digitize AC signals without significant spectral errors and

without adding noise to the digitized signal. Dynamic charac-

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

monic distortion (THD), are quantitative measures of this ca-

pability.

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. Typical values 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. Typical val-

ues are given in the Electrical Characteristics table. An alter-

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

tion components along with the random noise to yield a

signal-to-noise-plus-distortion ratio, 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 ADC10061 witha5V

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 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

In applications that require faster conversion times, the

Speed Adjust pin (pin 14 on the ADC10062, pin 17 on the

ADC10064) 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 cur-

rent 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 con-

version time is possible as well, although the linearity will be-

gin 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.

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