Chapter 4
Functional Details
Analog input resolution and range
The 16-bit A/D converter provides a resolution of 1/65,536 parts of full scale. The smallest reading of full scale
(1 part in 65,536) is called a Least Significant Bit (LSB). Four different bipolar ranges are controlled by
software:
Analog input ranges
Bipolar 1
LSB
±10 V
0.000305 V
±5 V
0.000153 V
±2.5 V
0.000076 V
±1.25 V
0.000038 V
The input range is controlled by a programmable-gain amplifier.
Conversion speed and amplification
The A/D chip always runs at full speed. The A/D converter and sample & hold circuit captures and digitizes a
signal in 5 µs. The conversion speed of the A/D remains constant in all conditions and at all throughput rates.
When you request a sample rate of say 20 kHz, the A/D converter is still converting the signal in 5 µs. The
20 kHz rate comes from the fact that conversions are being initiated only every 50 µs.
What factors limit conversion speed?
The first is clearly the A/D. A 5µs conversion speed translates to a maximum throughput of 200 kHz. The
second limiting factor can be the analog front end.
The front end may consist of a multiplexer and a programmable gain amplifier. The speed at which these
circuits can switch may also limit the throughput of the A/D board. That is, the rate at which it can acquire,
convert and transfer a signal with full accuracy. Accuracy is the key term here. The A/D can always run at full
speed, but has the front end settled and captured a true, accurate signal?
What about input range vs. speed?
The design of the analog front end is crucial to maintaining total throughput. Most A/D chips have a fixed input
range, typically ±5V. The analog front end amplifies low level signals and adjusts unipolar signals to match the
A/D converter's standard input. The PC-CARD-DAS16/16AO achieves 200 kHz in all of the four ranges for
single channel acquisitions.
Triggering and transfer
A trigger begins an acquisition/transfer cycle. There are three ways to trigger a PC-CARD-DAS16/16AO —
programmable pacer, software, or external. The trigger source selection is programmable.
The programmable pacer is the quotient of two 16-bit counters dividing a 10 MHz or 1 MHz wave derived
from a 10 MHz crystal oscillator which can be used to trigger any number of paced conversions. A single
conversion can be triggered by software at any time. External trigger, pacer clock and gate signals may also be
used to control conversions and synchronize to external events.
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