ADC10662CIWM datasheet(12/14 Pages) NSC

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Applications Information (Continued)

An A/D converter’s AC performance can be measured using

Fast Fourier Transform (FFT) methods. A sinusoidal wave-

form is applied to the A/D converter’s input, and the trans-

form is then performed on the digitized waveform. The re-

sulting spectral plot might look like the ones shown in the

typical performance curves. The large peak is the fundamen-

tal frequency, and the noise and distortion components (if

any are present) are visible above and below the fundamen-

tal frequency. Harmonic distortion components appear at

whole multiples of the input frequency. Their amplitudes are

combined as the square root of the sum of the squares and

compared to the fundamental amplitude to yield the THD

specification. Guaranteed limits for THD are given in the

table of Electrical Characteristics.

Signal-to-noise ratio is the ratio of the amplitude at the fun-

damental frequency to the rms value at all other frequencies,

excluding any harmonic distortion components. Guaranteed

limits are given in the Electrical Characteristics table. An al-

ternative definition of signal-to-noise ratio includes the distor-

tion components along with the random noise to yield a

signal-to-noise-plus-distortion ration, or S/(N + D).

The THD and noise performance of the A/D converter will

change with the frequency of the input signal, with more dis-

tortion and noise occurring at higher signal frequencies. One

way of describing the A/D’s performance as a function of sig-

nal frequency is to make a plot of “effective bits” versus fre-

quency. An ideal A/D converter with no linearity errors or

self-generated noise will have a signal-to-noise ratio equal to

(6.02n + 1.76) dB, where n is the resolution in bits of the A/D

converter. A real A/D converter will have some amount of

noise and distortion, and the effective bits can be found by:

where S/(N + D) is the ratio of signal to noise and distortion,

which can vary with frequency.

As an example, an ADC10662 with a 4.85 V

P-P, 100 kHz

sine

wave

input

signal

will

typically

have

signal-to-noise-plus-distortion ratio of 59.2 dB, which is

equivalent to 9.54 effective bits. As the input frequency in-

creases, noise and distortion gradually increase, yielding a

plot of effective bits or S/(N + D) as shown in the typical per-

formance curves.

8.0 SPEED ADJUST

The speed adjust pin is connected to an on-chip current

source that determines the converter’s internal timing. By

connecting a resistor between the speed adjust pin and

ground as shown in

Figure 4, the internal programming cur-

rent is increased, which reduces the conversion time. The

ADC10662 and ADC10664 are specified and guaranteed for

operation with R

SA = 14.0 kΩ (Mode 1) or RSA = 8.26k

(Mode 2). Smaller resistors will result in faster conversion

times, but linearity will begin to degrade as R

SA becomes

smaller (see curves).

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