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PARxCH Answers
This page contains answers to frequently asked questions about our
PARxCH family of 24 bit A/D products. For answers to questions
about data acquisition in general see the
General Acquisition FAQ.
For a list of the questions, please return to the main
FAQ Index
page. If you still have questions after reading this page,
email them to info@symres.com
and we'll be glad to answer them for you.
Q: Can I use a USB to parallel dongle with the PARxCH ?
Sorry, but no. The PARxCH driver talks directly to the parallel port
hardware. All the USB to parallel software we're aware of works by
intercepting user space calls and would not see the calls our driver
makes.
Add-in parallel port cards should work fine as long as they properly
emulate a true motherboard parallel port. Add-in cards using the
MosChip or NetMos chipset are ok for BPP mode only, but are known to
fail when using EPP mode or interrupts. See our
Third-Party page for a company
offering a PCMCIA parallel port card for a laptop that has worked for us.
Q: Is there a 2 channel PARxCH ?
No. The PARxCH family of products are available only in 1, 4, or 8
channel versions. For 2 or 3 channels, use the PAR4CH. For 5, 6, or 7
channels, use the PAR8CH.
Q: How much crosstalk is there between channels on the PAR4CH and PAR8CH ?
Almost none since each channel has its own individual A/D converter
chip. Basically, the channel to channel isolation is so complete that
the crosstalk is too small to measure.
Q: What is the analog input range of the PARxCH ?
Our PARxCH products support a +/-10v analog input range. This is a
popular standard for A/D systems.
Q: What is the overvoltage protection range of the PARxCH ?
On the PARxCH, overvoltage input protection is provided by input
series resistors and the protection diodes internal to the A/D
converters themselves. Usually the input series resistors are around
10K ohms. And since most internal IC protection diodes can pass at
least 10ma you get something like 100v of continuous protection.
Users needing protection beyond these sorts of limits should consider
their own external protection networks. However, be aware that at
high precision, many networks can degrade system resolution and TC
temperature performance. You should carefully evaluate the impact of
any protection network you select.
Q: Does the PARxCH have any digital input/output capabilities ?
Yes. Our PARxCH 1/4/8 channel 24 bit A/D systems have four digital
input bits and four digital output bits available on a DB15 connector
on the front panel. The output bits are latched while the input bits
are polled. In addition, the system also has a front panel LED that
can be toggled to provide a visual output. These bits are intended for
general purpose control applications of external equipment in
conjunction with analog data acquisition.
The digital input bit values can be accessed on the PAR1CH and PAR4CH
in a polled fashion by calling the ParXchUserIoRd library function
from software. While the PAR8CH supports this polled style of digital
input too, it has enhanced firmware to automatically sample the
digital input each time a new piece of analog data is acquired.
Our PAR24B 8 channel 24 bit A/D system has a digital output byte
available on a 13x2 pin header. The output value is latched, and can
be updated in the 100's of kHz range. The system also has an on board
LED to indicate the value of the lowest bit of the output byte.
Q: What factors affect the maximum achievable sampling rate on the PARxCH ?
For the PARxCH data acquisition systems, the maximum sampling rate is
in part determined by the PC and software you are running.
For example, consider saving data to the hard disk. If the PC and its
disk system is fast, then there will be less of a bottleneck in saving
the data. In general, we have found a 500 MHz or greater PC is
sufficient to sustain the 5 kHz rate. By comparison, a 25 MHz 386
PC104 card is only capable of something more like 800 Hz.
Note: The PC must also use one of the faster parallel port protocols, EPP
or ECP/EPP in order to achieve the 5 kHz rate. The BPP protocols are too
slow.
Q: What is the impedance of the PARxCH analog inputs ?
Around 10K to 20K ohms. While many users need high impedance inputs
(perhaps even in the 100+ Mohms range), we have found that it is not a
good idea to put them directly on the A/D system. For an explanation see
the Data Acquisition Background FAQ
"What is the trouble with high impedance analog inputs".
Q: What is the input sensitivity of the PARxCH ?
The input sensitivity for an A/D converter is the number of digital
output counts per analog input volt. While this definition seems
simple enough, some care is required with differential inputs.
With the PARxCH products, the output counts goes to positive full
scale when the + analog input pin is +10 volts and the - analog input
pin is at -10 volts. That is a total difference of 20 volts between
the + and - pins to achieve positive full scale output counts.
Conversely, the output counts goes to negative full scale when the
analog voltages are reversed with the + pin driven to -10 volts, and
the - pin driven to +10 volts. That is a total difference of -20
volts between the + and - pins to achieve negative full scale output
counts.
Since the number of counts between positive and negative full scale is
2**24, and the total voltage span between the two extremes is 40
volts, the number of counts per volt sensitivity is:
40 volts / 2**24 = 2.38 microvolts / count
If you are running at a sampling rate with a noise floor at the 21 bit
level, then the actual sensitivity you would be able to detect would
be:
40 volts / 2**21 = 19.07 microvolts / count
To achieve greater sensitivities requires an amplifier such as our
AMP4CH-DF, which is specifically designed to work with the PARxCH
series of products. However, keep in mind higher signal voltages
result in a better signal to noise ratio. Always run with the highest
signal levels possible to obtain the best resolution, and put the
amplifier as close to the sensor as possible. This is particularly
true on long cable runs.
Q: How do I calibrate the PARxCH A/D converters ?
Even though an input range of +/-10v, or other range as appropriate,
may be generally spoken about for an A/D converter, calibration is
always required for precise counts/volt measurements on a particular
system.
Calibration is required because input signal processing usually
involves resistors, and even precision tolerance resistors are only
available in stock catalog values. In addition, the PARxCH systems
bias their inputs with a permanent offset for best performance.
Our software for any particular system includes a program called
meter. The meter.c source code discusses how to perform a straight
line calibration for that system.
Q: What length parallel port cable can I use with the PARxCH and PAR24B ?
The parallel port cables supplied with the PARxCH and PAR24B are 6 ft
long, and this is the maximum length we usually recommend. The
supplied cable is a standard male to female DB25 with all 25 wires
connected straight through.
You may be able to run longer cables, and even string successive
cables together in a pinch. It depends on the characteristics of your
particular PC's parallel port. But generally a single cable of
shorter length will cause fewer troubles and is recommended.
In specific installations we may be able to increase the cable
length. The record is 50 ft with a PAR24B. Please contact us if
longer cable length is a design requirement for your installation.
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FAQ Index
for links to more answers.
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