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This page contains answers to frequently asked questions about our USBxCH 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 questions in other categories, return to the main FAQ Index page
If you still have questions after reading this page, Contact Us and we'll be glad to help

Q: What sampling rates are allowed on the USBxCH ?

The USBxCH 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 native rates are determined by the A/D converters and the master clock on the board which have been selected to minimize noise floors. Utilities for interpolating to other rates such as 1Hz and 100Hz are included with the system software.  With a GPS antenna attached the interpolated rates have sub-microsecond accuracy and are aligned with the GPS PPS ticks.  See the PDF User Manual "Sampling rates" chapter for more details.

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

The maximum rate is 9765.6 samples per second, or approximately 10kHz.   In order to sample at this rate without loss the PC becomes the limiting factor, and must save the acquired data quickly.  Use the Blast command line acquisition program and save directly to files. The graphical overhead of Scope will not be able to keep up.  Reducing the number of other competing programs and tasks running on the PC is also recommended.

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

Almost none since each channel has its own individual A/D converter. The channel to channel isolation is so complete that crosstalk is too small to measure.  To confirm this, put a full scale sine or square wave input on one channel and short the inputs on adjacent channels.  Measure the feed through to the shorted channels by running Scope with all 24 bits showing.

Q: What is the analog input range of the USBxCH ?

The USBxCH systems have a native +/-4v analog input range. Other input ranges such as +/-10v are supported by populating resistor divider pads in the analog front end signal conditioning circuitry.  Custom ranges are available on request when placing your order.

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

Overvoltage protection is provided by input series resistors and the protection diodes internal to signal conditioning op amps. The input series resistors normally have a value between 1K and 10K ohms.  This is enough to protect the input to +/-30 volts guaranteed.

Beyond these limits external protection networks comprised of zener diodes and gas tubes are recommended.  Be aware such networks can degrade system resolution and TC temperature performance at high precision.

Q: Does the USBxCH have any digital input/output capabilities ?

There are four digital input bits and four digital output bits on the front panel DB25 digital connector. The output bits are latched while the input bits are sampled synchronously in step with the analog data. There are also user programmable LEDs on the front panel for visual feedback. The digital input/output bits are intended for general purpose control of external equipment like triggering.

The digital inputs are automatically saved to the data files along with the analog data, similar to a "mixed mode" oscilloscope.

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

This is set by resistors in the on board input signal conditioning. The normal factory default is 51K ohms. Other values as high as 10M Ohms are possible on request. We strongly recommend using the lowest input impedance possible for your application to reduce noise.

For a few more comments, see the 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 involves resistors, capacitors, and op amps each with their own tolerances.

A calibration program is included with the USBxCH sofware. Use the Calibrate program to 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. The scale and offset are then specified in the INI files for the processing utilities.

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

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.

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

The USBxCH is designed to work with GPS antennas having RS232 NMEA and PPS ( pulse per second ) signals.  The NMEA signals provide coarse time and location, while the PPS provides accurate sub-microsecond timing.  The preferred NMEA strings are GGA, RMC, and ZDA.  See the GPS Time Stamping chapter of the PDF User Manual for details and diagrams showing the required setup.

We also offer the USBGPS, a Garmin 16x HVS antenna with the required cable, NMEA, and PPS setups for easy "ready to go" use.

Q: What is the typical USBxCH GPS time accuracy ?

The time accuracy for the USBxCH A/Ds is a guaranteed 800 nanoseconds. Because the USBxCH systems maintain their own clock tick and counter on board, recording time stamping automatically along with the analog data, they always have 800 nanosecond accuracy are not subject to any network or interrupt latencies.

Q: How does your USBxCH GPS time stamping work ?

The USBxCH systems maintain a dedicated high speed counter in the FPGA on board. Two count values are are saved in parallel with the analog data stream. The first is the number of counts per second between PPS pulses which calibrates the counter in terms of counts per second. The second is the number of counts from a PPS pulse to the next acquired sample.

With these two numbers the exact time of the acquired sample in seconds can be computed to within the delta t of the counter. Furthermore because the counts per second is recomputed every second, unless you have a huge temperature change over a one second period, the system tracks very well with temp.

Q: Can I use the USBGPS by itself to get time without the USBxCH ?

No. The USBGPS cabling and signals should be connected to the USBxCH digital IO connector to work correctly. The USBxCH digital IO works with both the RS232 and PPS signals together to perform accurate time stamping not possible with a GPS antenna simply connected to a PC.

Q: What NMEA messages are available with the USBGPS and what is their format ?

As programmed at the factory, the USBGPS provides 2 NMEA messages every second. The most important of these is the GGA message which typically looks like this:


The first field identifies the message, the next contains the time shown as hours, minutes, and seconds. The latitude and longitude in degrees, minutes, and seconds with their quadrants comprise the next four fields. The following two fields contain a GPS data quality flag and the number of currently acquired satellites. The rest of the fields are related to altitude, except for the last one which is a check sum.

The second NMEA message provided by the USBGPS is the RMC message which typically looks like this:


The first field identifies the message, the next field represents time as hours, minutes, and seconds all packed together, and the third is a validity indicator. It is followed by 4 fields containing location information and 2 fields containing speed and course data. The 10th RMC field contains the date written as DDMMYY. The last field is a check sum.

For a complete description of the supported NMEA messages and their formats, please see Section 4.2 of the Garmin Users Manual included in the docs directory of the USBxCH software.

Q: Why is the USBGPS time not working ?

For USBGPS diagnostics, we recommend running the USBxCH NmeaTime utility and examining the NMEA strings to determine when satellite lock has occurred. If the third field of the $GPRMC message is a 'V', it means the GPS information is invalid because satellite lock has not been achieved. When this field changes to 'A', the GPS info is acceptable.

You should also see the red led on the front of the USBxCH begin blinking which indicates a PPS signal is being sent. If it doesn't you may need to use the SR GpsProg utility to reprogram the USBGPS back to factory settings to ensure that PPS is enabled.

Q: How can I set the PC time with the USBGPS ?

The USBxCH NmeaTime command line utility includes an option for setting the PC from the GPS time. If you have permission to change the PC time, this program will reset the PC time immediately after starting and at the start of each new GMT day thereafter. Remember this will not work unless you have proper file and execution permissions. Copyright © Symmetric Research