ADC10461 datasheet(14/16 Pages) NSC

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Applications Information (Continued)

acteristics such as signal-to-noise ratio (SNR) and total har-

monic distortion (THD), are quantitative measures of this

capability.

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

fundamental frequency to the rms value at all other frequen-

cies, excluding any harmonic distortion components. Guar-

anteed limits are given in the Electrical Characteristics table.

An alternative definition of signal-to-noise ratio includes the

distortion 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

distortion and noise occurring at higher signal frequencies.

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

of signal frequency is to make a plot of “effective bits” versus

frequency. 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 ADC10461 with a 4.85 V

P-P, 100 kHz

sine wave input signal will typically have a signal-to-noise-

plus-distortion ratio of 59.2 dB, which is equivalent to 9.54

effective bits. As the input frequency increases, noise and

distortion gradually increase, yielding a plot of effective bits

or S/(N + D) as shown in the typical performance curves.

8.0 SPEED ADJUST

In applications that require faster conversion times, the

Speed Adjust pin (pin 14 on the ADC10462, pin 17 on the

ADC10464) can significantly reduce the conversion time.

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

current is increased, which reduces the conversion time. As

an example, an 18k resistor reduces the conversion time of

a typical part from 600 ns to 350 ns with no significant effect

on linearity. Using smaller resistors to further decrease the

conversion time is possible as well, although the linearity will

begin to degrade somewhat (see curves). Note that the

resistor value needed to obtain a given conversion time will

vary from part to part, so this technique will generally require

some “tweaking” to obtain satisfactory results.

For applications that require guaranteed performance using

the speed adjust pin, the ADC10662 and ADC10664 are

tested and guaranteed for static and dynamic performance

with a fixed value of speed-up resistor.

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