D 5 D 4 D 3 D 2 D 1 D 0 MSB (LSB + 11) LSB (MSB + 11) The MCP4725 is a low-power, high accuracy, single channel, 12-bit buffered voltage output Digital-to-Analog Converter (DAC) with non-volatile memory (EEPROM). Its on-board precision output amplifier allows it to achieve rail-torail analog output swing. The MCP4725 has a two-wire I2C™ compatible serial interface for standard (100 kHz), fast (400 kHz) or high-speed (3.4 MHz) mode. The MCP4725 is an ideal DAC device where design simplicity and small footprint is desired. Applications • Set Point or Offset Trimming • Sensor Calibration • Closed-Loop Servo Control • Low Power Portable Instrumentation • PC Peripherals • Data Acquisition Systems Click here for more information on the MCP4725 converter that has an input full-scale range of 4.096V: offset error = ± 3LSB = ± 3 mV, gain error = ± 5LSB = ± 5 mV, These specifications actually claim that the converter can have (worst case) an 8 mV (or 8 code) error introduced through the conversion process. This is not to say that the error occurs at the LSB, LSB-1, LSB-2, LSB-3, LSB-4, LSB-5, LSB-6 and LSB-7 positions in the output-bit stream of the converter. The errors can be up to eight times one LSB, or 8 mV. Precisely stated, the transfer function of the converter could have up to eight codes missing out of 4,096 codes. These codes will be missing at the lower or upper range of the codes. For instance, a converter with an error of +8 LSB ((+3 LSB offset error) + (+5 LSB gain error)) will produce possible output codes of zero to 4,088. The lost codes are from 4088 up to 4,095. This is a small, incremental error of 0.2% at full-scale. In contrast, a converter with an error of -3 LSB ((-3 LSB offset error) – (-5 LSB gain error)) will produce codes from three up to 4,095. The gain error in this situation produces an accuracy problem, not a loss of codes. The lost codes are 0, 1 and 2. Both of these examples illustrate the worst possible scenario. Typically, the offset errors and gain errors The real-life performance enhancements, due to incremental improvements in an ADC’s offset or gain specifications, are negligible to non-existent. To some designers this seems like a bold assumption, if precision is one of the design objectives. It is easy to implement digital calibration algorithms with your firmware. However, more importantly, the front-end amplification/signal conditioning section of the circuit typically produces higher errors than the converter itself. This discussion puts a new light on the conclusions reached at the beginning of this article. In fact, the 12-bit converter, as specified above, has an accuracy of approximately 11.997 bits. The good news is that a microprocessor or microcontroller can remove this offset and gain error with a simple calibration algorithm. 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 • 18 · 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=1335&dDocName=en532229 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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