Answers to frequently asked questions about the SERxCH family of products
These include the
For answers to questions about data acquisition in general see the
General Acquisition FAQ
For a list of questions in other categories,
return to the main
If you still have questions after reading this page,
and we'll be glad to help
Q: Where can I find more info and FAQ answers on the SERxCH ?
See the PDF User Manuals on the
downloads page for more specifications and discussions.
Q: What is a ratiometric application ?
Ratiometric applications use sensors that rely on the ratio of two quantities.
Perhaps the most popular class of such sensors are 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 ratiometric sensors require three connections: one for the minimum
value ( ground ), another for the maximum value ( the stimulus ), and the third
for the sensor value ( the wiper ) which is a ratio between the sensor value
and stimulus voltage.
All the SERxCH products have buffered A/D reference voltages to provide
stimulus to ratiometric sensors. No other external circuitry is required to
drive the sensor. And the beauty of ratiometric measurement is it removes all
sensitivity to temperature drift and TC effects !!
For further details, see the PDF User Manuals which are available for free from
the User Manuals section of our
downloads page. The Examples chapters have "hands on" demos of simple
Q: Can I use the SERxCH with Excel, MatLab, VisualBasic, LabView, TestPoint ?
Third party products like Excel, MatLab, VisualBasic, and LabView can be used
with the SERxCH software in one of two ways:
First, acquire data to files in ASCII format with a SERxCH program like DVM.
These files contain simple columns of data which can be easily imported into
programs like Excel or MatLab. The ASCII data files are essentially
spreadsheets already in fixed width or csv format.
Second, for programs like Visual Basic and LabView, you can output data from
our acquisition programs to a pipeline. Downstream programs can then open the
pipeline and have "real time" data feeds for further processing and display.
Customers should be aware they are responsible for interfacing to downstream
software themselves. SR does not provide free consulting for third party
Q: What analog input voltage ranges are possible for the SERxCH ?
The SER1CH has one single ended channel.
For the SER1CH, the single ended input must be between 0 and +5 volts, also as
measured with respect to analog ground ( AGND ). The SER1CH also features a
VREF output for use as a stimulus source.
If you need to work with other voltage ranges, you must add external circuitry
such as a voltage divider or amplifier to scale your input into the SERxCH 0 to
+5 range. See the the SERxCH PDF User Manuals for specific examples.
For an even more extensive overview, see the PDF paper
Scaling and Biasing Analog Signals.
Q: Why does waving my hand over the SERxCH change the DVM voltage ?
The input impedance of the SERxCH is 10M ohms. Waving your hand over the
analog inputs when nothing is connected will induce a measurable charge and
voltage change by capacitive coupling. DVM reflects this change.
Connect the analog input to the AGND or VREF signals and you will notice the
effect disappears. This behavior is typical of all precision, high input
impedance voltmeters. Floating inputs tend to wander 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 SERxCH inputs ?
Yes! Although the input circuitry has resistors and capacitors to help guard
against static shocks on the analog input connectors, static discharge is
brutal to any electronics. This includes small sparks generated from walking
over a carpet. Static discharge events will result in degradation of the
electronics even if total failure does not occur. For example, every
time a resistor receives a static shock a little bit of its metalized film is
vaporized and its resistance changes. With enough shocks it may blow to open
like a fuse would.
Before touching the analog inputs, take a moment and discharge yourself by
touching a nearby metal object such as a metal desk or computer case. Then
touch shield pad of the SERxCH and then finally the analog
inputs for any connections that must be made. The moment spent consistently
performing this simple routine will significantly reduce static damage.
Q: Do the SERxCH have any digital buffering ?
No, the SERxCH do 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 the PC is slow and cannot read the current conversion, it
will be lost when the next conversion is performed. At the acquisition rates
of the SERxCH it is assumed the PC can keep up with the digital bandwidth
required. Most COM ports are interrupt driven and have deep buffers within the
Other SR products such as the USBxCH A/D's feature sizable FIFO buffers so the
PC does not have to read results immediately.
Q: Is there any antialiasing filter on the SERxCH ?
No. There is no op amp antialias filter in the SERxCH front end circuitry.
However, the internal digital filter of the LTC2400 and LTC2498 A/D converters
have excellent 50 or 60Hz rejection. For low frequency measurements, powerline
noise is the most common antialias filter requirement. See the user manuals
for more information about 50/60Hz rejection.
For rejection of higher frequency contamination, external single pole RC
filters are usually recommended.
Q: Do the SERxCH systems perform GPS time stamping ?
No. The current SERxCH family does not. The acquisition programs like DVM
will time stamp acquired data with PC time, but this is far less accurate than
GPS time. However, for systems sampling at 1Hz rates it is often sufficient,
and full tilt GPS time would be excessive.
If you need micro-second time stamping, consider systems like the USBxCH family
which are designed for precision GPS time stamping on board.
Q: What format are the SER1CH-UA 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.
For the SER1CH-UA, 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.
Q: What kind and length of RS232 serial cable does the SER1CH-UA require ?
The SERxCH systems use standard straight through DB9 male to female cables,
sometimes 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, we recommend
using shielded cable, 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.
For the SER1CH-UA you will need 5 conductors. One each for ground, power,
CTS, RTS and DTR. You may also wish to route power on custom multiconductor
Installations may use up to 1000 feet of standard CAT5e cable for custom builds
without trouble. Digital edges on the SERxCH products are kept slow so there
are no problems with transmission line reflections. Use pure copper CAT5e
cable ( and not aluminum core ! ) to reduce DC losses due to wire resistance.
Q: Will USB to RS232 dongles work with the SER1CH-UA ?
For the SER1CH-UA, only dongles that support the RS232 signals RTS, CTS, DTR
will work. Be aware, many inexpensive dongles support only Rx and Tx.
For the SER1CH-UA we recommend the Keyspan dongle which is clearly advertised
as supporting all the RS232 signals including RTS, CTS, DTR.
Check first on any dongle for the SER1CH-UA , don't make assumptions.
Q: Why is the SER1CH-UA consuming more than 10ma ?
The SER1CH-UA is normally ultra low power, less than 1 milliamp. However if
you have the green power LED enabled, the power will jump to about 10ma.
Bright cheerful LEDs are power hungry! Remove the jumper to
disable the LED and the power will be reduced.
And really ... if your A/D board is buried 1000 feet down a borehole, who will
ever see the LED anyway, the borehole leprechaun ? The LED is mainly useful
for bench testing during development.
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