What Is SC Express?

Publish Date: Feb 08, 2012 | 2 Ratings | 2.00 out of 5 |  PDF

Table of Contents

Introduction to the SC Express Family

The NI SC Express family offers PXI Express data acquisition with integrated signal conditioning for measuring strain gages, bridge-based transducers, thermocouples, microphones, accelerometers, and high-voltage analog input signals. Increased accuracy, high sensitivity and data throughput, and tight synchronization are key features that make this family ideal for advanced measurement systems from low to high channel counts.

Figure 1. SC Express data acquisition (DAQ) modules include integrated signal conditioning
 for measuring sensors and high voltages.

Achieving Better Measurement Accuracy

Many sensors require signal conditioning before output can be accurately digitized. Examples of signal conditioning include amplification, excitation, isolation, and filtering. For many years, engineers implemented signal conditioning through separate front-end systems cabled to a data acquisition system that converted the conditioned analog signals to digital. New technologies and miniaturization have recently made it possible for companies to integrate signal conditioning and analog-to-digital conversion into the same device to deliver higher-accuracy measurements by eliminating error-prone cabling and connectors. SC Express modules for each measurement type have unique design implementations to allow better accuracy, including low isothermal error for thermocouple measurements, anti-aliasing and low pass filtering for dynamic signal acquisition, and internal gas cell calibration for Fiber Optic Sensor measurements. 

PXI Express Dedicated Data Throughput

SC Express modules are built on the x1 PXI Express bus with dedicated bandwidth per device up to 250 MB/s. Because of the added bandwidth provided by PXI Express, the SC Express modules offer simultaneous sampling options using the same channel counts and connectivity as multiplexed devices.  Unlike multiplexed devices that reduce sampling rates as you add more channels, you can use simultaneous sampling devices to maintain sampling rates as you expand the number of channels. 

Figure 2. SC Express modules synchronize with more than 1,500 I/O types on the PXI platform.

PXI Platform Advanced Timing and Synchronization

The PXI platform has delivered advanced timing and synchronization to measurement systems for more than 10 years. Building on PXI capabilities, PXI Express provides the additional timing and synchronization features of a 100 MHz differential system clock, differential signaling, and differential star triggers. By using differential clocking and synchronization, PXI Express systems benefit from increased noise immunity for instrumentation clocks and the ability to transmit at higher frequency rates. The SC Express family uses PXI Express to deliver tight synchronization between modules in one chassis or multiple chassis. When synchronized with the more than 1,500 modules in the PXI platform, SC Express gives you the flexibility to develop a measurement system that meets the needs of your application. 

Measuring Strain Gages and Bridge-Based Transducers on PXI Express

The NI PXIe-4330 and NI PXIe-4331 24-bit simultaneous bridge input modules offer 0.03 percent typical accuracy and up to 102.4 kS/s per channel sample rate for high-performance strain, load, force, and torque measurements. The bridge input modules achieve high accuracy with a ratiometric design where the analog-to-digital converter (ADC) references the excitation voltage. This approach removes the measurement’s dependence on the stability and accuracy of the excitation voltage. In addition, unique delta-sigma ADCs are used to provide both excellent AC performance and DC accuracy, a distinctive feature considering traditional delta-sigma ADCs provide only good AC specifications. 

Figure 3. You can use the NI PXIe-433x with the TB-4330 front-mounting terminal block for screw terminal connectivity.

The NI PXIe-433x modules can perform quarter, half, and full bridge-based measurements with automatic synchronization features; the included driver software ensures tight synchronization across multiple modules and chassis with inter-channel skews as low as 5 ppb. These devices offer per channel excitation from 0.625 to 10 V with remote sensing to compensate for error caused by resistance in the wires connecting the voltage source to the bridge. For added flexibility, built-in bridge completion (120 Ω, 350 Ω, and 1 kΩ) and shunt calibration (50 kΩ, 100 kΩ) are software-selectable on a per channel basis. You should use the NI PXIe-433x modules with the NI TB-4330 front-mounting terminal block to directly connect signals and sensors. 

Figure 4. The NI PXIe-433x modules have built-in antialiasing filters to remove high-frequency noise.

The 8-channel bridge input modules can scale to a 136-channel system in a single chassis and thousands of channels when synchronizing across multiple chassis. The high accuracy, simultaneity, and synchronization features make the NI PXIe-433x modules ideal for high-channel-count structural test applications, such as wind tunnels, bridge monitoring, and crash tests.

Measuring Thermocouples on PXI Express

With the NI PXIe-4353 thermocouple module, you can measure temperatures with .30 °C typical accuracy and 24-bit resolution on 32 channels. The NI TB-4353 isothermal terminal block, which provides powerful cold-junction compensation (CJC), achieves the high-accuracy levels. CJC improves your measurement accuracy by providing the temperature at the cold junctions and applying the appropriate correction. The isothermal terminal block minimizes error with a unique design that optimizes thermal conductivity, making it possible for the nearby CJC thermistors to accurately measure the temperature at the thermocouple junctions. The TB-4353 front-mounting terminal block includes eight CJC thermistors that are distributed on the terminal block to closely measure the cold junctions across the terminal block. 

Figure 5. The NI PXIe-4353 multiplexes 32 thermocouple channels, eight CJC channels, and two auto-zero channels through three ADCs.

For increased performance, the NI PXIe-4353 shares its inputs across three different ADCs and supports multiple timing modes (90 S/s/ch max) to balance the trade-off between speed versus accuracy and noise rejection. The module also includes 300 Vrms CAT II bank isolation to allow flexibility in the environments and eliminate crosstalk. With bank isolation, you can connect floating thermocouples and thermocouples with a common mode voltage of up to 300 V relative to the chassis ground. Additionally, the module has two auto-zero channels to compensate for offset error, as well as open thermocouple detection circuitry on each channel. 

The 32-channel thermocouple module can scale to a 544-channel system in a single chassis and thousands of channels when synchronizing across multiple chassis. The increased accuracy and scalable platform make the NI PXIe-4353 module well-suited for precision measurements from a single channel to large systems for applications such as fuel cell testing and monitoring thermal chambers.

Measuring High Voltages on PXI Express

The NI PXIe-4300 simultaneously measures eight differential high-voltage analog inputs up to ±300 V.  This module features 300 Vrms channel-to-channel CAT II isolation and 3,300 Vrms channel-to-earth five-second withstand to protect users and equipment from hazardous voltages. Measurement Category II is for measurements performed on circuits directly connected to the electrical distribution system. This category refers to local-level electrical distribution, such as that provided by a standard wall outlet, for example, 115 V for the United States or 230 V for Europe.

With a 16-bit ADC per channel and a 250 kS/s per channel sampling rate, the NI PXIe-4300 module can stream 4 MB/s per module and 68 MB/s in a single chassis. Because the NI PXIe-4300 uses PXI Express technology, you can simultaneously acquire channels from dozens of modules at maximum sample rate without reaching communication bus limitations. To remove high-frequency noise, the NI PXIe-4300 also includes programmable lowpass filters (10 kHz, 100 kHz, bypass) per channel. 

You can choose the NI TB-4300 terminal block for ±10 V signal inputs or NI TB-4300B terminal block for ±300 V signal inputs. The TB-4300 is strictly a feedthrough terminal block. The TB-4300B provides 30:1 attenuation to the input signal with additional high-voltage protection circuitry. Scaling constants are stored in the EEPROM and used by software to automatically apply scaling to the signal band of the selected range. The TB-4300 and TB-4300B are front-mounting terminal blocks with screw terminal connectivity. 

This 8-channel analog input module can scale to a 136-channel system in a single chassis and thousands of channels when synchronizing across multiple chassis. The ability to measure 300 V with CAT II channel-to-channel isolation makes the NI PXIe-4300 module ideal for automotive testing, such as fuel cell and battery tests.

Measuring Microphones and Accelerometers with PXI Express

The PXIe-4499 and PXIe-4497 are high accuracy data acquisition devices specifically designed for high channel count sound and vibration applications.  Each device is capable of simultaneously sampling its 16 channels at rates of up to 204.8kS/s.  The dedicated delta-sigma ADCs allow you to scale your measurement system without having to sacrifice sampling rate or synchronization.

Clipped or overranged waveforms can cause distortion if a signal exceeds the input range.  The PXIe-449x devices feature analog and digital overload detection to ensure the integrity of your data.  These devices are capable of detecting the overload conditions before the gain and offset correction are applied and can correct for them before they reach the ADC.

The AC input coupling on the PXIe-449x enables users to take full advantage of the dynamic range, even with a significant amount of unwanted offset.  When AC coupling is enabled, a highpass resistor-capacitor (RC) filter is inserted in the front end conditioning, resulting in attenuation of the low-frequency response of the AI circuitry. 

You can easily connect microphones and accelerometers via the InfiniBand cables (pictured below).  The PXIe-449x also provide excitation for IEPE sensors and support Transducer Electronic Data Sheets (TEDS).  Furthermore, National Instruments has partnered with GRAS Sound and Vibration and PCB Piezotronics to help you take the guesswork out of sensor compatibility. 

Measuring Fiber Bragg Grating Optical Sensors with PXI Express

Extreme electrical conditions, such as potential exposure to high voltage or ambient electromagnetic interference (EMI), are a common source of noise in measurements taken with traditional electrical sensors. The NI PXIe-4844 Optical Sensor Interrogator (OSI) for Fiber Bragg Gratings eliminates these concerns by bringing the EMI immune and non-conductive advantages of Fiber Optic Sensors to the PXI platform.  Traditional sensors may be problematic in corrosive or explosive environments. The chemically inert, electrically passive sensors used by PXIe-4844 OSI module make it immune to the risk of sensor degradation or sparks.  Systems with mixed measurements and complex sensor topologies can incur costly field wiring and cabling challenges.  Fiber Optic Sensors simplify cabling by incorporating up to 20 different sensors on a single fiber. All of these advantages, along with the best in class sensitivity and accuracy afforded by its internal gas cell calibration component, make the NI PXIe 4844 an ideal addition to any PXI system.

Making Sensor Measurements Easier

The SC Express family delivers ease-of-use features in hardware and software to reduce setup and development time. Integrated signal conditioning and smart terminal blocks simplify hardware setup and software features, such as new bridge channel types and multidevice tasks, reduce software development time.

Integrated Signal Conditioning

Integrating signal conditioning and ADCs onto the same device results in a smaller footprint and higher measurement performance. Integrated signal conditioning also provides simplified cable management and calibration due to fewer components, which drastically reduces the installation and maintenance cost of a large-channel-count measurement system.

SC Express Module

SC Express Terminal Block/Connectivity

NI PXIe-4330 Bridge Input (25 kS/s/ch)

NI TB-4330

NI PXIe-4331 Bridge Input (102.4 kS/s/ch)

NI PXIe-4353 Thermocouple Input

NI TB-4353

NI PXIe-4300 Isolated Analog Input

NI TB-4300 (10 V)

NI TB-4300B (300 V)

NI PXIe-449x Microphone and Accelerometer Input InfiniBand Cables
NI PXIe-4844 Optical Sensor Interrogator Fiber Optic Cabling

Table 1. Each SC Express module has its own compatible terminal block or connectivity options.

Smart Terminal Blocks

The SC Express front-mounting terminal blocks house signal conditioning for certain measurement types and include screw terminal connectivity. Each module type has its own terminal block, with the NI PXIe-4300 high-voltage analog input module having two different terminal blocks, as shown in Table 1. The front-mounting terminal blocks have the same external style and appearance, so they are designed with keying to match only the appropriate modules. This prevents users from accidentally connecting the high-voltage terminal blocks to lower-voltage sensor modules. 

Figure 6. SC Express front-mounting terminal blocks are designed with keying to match only appropriate modules.

The SC Express front-mounting terminal blocks are not only automatically recognized in software, but they are also hot swappable. This makes troubleshooting easier because you can connect and remove terminal blocks without powering down the PXI measurement system. Lastly, each terminal block includes alignment fins that guide the connector onto the PXI Express module preventing bent pins. The smart terminal blocks add to productivity by decreasing setup and troubleshooting time. 

New Force, Pressure, and Torque NI-DAQmx Channel Types

NI-DAQmx driver software includes new features for bridge-based measurements, including custom channel types for force, pressure, and torque measurements. This allows users to select the appropriate measurement type from the DAQ Assistant or the NI-DAQmx API. Measurements are returned in the physical units that you select from N, lb, Pa, psi, Nm, and so on. These channel types include offset nulling and shunt calibration options within the DAQ Assistant and the NI-DAQmx API. With the new channel types, you can choose to enter two-points, a table, or polynomial coefficients depending on the specifications provided by the sensor manufacturer. National Instruments continues to make advancements in software to make data acquisition and sensor measurements easier to program. 

Figure 7. The NI-DAQmx driver includes easy programming for strain, force, pressure, and torque measurements.

NI-DAQmx Support for Multidevice Task

National Instruments has also enhanced the NI-DAQmx driver with an innovative approach to configure multiple boards in the SC Express family for synchronization in a single chassis. You can use a single NI-DAQmx multidevice task to add channels from multiple boards with multiple measurement types. You can program an entire 18-slot PXI chassis with up to 544 channels using the simple NI LabVIEW code shown in Figure 8. 

Figure 8. Acquire from 1 to 544 channels with the same NI-DAQmx code.

Conclusion

The SC Express family includes the highest performing data acquisition and signal conditioning on the PXI platform. With increased accuracy, high data throughput, and best-in-class synchronization, you can design a flexible and scalable data acquisition system for low- to high-channel-count applications. National Instruments advanced PXI Express technology and ease-of-use software features give you the power to decrease development time and meet the most demanding measurement requirements.  

Resources

View the Specifications and Pricing for SC Express

Learn How SC Express Delivers Higher Measurement Performance

Back to Top

Bookmark & Share

Ratings

Rate this document

Answered Your Question?
Yes No

Submit