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SER1CH-UA Answers
This page contains answers to frequently asked questions about our
SER1CH-UA family of low power 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: Where can I find more info and FAQ answers on the SER1CH ?
See the FAQ chapter of the SER1CH-UA User Manual in PDF format which is available
from the User Manuals section of our downloads page.
Q: What is a ratiometric application ?
Ratiometric applications are those which, as the name suggests, use
sensors that rely on the ratio of two quantities. Perhaps the most
popular class of ratiometric sensors is potentiometers. These include
the classic 10-turn pot which can measure angular position, linear
pots and string gauges for measuring lengths, tilt meters, and
pressure transducers.
Typically, these ratiometric sensors all require three connections.
One defines the minimum value, another defines the maximum value, and
the third provides the measured value which is constrained to stay
between the defined min and max.
The SER1CH-UA data acquisition systems are exceptionally well suited
for working with ratiometric sensors since their three connections
(VGND, VIN, VREF) can be hooked up directly. VGND defines the sensor
minimum, VREF defines the sensor maximum, and VIN is the measured
value.
One obvious benefit of using the SER1CH-UA to drive the ratiometric
sensor is a simpler setup since no additional equipment is required to
power the sensor. A less obvious benefit, is more stable results.
Because both the sensor and the A/D converter are using the exact same
voltage range, any thermal variations in the reference voltage cancel
out. This removes a large source of measurement noise and produces
better results.
For further details, please see the SER1CH-UA User Manual which is
available for free from the User Manuals
section of our downloads page. See the "Ratiometric Measurements"
chapter (8) for an extensive discussion including the mathematical
equations involved as well as experimental plots showing the improved
stablity. Also see the "Ratiometric potentiometer" section (10.3) of
the "Example Applications" chapter for instructions and photos of the
SER1CH-UA-BANA being used with linear and 10-turn pots.
Q: What is the difference between the SER1CH-UA, SER1CH-UA-DSUB, and SER1CH-UA-1in ?
In terms of software and functionality, the SER1CH-UA-BANA, SER1CH-UA-DSUB,
and SER1CH-UA-1in are identical. The only difference is in their shape
and connectors.
The SER1CH-UA-BANA is rectangular and has a 2.1mm jack for power, a female
DB9 for RS232 serial communication, and banana jacks for the reference
voltage, analog input, and ground. This version is well suited for
individual use in labs and schools.
The SER1CH-UA-DSUB is rectangular and has a female DB15 for power and
RS232 serial communication and a male DB9 for the reference voltage,
analog input, and ground. This version is best for use in labs or
industrial plants as part of a permanent installation where it is
hooked up to other equipment with custom cables made by the customer.
The SER1CH-UA-1in is very narrow and has a 8-pin right angle header
for power and RS232 serial communication and a 6-pin right angle
header for the reference voltage, analog input, and ground. This
version is best for use in restricted spaces such as boreholes where
it is part of a permanent installation and is hooked up to other
equipment with custom cables made by the customer.
Q: Can I use the SER1CH-UA with Excel, MatLab, VisualBasic, LabView, TestPoint ?
Third-party products like Excel, MatLab, VisualBasic, LabView or
TestPoint typically work with the provided SER1CH-UA software in one
of two ways.
The SER1CH-UA software outputs acquired data in ASCII format. If the
bare, no header, format is selected, DVM.asc contains simple columns
of data which can be easily imported into programs like Excel or MatLab.
In addition, the SER1CH-UA software includes a library of underlying
support functions for performing tasks like opening, starting, and
reading data from the SER1CH-UA. These functions are written in C and
are made available as a DLL. This allows them to be called from other
products such as Visual Basic, Fortran, LabView or TestPoint as well
as other programming languages as long as you follow C calling
conventions.
Symmetric Research does not provide programming advice for SER1CH-UA
users writing their own programs for third-party products.
Q: What format are the SER1CH-UA DVM.asc output files in ?
ASCII with the data listed in columns. You can read the files in any
text editor. The format is simple and can be imported into many other
programs.
Two ASCII formats are possible, verbose and bare. Verbose has a
header with information about the DVM acquisition parameters at the
top. Bare format has just the data in columns.
Note: The DVM.out ASCII format is different from the binary .OUT
format used by the PARxCH A/D's.
Q: What kind and length of RS232 serial cable does the SER1CH-UA require ?
Use a standard straight through DB9 male to female cable. Sometimes
this is called a DB9MF extension cable. Do not use null modem
cables. They swap signal wires and will not work correctly.
If you are using multiwire jacketed cable to make a custom cable, you
will need 5 conductors. One each for ground, power, CTS, RTS, and
DTR. We also recommend shielded cable for this connection, with the
shield connected to the outer housing on the Dshell connectors on each
end. The shield will conduct away any static discharges to the
enclosures on each end.
Split out the power conductor and route it to the 2.1mm power
connector. The other signals can go to the DB9 connector. If you are
making a permanent installation consider using the SER1CH-UA-DSUB
model which has all connections on Dshell connectors.
We have tried 100 feet of RS232 serial port cable. Signal quality of
the digital signals was good and the system ran without trouble.
Longer cables may work, but have not been tested.
Q: Will USB to RS232 dongles work with the SER1CH-UA ?
Only dongles that support the RS232 signals RTS, CTS, DTR can be used
with the SER1CH-UA. Be careful, many dongles support only RX and TX.
One USB to RS232 dongle we have used successfully is from Keyspan. It
is clearly advertised as supporting all the RS232 signals
including RTS, CTS, DTR. Various other dongles have failed, so don't
make assumptions.
Q: What analog input voltage ranges are possible for the SER1CH-UA ?
The SER1CH-UA analog input voltage may range from 0 to +5 volts. This
is the voltage as measured on the middle red VIN banana with respect
to the black ground GND banana. Valid count values are returned only
for voltages within this range.
If you have another voltage range, then you must add external
circuitry such as a voltage divider or amplifier to scale the voltage
into the 0 to +5 range. See the Scaling and biasing chapter in the user
manual for examples.
Many sensor circuits can use the voltage reference provided on the
VREF red banana jack to scale their ranges naturally into the full
range of the SER1CH-UA. See the Ratiometric measurements chapter for
examples.
Q: Why does waving my hand over the SER1CH-UA change the DVM voltage ?
The input impedance of the SER1CH-UA is 10M ohms. Waving your hand
over the input banana jacks when nothing is connected will induce a
measurable charge and voltage change by capacitive coupling. DVM
reflects this change.
Connect the analog input VIN to the VREF or GND jacks and you will
notice that the effect disappears. This behavior is typical of all
precision, high input impedance voltmeters. Floating inputs tend to
wander as as various charged entities are brought near their inputs.
Connect the input to specific low impedance voltages to make accurate
measurements.
Q: Do static shocks hurt the SER1CH-UA inputs ?
Yes. Although the input circuitry has resistors and capacitors to
help guard against static shocks on the banana jack connectors, static
discharges are brutal to any electronics.
This includes small sparks generated from walking over a carpet.
Static discharge events will result in a degradation of the
electronics even if total failure does not occur.
Before touching the banana jack inputs, take a moment and discharge
yourself by touching a nearby metal object such a metal desk or a
computer case. Then touch the metal case of the SER1CH-UA and then
finally the banana jack inputs for any connections that must be made.
The moment spent consistently performing this simple routine will
significantly reduce static damage.
Q: Why is the SER1CH-UA consuming more than 10ma ?
You probably have the green power LED enabled, which consumes
considerably more than the SER1CH-UA base power consumption. The
green LED jumper must be disabled to minimize power. For more
information, see the Batteries and minimizing power chapter in
the user manual.
Q: Does the SER1CH-UA have any digital buffering ?
No, the SER1CH-UA does not have buffering memory for accumulating
converted results. After a conversion, the digital result is saved on
the A/D chip. The PC must read that result. If another conversion is
performed, the previous digital result is lost.
Other Symmetric Research A/D products such as the PARxCH A/D's feature
sizable FIFO buffers so the PC does not have to read results
immediately. At the acquisition rates of the SER1CH-UA it is assumed
the PC can keep up with the digital bandwidth required.
Q: Is there any antialiasing filter on the SER1CH-UA ?
There is no op amp antialias filter in the front end signal
conditioning of the SER1CH-UA. However, the internal digital filter
of the LTC2400 A/D converter has excellent 60 or 50Hz rejection. For
low frequency measurements, powerline noise is the most common
antialias filter requirement. See the user manual chapter 60/50Hz
Rejection for more information.
For rejection of higher frequency contamination, single pole RC
filters are usually recommended. There are pads for installing a
variety of RC combinations on the SER1CH-UA front end signal
conditioning. See the Circuit Diagrams chapter for more information.
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FAQ Index
for links to more answers.
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