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Bits, Noise, and Linearity; the Imperfections of ADCs

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Differential vs. single-ended

To what is a potential measurement referenced? Think of a battery you hold in your hand. The difference in potential across the battery is 1.5 V or 9 V or whatever its rating may be. But which end is 0? NEITHER! Either end can be DEFINED to be zero, but neither end is inherently the zero reference! Typically, one defines zero as the potential of the watertable underlying the structure in which an experiment is performed. Most ADCs allow the user to select whether the defined/ground connection from the wall plug is the measurement reference (single-ended input), or if the difference between the high and low sides of a potential difference are to be measured independent of the wall ground (double-ended or differential input). Typically, ADC cards are wired such that one can measure twice as many single-ended as differential inputs. Because common mode noise (noise that effects both the signal to be measured and the reference against which it is measured) is common (for example, from power line noise), differential input is usually quieter than single-ended.



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