PDF1. Overview of SC Express Family
The NI SC Express family offers PXI Express data acquisition (DAQ) modules with integrated signal conditioning for high-performance sensor measurements. National Instruments used the industry-standard PXI platform and implemented unique analog design technologies to provide increased accuracy, high bandwidth, and tight synchronization for advanced measurement systems that scale from tens to thousands of channels.
Figure 1. The PXI Express sensor measurement system is powerful and scalable.
2. High Measurement Accuracy
SC Express Design Minimizes Isothermal Error in Thermocouple Measurements
You can attribute thermocouple measurement error to many sources including noise, resolution, offset error, and gain error of the device, as well as external noise, thermocouple error, and cold-junction compensation (CJC) error. CJC error is often the largest error in thermocouple measurement devices. The NI PXIe-4353 thermocouple module compensates for the temperature at the cold junction by measuring the thermistor that is in close proximity. From this reading, NI-DAQmx driver software automatically determines the proper offset and applies it to the thermocouple measurements.
Figure 2. The TB-4353 has eight CJC thermistors to measure any temperature gradients across the terminal block.
The primary factors in CJC errors are thermistor error and isothermal error. The NI TB-4353 isothermal terminal block uses a high-accuracy thermistor that contributes only .15 °C of error when used at room temperature, including error from the measurement device. Isothermal error is the temperature difference between the thermistor and the actual temperature of the cold junction. The system environment and design of the measurement device impact this temperature difference. The TB-4353 has a unique design that allows heat to dissipate evenly between the cold junctions and the thermistors. It also features eight thermistors that are distributed on the terminal block and measured by the NI PXIe-4353 to further increase the accuracy of the CJC measurement. These CJC channels have a dedicated analog-to-digital converter (ADC), so they are measured in parallel with the thermocouple measurements.
To Learn More about Using SC Express for Accurate Thermocouple Measurements, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “SC Express Increases Thermocouple Measurement Accuracy” White Paper
Measuring Wheatstone Bridge-Based Sensors with the Ratiometric Approach
Many sensors, including strain gages, load cells, pressure sensors, and torque sensors, are based on the concept of a Wheatstone bridge. The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers. The output voltage of the bridge scales with the excitation voltage. However, the ratio of the bridge output (VCH) and the excitation voltage (VEX) remains fixed over variations in excitation voltage, and it is this unitless ratio (VCH/VEX) that is of interest. To accurately measure the ratiometric output of a bridge-based sensor, you must know both the bridge output voltage (VCH) and the excitation voltage.
Figure 3. The output of a Wheatstone bridge is the measurement between the middle nodes of the two voltage dividers.
The NI PXIe-433x bridge input modules uses circuitry that continuously measures the excitation voltage and applies it as a reference to its ADC. In this way, variations in the excitation voltage are compensated for, and the module returns data as a ratio of the bridge output voltage and the excitation voltage. This approach removes the measurement’s dependence on the stability and accuracy of the excitation voltage. Due to the ratiometric design, measurements with the NI PXIe-433x bridge input modules are less susceptible to temperature drift and other variations in the excitation voltage.
To View More about Ratiometric Bridge-Based Measurements, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “Measuring Bridge-Based Sensors with the Ratiometric Approach” White Paper
24-Bit Delta-Sigma Modulation
The NI PXIe-433x bridge input modules and NI PXIe-4353 thermocouple input module take advantage of 24-bit delta-sigma ADCs for high-precision strain, bridge-based, and temperature measurements. The differentiating aspects of delta-sigma ADCs are the use of oversampling in conjunction with decimation filtering, and quantization noise shaping. A delta-sigma ADC behaves as a lowpass filter for the signal and a highpass filter for the quantization noise, thus pushing the noise to higher-frequency regions. This phenomenon is called noise shaping, and is taken advantage of by employing digital decimation that filters high frequencies from the modulator output and removes the quantization noise. Noise power is reduced in the frequency band of interest, which means a higher signal-to-noise ratio (SNR) or a greater dynamic range. Delta-sigma modulation has several benefits including better antialiasing and higher resolution.
To See the Benefits of Delta-Sigma Modulation, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “Benefits of Delta-Sigma Analog-to-Digital Conversion” White Paper
3. Maximum Bandwidth
PXI Express Increases Data Throughput
By taking advantage of PCI Express technology in the backplane, PXI Express increases the available PXI bandwidth from 132 MB/s to 6 GB/s for a more than 45-times improvement in bandwidth while still maintaining software and hardware compatibility with PXI modules. The most notable PCI Express advancement over PCI is its point-to-point bus topology. The shared bus used for PCI is replaced with a shared switch, which provides each device its own direct access to the bus. Unlike PCI, which divides bandwidth between all devices on the bus, PCI Express provides each device with its own dedicated data pipeline. Data is sent serially in packets through pairs of transmit and receive signals called lanes, which allow 250 MB/s bandwidth per direction, per lane. Multiple lanes can be grouped together into x1 (by one), x2, x4, x8, x12, x16, and x32 lane widths to increase bandwidth to the slot. PCI Express dramatically improves data bandwidth compared to PCI buses, minimizing the need for onboard memory and allowing faster data streaming.
PXI Express Sensor Measurements
SC Express modules are based on x1 PXI Express, which offers 250 MB/s of bandwidth per device to stream data back to the controller. Because of the added bandwidth provided by PXI Express, the SC Express modules offer simultaneous sampling options using the same channel counts as multiplexed devices. Unlike multiplexed devices that reduce sampling rates as you add more channels, you can use the simultaneous sampling devices to maintain sampling rates as you expand the number of channels. As seen in Figure 4, the SC Express modules can scale to high channel counts at maximum sample rates without exceeding the communication bus bandwidth.
Figure 4. SC Express modules scale to high channel counts without exceeding communication bus bandwidth.
Data Streaming Performance
When the communication bus is not a limiting factor in measurement speed or the number of channels, the processing speed or the streaming-to-disk performance becomes the obstacle to system performance. The NI-DAQmx driver allows data to stream directly to the Technical Data Management Streaming (TDMS) file format, providing logging throughput up to 400 MB/s rendering terabytes of data quickly. You can use the NI LabVIEW DataFinder Toolkit or NI DIAdem data management software to easily manage, analyze, visualize, and report meaningful information from these large data sets.
4. World-Class Synchronization
PXI Express Timing and Synchronization
As the latest evolution of the PXI platform, PXI Express has been designed to deliver a higher degree of synchronization to measurement I/O devices while remaining backward-compatible. The 100 MHz differential clock in the PXI Express backplane allows multiple devices to base their sample clocks on the same reference. The backplane trigger lines allow for coordinated triggering of all the devices in the chassis to the same 100 MHz clock edge. As with previous PXI chassis, a timing and synchronization module can replace the backplane clock with a higher precision time base in a single PXI Express chassis or it can also be used to synchronize measurement devices in multiple PXI Express chassis. If the application requires multiple chassis to be synchronized over large distances, then GPS or IRIG-B PXI timing modules can synchronize multiple chassis by a common clock signal.
Save Development Time with NI-DAQmx Multidevice Tasks
National Instruments has enhanced the NI-DAQmx driver with an innovative approach to configure multiple SC Express modules for synchronization. You can program a full chassis of any single SC Express module type with one multidevice task. With multidevice tasks, you can scale your application from one to 544 channels using the same NI-DAQmx code. You can also use a single NI-DAQmx task to automatically synchronize channels across multiple boards with different measurement types, as shown in Figure 5.
Figure 5. This block diagram shows example LabVIEW code for acquiring multiple modules with different measurement types.
For example, an 18-slot chassis with a combination of NI PXIe-433x bridge input modules and NI PXIe-449x dynamic signal acquisition (DSA) modules can be programmed with a single task resulting in less than 150 ns of module-to-module skew. The combinations in Table 1 support multidevice tasks across measurement types.
|
SC Express Support for Multidevice Tasks |
Module-to-Module Skew |
|
NI PXIe-4300: High-Voltage Analog Input Module |
<50 ns |
|
NI PXIe-433x: Bridge Input Modules |
<150 ns |
Table 1. These PXI Express module combinations support multidevice tasks.
In addition, you can synchronize SC Express modules with more than 1,500 modules on the PXI platform by passing timing clocks in software.
To Read More about Synchronization with SC Express, Download the Free “Achieving Higher Performance with SC Express” Resource Kit or Read the “Synchronization Explained” White Paper
5. Conclusion
National Instruments designed the SC Express family to measure sensors and high voltages with high accuracy, maximum bandwidth, and world-class synchronization. The SC Express family offers the most advanced signal conditioning on PXI Express and helps you achieve high-performance measurements while reducing development time.
