Datasheet

ADC0801, ADC0802

ADC0803, ADC0804, ADC0805

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SNOSBI1B –NOVEMBER 2009–REVISED FEBRUARY 2013

FUNCTIONAL DESCRIPTION

Understanding A/D Error Specs

A perfect A/D transfer characteristic (staircase waveform) is shown in Figure 45. The horizontal scale is analog

input voltage and the particular points labeled are in steps of 1 LSB (19.53 mV with 2.5V tied to the VREF/2 pin).

The digital output codes that correspond to these inputs are shown as D−1, D, and D+1. For the perfect A/D, not

only will center- value (A−1, A, A+1, . . . . ) analog inputs produce the cor- rect output digital codes, but also each

riser (the transitions between adjacent output codes) will be located ±1⁄2 LSB away from each center-value. As

shown, the risers are ideal and have no width. Correct digital output codes will be provided for a range of analog

input voltages that extend ±1⁄2 LSB from the ideal center-values. Each tread (the range of analog input voltage

that provides the same digital output code) is therefore 1 LSB wide.

Figure 46 shows a worst case error plot for the ADC0801. All center-valued inputs are guaranteed to produce the

correct output codes and the adjacent risers are specified to be no closer to the center-value points than ±1/4

LSB. In other words, if we apply an analog input equal to the center-value ±1/4 LSB, we guarantee that the A/D

will produce the correct digital code. The maximum range of the position of the code transition is indicated by the

horizontal arrow and it is specified to be no more than 1/2 LSB.

The error curve of Figure 47 shows a worst case error plot for the ADC0802. Here we guarantee that if we apply

an analog input equal to the LSB analog voltage center-value the A/D will produce the correct digital code.

Next to each transfer function is shown the corresponding error plot. Many people may be more familiar with

error plots than transfer functions. The analog input voltage to the A/D is provided by either a linear ramp or by

the discrete output steps of a high resolution DAC. Notice that the error is continuously displayed and includes

the quantization uncertainty of the A/D. For example the error at point 1 of Figure 45 is +1⁄2 LSB because the

digital code appeared 1⁄2 LSB in advance of the center-value of the tread. The error plots always have a

constant negative slope and the abrupt up- side steps are always 1 LSB in magnitude.

Figure 45. Clarifying the Error Specs of an A/D Converter Accuracy=±0 LSB: A Perfect A/D

Figure 46. Clarifying the Error Specs of an A/D Converter Accuracy =±1⁄4 LSB

Product Folder Links: ADC0801, ADC0802 ADC0803, ADC0804, ADC0805