USBxCH FAQ 

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This page contains answers to frequently asked questions about our USBxCH family of 24 bit A/D products. Additional answers can be found in the FAQ chapter of the PDF User Manual.

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: Why doesn't the USB driver work after waking the PC from sleep ?

Some versions of the standard USB hub driver do not properly handle wake up for self-powered USB peripherals and tend to crash the system. To avoid this unwanted behavior, the USBxCH driver stops itself before the PC goes to sleep. It must be manually restarted once the PC has woken up. This can be done with either the utility program DevMan included in the system software or with Windows Device Manager. For DevMan, run the wakeall command. For Device Manager, go to the SrInstrumentation class and disable and then enable the USBxCH driver to remove the yellow exclamation mark.

 
Q: What sampling rates are allowed on the USBxCH ?

The USBxCH data acquisition systems can sample at 14 different rates. These are 3.3, 6.5, 13.0, 19.5, 32.6, 39.1, 65.1, 78.1, 130.2, 651.0, 1302.1, 2604.2, 4882.8, and 9765.6 Hz or samples per second. Two additional rates (19531.3 and 39062.5) are theoretically possible, but can not be achieved in practice.

The allowed rates are determined by the A/D converters and the master clock on the board. The USBxCH master clock selects the maximum input oversampling rate for the A/D's which improve signal to noise ratio even more by averaging down by specified amounts to the final output rate. See the Sampling rates chapter of the user manual for more details.

 
Q: What is the maximum achievable sampling rate on the USBxCH ?

The maximum sampling rate that can be achieved is 9765.6 samples per second, or roughly 10kHz. In order to continuously sample at this rate, the PC becomes the limiting factor. The PC needs to save all acquired data quickly. Please use the Blast command line acquisition program since it imposes less software overhead than the graphical Scope program. We also recommend reducing the number of competing programs and tasks that are being run on the PC at the same time.

 
Q: How much crosstalk is there between channels on the USB4CH ?

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 USBxCH ?

Our USBxCH products support a +/-4v analog input range. Other input ranges can be achieved by populating the resistor divider pads in the USBxCH analog front end circuitry.

 
Q: What is the overvoltage protection range of the USBxCH ?

On the USBxCH, 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 USBxCH have any digital input/output capabilities ?

Yes. Our USBxCH 24 bit A/D systems have four digital input bits and four digital output bits available on a DB25 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 show the automatically sampled digital input each time a new piece of analog data is acquired. They can also be accessed in a polled fashion by calling the UsbXchUserIoRd library function from software.

 
Q: What is the impedance of the USBxCH analog inputs ?

This is set by resistors on the board to 51K 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 USBxCH ?

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 USBxCH products, the output counts goes to positive full scale when the + analog input pin is +4 volts and the - analog input pin is at -4 volts. That is a total difference of 8 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 -4 volts, and the - pin driven to +4 volts. That is a total difference of -8 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 16 volts, the number of counts per volt sensitivity is:

   16 volts / 2**24 = 1.0 microvolts / count

If you are running at a sampling rate with a noise floor at the 20 bit level, then the actual sensitivity you would be able to detect would be:

   16 volts / 2**20 = 16 microvolts / count

To achieve greater sensitivities requires modifying the gain resistors on the USBxCH circuit board. 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 USBxCH A/D converters ?

Even though the USBxCH specs correctly define its input range as +/-4v, calibration is always required for precise counts/volt measurements on any particular system. This is because input signal processing usually involves resistors, and even precision tolerance resistors are only available in stock catalog values.

Software included with the USBxCH contains a calibration program called CAL that comes in both graphical and command line versions. CAL helps calibrate the USBxCH by determining the scale and offset values needed to convert each channel from A/D counts to convenient units such as volts.

A precision voltage reference such as the VREF-399 can be useful for supplying known voltages to the CAL calibration program.

 
Q: What GPS antennas can I use with the USBxCH ?

The USBxCH are designed to work with GPS antennas that provide both a high precision Pulse Per Second (PPS) signal and industry standard serial NMEA messages with coarse time and location. We recommend using the GGA and RMC or ZDA messages. See the GPS Time Stamping chapter of the user manual for more details and for diagrams showing the required electrical connections. Or, purchase a ready-to-go USBXCH-GPS-DB25 antenna. This is a Garmin 16x HVS terminated to a 25-pin D-shell connector for easy connection to the USBxCH and programmed with the proper NMEA and PPS characteristics.

 
Q: What length USB cable can I use with the USBxCH ?

The USB cables supplied with the USBxCH are 6 ft long. Shorter 3 ft cables can be substituted on request. 6 ft is the maximum length allowed by the USB spec. For longer runs, intermediate USB hubs are required.

Return to the FAQ Index for links to more answers.