Frequently Asked Questions

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  • Are IVI Instrument Drivers Compatible with Linux?
    Discussing the feasibility of using an IVI instrument driver under a Linux OS.
     
    Due to the necessity to have ultimate control over the stability and maintenance of their test system, many users prefer to avoid designing their SW to be dependant on a Windows operating system.  The alternative of choice tends to be Linux.  For users who wish to use LXI instrumentation, will have this question of whether or not the IVI instrument driver that comes standard with their LXI hardware work in Linux.  To understand this, please refer to the following diagram:

     

    First of all, it is important to note that there are two levels of IVI compliance.  IVI – COM and IVI-C.  Although only one of the two is required for LXI instruments, it is quite common industry wide, to deliver both with LXI instruments.  Part of the reason for this is industry tools commonly used to develop IVI drivers generate an IVI-COM driver at the core, and automatically wrap the IVI-COM driver with an IVI-C driver, thus creating both deliverables.  This model ultimately poses problems for Linux users
     
    COM in general is a Microsoft technology, and is an architecture that is heavily dependent on characteristics specific to the Windows operating system.  So, as a rule of thumb, IVI-COM driver will not work on a Linux OS.
     
    If we continue with the diagram above, IVI-C will not work under Linux either due to the fact that the IVI-C driver is dependent on the IVI-COM driver.  In theory, a pure IVI-C driver could work under Linux, but there is one thing that is worthy of considering.  IVI-C instrument drivers require some kind of VISA drivers to be functional.  While there are some Linux based VISA libraries available in the industry, they are limited in functionality, and support.  While this approach is possible, it is challenging.
     
    VTI Instruments is committed to providing it’s customer base with an open software architecture.  What this means is we strive to allow our customers to develop in the OS and ADE of their choice.  When considering this problem upon designing the first LXI instrument, it was necessary to find a way to deliver an IVI driver as well as a system that was easily integrated onto a Linux OS.
     
    The solution involved creating a core driver that contained all of the functionality to control the device.  This is illustrated in the diagram above.  The higher level IVI-COM and IVI-C drivers are nothing more than wrappers to the true VTI Instrument core driver.  The advantage to this approach is the core driver can be delivered to customers who prefer Linux to Windows.  While they don’t get an IVI compliant driver, they do get a driver that delivers the same exact functionality without any of the caveats introduced by IVI to Linux users.
  • Can I Use Two DACs in Series to Increase Output Voltage?
    In many applications, the maximum output voltage of one DAC is not sufficient to meet the requirements of the specific test. In these cases, when wired correctly, two DACs can be used together to increase the total output voltage. You can increase the specified output voltage in a system by utilizing two or more isolated DAC channels from one or more output devices wired in series. Isolated DAC outputs can be found on the EX1200-3604/3608 card as well as the VM3618.  Make sure to check the "Maximum output (series channels)" or the "Isolation" in the product specification, as there is a limit to the output voltage that can be achieved in this way.  Also, make sure that the output channels are isolated.  For example, the VM3608/VM3616 are NOT isolated and may not be used in series. 

    Because the output is unipolar, we will reference our voltages to the RETURN signal pins. The example below shows how to wire two channels together to increase the output range from 0-32V to 0-64V.

  • Control Hardware (Stimulus) – Analog Output
    The control hardware performs an opposite task from the measurement hardware, interpreting digital words (commands) from the computer and outputting the appropriate electrical signals (voltages, currents, pulses, waveforms). These signals control fans, motors, valves, and heaters or route power and signals to external devices. Control hardware can be used for three types of control: analog output, digital output, and switching. Analog Output The D/A converter performs the opposite function of an A/D converter. It interprets commands from the computer and outputs the proper dc voltage or current. The output stays at this output level until the computer tells the D/A converter to output a new value. The voltage or current from the D/A converter can be used to control the speed of a fan, the position of a valve, or the flow rate of a pump. D/A converters are used in applications that require precise control of external devices. One specialized type of D/A converter is the arbitrary waveform generator.

    This device contains memory and a clock and can output a series of dc voltages at varying rates. The memory is used to store these voltage values and the clock determines the output rate. When the clock rate is fast enough, these dc voltages or waveforms take shape of sine, square, or ramps output.



    VTI Instruments offers a wide variety of analog control hardware in both the LXI and VXI form factors.
  • Control Hardware (Stimulus) – Digital Output
    The control hardware performs an opposite task from the measurement hardware, interpreting digital words (commands) from the computer and outputting the appropriate electrical signals (voltages, currents, pulses, waveforms). These signals control fans, motors, valves, and heaters or route power and signals to external devices. Control hardware can be used for three types of control: analog output, digital output, and switching.

    The digital output card interprets a command from the computer and outputs a high or low voltage on each of its channels (bits). It is commonly used to turn on/off small lights or to send digital words to machinery. The pulse output card is a combination of a digital output card and an arbitrary waveform generator. Like the waveform generator, it contains a clock. It outputs a series of pulses at varying rates. The most common use of a pulse output card is to control stepper motors. That is the reason this card is sometimes called a stepper motor controller. Stepper motors are used in applications to move an object. These motors vary in size from miniature to large overhead cranes. The number of pulses determines the distance traveled while the frequency of the pulses is the speed at which this distance is traversed. Sophisticated stepper motor controllers can be programmed to accelerate and decelerate the motors.

  • How Can I Decrease Test Time in High Channel Count Switch & Measure Applications?
    In the area of functional test, often test time is a critical requirement. Latency in commanding signal switch paths, stimulus devices, and the measurement can have a significant impact on the overall time it takes to complete a full functional test of a particular DUT. 

    Using the VXI bus in combination with the VM2710 DMM and SMIP family of switching products, scanlists can be used to greatly reduce the latency introduced by using the host PC to control switch closures. Specifically, the user can pre-load the SMIP switches with a list of relays, or channels that are to be measured by the DMM. After that, hardware triggers are used instead of the host PC to control the sequencing of events. The process is as follows: 

    DMM Setup (1 Communication Transaction From PC)
    Trigger System Setup (1 Communication Transaction From PC)
    Load Scanlist (1 Communication Transaction From PC)
    Initiate Scan (1 Communication Transaction From PC)
    From here, the HW repeats the following routine 

    Switch Closure. Once Settled Trigger is Sent to the DMM
    DMM takes a measurement. Once Completed a Trigger is Sent to the Switch
    Repeat….
    The time saved with this approach is directly related to the number of channels that need to be measured. For example, if a total of 500 channels need to be measured, a well planned routine can turn 504 communication transactions from the PC into only 4, and thus greatly improving the time the test takes to complete. 

    The EX1200 is also a scanning DMM platform w/signal switching. This particular product exemplifies how this same concept can be taken one step further to completely optimize test times. In addition to being able to include switch closures in the scan list, this platform provides an interface to include DMM and stimulus setup as part of the scan. For tests that require multiple measurement functions out of the DMM, this is a valuable feature. The process would be as follows: 

    Load Scanlist (1 Communication Transaction)
    Initiate Scan (1 Communication Transaction)
    From here, the HW repeats the following routine 

    Switch Closure. Once Settled Trigger is Sent to the DMM
    DMM takes a measurement. Once Completed a Trigger is Sent to the Switch
    Repeat….
    DMM Receives a Separate Trigger Instructing it to Change Measurement Function. Once Completed a Trigger is Sent to the Switches
    Switch Closure. Once Settled Trigger is Sent to the DMM
    DMM takes a measurement. Once Completed a Trigger is Sent to the Switch
    Repeat….
    Now, a test consisting of thousands of channels and many different measurement types can be completed in the most efficient amount of time as possible, and virtually independent of a host PC.
  • What is AC Isolation with Respect to Signal Switching?
    Switches are designed to distribute a signal. However, in doing so, they will distort the signal to some extent. This distortion should be well defined and quantified in the switch specification.  This information allows users to accurately choose a switch that is appropriate given the needs of their application.

    High frequency signals can sometimes couple across an open relay. The quantification of a switches ability to reject this undesired phenomenon is called isolation and is specified in dB to express the magnitude of the coupled signal.  The greater this number, the better the isolation.



    Isolation will typically be expressed from input to output of the worst case channel, with the relay open. The isolation specification is important to consider since it determines the amount of noise that is induced to the output of a channel from the input of that channel when the channel is open.
  • What is an A/D Converter?
    The A/D converter, a key element in a DAC system, is used to convert dc voltages from transducers into digital words (data). The voltage represents a temperature, pressure, flow, pH, or speed and must be converted to a digital word before it can be passed to an intelligent device like a computer.

    A voltmeter performs the same task as an A/D converter. A multi-meter is a superset of a voltmeter and A/D converter. In addition to measuring dc voltages, the multi-meter can measure ac voltage, resistance, and sometimes current. The A/D converter is specified in bits.

    The number of bits defines the resolution, the smallest voltage change that the A/D converter can distinguish. If an A/D converter is 8 bits, it can distinguish up to 28 or 256 parts. If the A/D has a range of (0 - 10) V, it can sense changes in steps of 10/256 = 0.0391 V. Voltmeters are usually specified in digits.


  • What is Common Mode Noise?
    Common mode noise is electrical interference on the two signal lines that causes both lines to change in potential relative to ground. Common mode noise most often results when the ground potential between the measuring instrument and the device being measured are different. The difference in grounds results in a ground loop, a current flowing through ground and the low lead. Once this current appears in the low lead wire it will cause a voltage because the wire has some resistance. The longer the lead, the more lead resistance and the greater the voltage error.

    TIP: To reduce common mode noise, use a guarded voltmeter. Tie the guard to the low side of the device being measured. This will shunt any ground loop currents away from the high and low measurement wires.

  • What is Digital Noise?
    Sometimes digital circuitry on measurement devices can affect the analog front end of the device. Digital circuitry passes information in the form of voltages, switching from high to low at high speeds. This activity creates high frequency noise that can easily be transferred to analog signals that come in close proximity.

    TIP: Never allow sensitive signals to pass near digital circuitry like that found in a computer. (Cards plugged into a PC may be susceptible to digital noise present in the computer).
  • What is LXI?

    LXI Overview
    The functional test and data acquisition community has been instrumental in establishing many industry standards, ranging from communications interfaces to instrumentation backplanes. The need for ever increasing bandwidths and higher data transfer rates has helped drive the latest industry initiative, LAN eXtensions for Instrumentation (LXI).

    LXI is a powerful test instrumentation platform supported by the world’s leading instrumentation companies. The LXI Consortium was formed in the fall of 2004 by VTI Instruments (formerly known as VXI Technology) and Agilent Technologies. Membership of LXI Consortium has since grown to over 40 leading test and measurement companies from around the globe.

    LXI Features
    LXI combines the synchronization and triggering features inherent in VXIbus and IEEE-1588 devices, with the benefits of Ethernet and GPIB technologies. The standard defines a platform for small to medium channel density and distributed modular instruments using low-cost, open-standard LAN (Ethernet) as the system backbone. LXI was developed to offer the size and integration advantages of modular instruments without the cost constraints of card-cage architectures. The standard continues to evolve, leveraging current and future LAN functionality, far exceeding legacy T&M connectivity capabilities.

    Key attributes that set LXI apart from other architectures are:

    • Speed, simplicity, worldwide reach, low implementation cost, and backward compatibility of LAN.
    • Quick, easy configuration through the intuitive web interface built into compliant instruments.
    • Simplified programming and greater software reuse through IVI drivers.
    • The ability to create hybrid systems that include LXI, GPIB, VXI and now PXIe.
    • Enhanced system performance and event handling via hardware- and LAN-based triggering modes.
    • Synchronization of local and remote instruments through the IEEE-1588 precision time protocol.


    The LXI Standard Specification can be downloaded from the official website - http://www.lxistandard.org

  • What is Maximum Carry Current with Respect to Signal Switching?

    Maximum carry current is the amount of current that can safely pass after closing or prior to opening the contacts.  This is specification is differentiated from the switching voltage and current tolerances, and is only valid when the relay is closed, and settled.

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