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AD1200 Reference
next conversion but also the burden OF starting the conversion.
In addition, input signals can be regularly sampled at
known intervals thus providing information not only about the
magnitude of the input but also about how it changes with time.
The
pacer
conversions, this concept is explained below.
The timer circuitry can be programmed to select either the
on board 600kHz oscillator or an external, user supplied, TTL
clock as fundamental frequency for the pacer clock. The timer
register can be programmed to select 1 of 57 different factors by
which to divide the fundamental frequency to produce the
resulting pacer clock signal.
This pacer clock signal is used mainly as a strobe to force
A/D conversions but can also be used in the top of range
AD1200 card to strobe data into either D/A converter and in or
out of the TTL compatible digital ports.
____________________________
A/D Throughput and Pipelining.
The throughput of the A/D converter is how many
conversions can be performed within a given period of time.
The raw A/D conversion time, 25 microseconds for the AD1221
& AD1211 cards and 8 microseconds for the AD1220, AD1210
& AD1200 cards, places the fundamental limit on how many
conversion can be performed in one second. The settling time of
the analogue input multiplexer, when the channel selected for
conversion is changed, and the settling time of the programmable
gain amplifier, when a new gain is selected, are also a factor.
However, a great consideration is the method used to transfer the
data from the card into the PC.
From the above discussions about programmed I/O,
interrupts, DMA and the use of the pacer clock it is apparent
that the slowest method of taking A/D readings is programmed
i/o. This method also has the disadvantage of requiring the most
work from the user when writing programs. The processing
power of the PC is of great importance and a 4.77MHz 8086 PC
Page 14
clock
allows
pipelining
of
Introduction
successive
A/D
Chapter 1
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