ADC0801 datasheet(17/41 Pages) NSC

17 / 41 page

Functional Description (Continued)

Figure 2 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 guaranteed 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 guar-

anteed to be no more than 1⁄2 LSB.

The error curve of

Figure 3 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 con-

tinuously displayed and includes the quantization uncertainty

of the A/D. For example the error at point 1 of

Figure 1 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 upside steps are

always 1 LSB in magnitude.

Transfer Function

DS005671-81

Error Plot

DS005671-82

FIGURE 1. Clarifying the Error Specs of an A/D Converter

Accuracy=±0 LSB: A Perfect A/D

Transfer Function

DS005671-83

Error Plot

DS005671-84

FIGURE 2. Clarifying the Error Specs of an A/D Converter

Accuracy=±1⁄4 LSB

ADC0801 Datasheet(PDF) 17 Page - National Semiconductor (TI)