AD676

REV. A

–13–

AC PERFORMANCE

AC parameters, which include S/(N+D), THD, etc., reflect the

AD676’s effect on the spectral content of the analog input sig-

nal. Figures 12 through 16 provide information on the AD676’s

ac performance under a variety of conditions.

As a general rule, averaging the results from several conversions

reduces the effects of noise, and therefore improves such param-

eters as S/(N+D). AD676 performance may be optimized by

operating the device at its maximum sample rate of 100 kSPS

and digitally filtering the resulting bit stream to the desired signal

bandwidth. This succeeds in distributing noise over a wider

frequency range, thus reducing the noise density in the fre-

quency band of interest. This subject is discussed in the follow-

ing section.

OVERSAMPLING AND NOISE FILTERING

The Nyquist rate for a converter is defined as one-half its sam-

pling rate. This is established by the Nyquist theorem, which re-

quires that a signal he sampled at a rate corresponding to at

least twice its highest frequency component of interest in order

to preserve the informational content. Oversampling is a conver-

sion technique in which the sampling frequency is more than

twice the frequency bandwidth of interest. In audio applications,

the AD676 can operate at a 2

In quantized systems, the informational content of the analog

input is represented in the frequency spectrum from dc to the

Nyquist rate of the converter. Within this same spectrum are

higher frequency noise and signal components. Antialias, or low

pass, filters are used at the input to the ADC to reduce these

noise and signal components so that their aliased components

do not corrupt the baseband spectrum. However, wideband

noise contributed by the AD676 will not be reduced by the

antialias filter. The AD676 quantization noise is evenly distrib-

uted from dc to the Nyquist rate, and this fact can be used to

minimize its overall affect.

The AD676 quantization noise effects can be reduced by

oversampling–sampling at a rate higher than that defined by the

Nyquist theorem. This spreads the noise energy over a band-

width wider than the frequency band of interest. By judicious

selection of a digital decimation filter, noise frequencies outside

the bandwidth of interest may be eliminated.

The process of analog to digital conversion inherently produces

noise, known as quantization noise. The magnitude of this noise

is a function of the resolution of the converter, and manifests it-

self as a limit to the theoretical signal-to-noise ratio achievable.

This limit is described by S/(N+D) = (6.02n + 1.76 + 10 log

is the sampling frequency, and Fa is the signal bandwidth of in-

terest. For audio bandwidth applications, the AD676 is capable

of operating at a 2

oversample rate (96 kSPS), which typically

produces an improvement in S/(N+D) of 3 dB compared with

operating at the Nyquist conversion rate of 48 kSPS. Over-

sampling has another advantage as well; the demands on the

antialias filter are lessened. In summary, system performance is

optimized by running the AD676 at or near its maximum sam-

pling rate of 100 kHz and digitally filtering the resulting spec-

trum to eliminate undesired frequencies.

DC CODE UNCERTAINTY

Ideally, a fixed dc input should result in the same output code

for repetitive conversions. However, as a consequence of system

noise and circuit noise, for a given input voltage there is a range

of output codes which may occur. Figure 9 is a histogram of the

codes resulting from 1000 conversions of a typical input voltage

by the AD676 used with a 10 V reference.

2

1

0

–1

DEVIATION FROM CORRECT CODE – LSBs

800

0

200

400

600

Figure 9. Distribution of Codes from 1000 Conversions,

Relative to the Correct Code

The standard deviation of this distribution is approximately 0.5

LSBs. If less uncertainty is desired, averaging multiple conver-

sions will narrow this distribution by the inverse of the square

root of the number of samples; i.e., the average of 4 conversions

would have a standard deviation of 0.25 LSBs.