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DIGITAL SIGNAL PROCESSING (DSP) 201
version hardware is thus operated at random levels within the operating range. A
web site describing this method is www.chipcenter.com/TestandMeasurement/
tn024.html.
Digital noise This can be added to the A/D output. This technique is perhaps the
easiest to perform and it can be done with hardware or within the DSP processor.
Here are some dithering web sites:
www.cinenet.net/ spitzak/conversion/dithering.html
www.audioease.com/Pages/Barbabatch/TechInfo.html#aDithering
www.edi.lv/dasp-web/sec-6.htm

Sample and Hold (S/H)

Analog inputs might be changing when they are sampled. Even after filtering out the
high-frequency components in the antialias filter, the input to an A/D might be chang-
ing while the A/D is performing its function. Some A/D converters might be thrown off
by a changing input, delivering an erroneous output. If the A/D converter must have a
stable input during the conversion process, then the converter itself generally has a sam-
ple and hold (S/H) amplifier built right into the A/D converter. If it does not, we would
have to add one before the A/D input. The S/H amplifier has a hold input that controls
the hold function. When low, the S/H amplifier’s output simply follows the input. When
high, it takes a quick snapshot of the S/H analog input value and freezes the S/H ampli-
fier output at that value. The S/H maintains this value long enough for the A/D to con-
vert it to a digital value.
Further information on S/H amplifiers can be downloaded from www.national.com/
an/AN/AN-775.pdf and www.om.tu-harburg.de/Download/Datasheets/Linear/NE_
SE5537.pdf. Check the application sections and the tips on acquisition.

Antialias Filters

Now that we’ve got some idea what has to be inside the A/D block in our DSP system,
what about the antialias filter? Well, the news here is even a bit tougher. We made a
statement a while back that the antialias filter should be a low-pass filter that filters out
all frequencies above the Nyquist Frequency. The ideal antialias filter would pass all
frequencies (untouched) up to the Nyquist Frequency. Above that breakpoint, the