Project Analog - February 2008 - (Page 17) The Number of bits vs. LSb errors Contributed by Microchip Technology www.microchip.com Contents Viewpoint Digital potentiometer application circuits the converter finally transmits as MSB-11 (or LSB). The terminology, LSB, is very specific. It describes the last position in the digital stream. It also represents a fraction of the full-scale input range. For a 12-bit converter, the LSB value is equivalent to the analog full-scale input range, divided by 212 or 4,096. If I put this in terms of real numbers, I have an LSB size of 1 mV with a 12-bit converter that has full-scale input range of 4.096V. However, the most instructive definition of LSB is that it can represent one code out of the 4096 codes possible. Going back to the specifications and translating them into a 12-bit W hat do the Least Significant Bits (LSB) specifications mean when you are looking at Analog-toDigital converters (ADC)? A fellow engineer told me that a 12-bit converter, from X manufacturer, had just seven usable bits. So, essentially the 12-bit converter was only a 7-bit converter. He based this conclusion on the device’s offset and gain specifications. The maximum specifications were: offset error = ± 3 LSB, gain error = ± 5 LSB, At first glance, I thought he was right. From the list above, the worst specification is gain error (± 5 LSB). Applying simple mathematics, 12 bits minus 5 bits of resolution is equal to 7 bits, right? Why would an ADC manufacturer introduce such a device? The gain-error specification motivates me to purchase a lowercost, 8-bit converter, but that doesn’t seem right. Well, as it turns out, it wasn’t right. Let’s start out by looking at the definition of LSB. Think of a serial 12-bit converter, it produces a string of 12 ones or zeros. Typically, the converter’s first transmitted digital bit is the Most Significant Bit (MSB) (or LSB + 11). Some converters transmit the LSB first. We will assume that the MSB is first in this discussion (shown in Figure 1). The second bit is MSB-1 (or LSB+10), the third bit is MSB-2 (or LSB+9), etc. At the end of this string of bits, 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 17 · ProjeCT ANALog · feb 08 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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