Functional Details; Analog Input Resolution And Range; Conversion Speed And Amplification; Triggering And Transfer - Measurement Computing PC-CARD DAS16AO User Manual

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:
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.
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
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Chapter 4