Choosing DAQ hardware: PCI board, USB I/O or Ethernet I/O

The same measurement task can often be solved with a plug-in PCI or PCI Express DAQ board inside the test PC, a USB I/O module next to a laptop, or an Ethernet I/O module next to the test object. The three form factors behave very differently in sample rate, timing, isolation and cabling. This article goes through the criteria that matter when choosing DAQ hardware for a test or lab setup, and shows where the ICP DAS boards and modules fit.

Sample rate and simultaneous sampling

The sampling theorem requires a sample rate of more than twice the highest frequency in the signal. In practice you sample several times faster, and you need either an anti-alias filter or a signal with no content above half the sample rate.

Check whether a rate is quoted per channel or in total. Many multi-channel inputs share one converter between channels. The ET-2217H, for example, samples 8 kHz in total: 1 kHz per channel with all eight channels active, or 8 kHz on a single channel. A scanned input also adds a small time offset between channels, which matters when you compare phase, for example between voltage and current.

Simultaneous sampling removes that offset. The PCIe-8620 and PCIe-8622 boards sample 8 or 16 channels simultaneously at 200 kS/s per channel with 16-bit resolution. Over Ethernet, the PET-7H16M offers 8 simultaneously sampled 16-bit channels at 200 kS/s each, and the PET-7H24M 4 differential 24-bit channels at 128 kS/s each.

Example: eight ±10 V sensor outputs with content up to 5 kHz need well over 10 kS/s per channel, captured at the same instant. The ET-2217H is too slow for this. A PCIe-8620 in the test PC or a PET-7H16M next to the rig covers it with margin.

Latency and data transfer

Sample rate and latency are different things. A board or a streaming module can capture data at a precise, hardware-timed rate and still deliver it to your program in blocks.

• PCI and PCIe boards buffer samples in an on-board FIFO, for example 8,192 samples on the PCI-1802LU and PCI-2602U, and are read by polling, interrupt or DMA depending on the model. This is the shortest path between the signal and your test software.
• USB I/O such as the USB-2026 uses USB 2.0 full speed (12 Mbps) and suits readings at tens or hundreds of samples per second, such as its 200 Hz fast mode, rather than waveform capture.
• Ethernet I/O usually works in command-response mode: the module samples into its Modbus register table, and the host reads the latest values when it asks. The ET-2217H and the PET-7H modules also support TCP streaming of continuous sample blocks. Streaming gives high throughput, but the block size adds delay, so it suits recording better than fast closed-loop control.

Isolation, distance and channel count

Isolation protects the PC and the measurement against ground potential differences and transients from motors or power electronics. Examples are bus-type isolation on the PCIe-8620 (2500 VDC) and PISO-813U (3750 Vrms), 3000 VDC I/O isolation on ET-2200 analog modules such as the ET-2217H, 3000 VDC channel-to-channel isolation on the ET-2217CI, and 3000 VDC intra-module isolation on the USB-4018HS.

Distance decides where the electronics should be. With a PC board, every sensor cable runs back to the PC, which picks up noise on long runs of low-level signals. USB cables are limited to about 5 m without active extenders. Ethernet allows 100 m per copper segment, and the 2-port switch on ET-2200 modules lets you daisy-chain modules along a test line.

Channel count is simple on a board as long as one board is enough. The PCI-AD64SU takes 64 single-ended or 32 differential 16-bit inputs, with 1 MS/s on a fixed channel and 250 kS/s when scanning. Its Card ID switch lets software tell identical boards apart in one PC.

Software and drivers

Check driver support before you compare specifications. Boards such as the PCI-1802LU and PCI-1602U are supported on Windows, Linux and DOS, with a DLL, an ActiveX control and sample programs for C++, Delphi, Visual Basic, C#.NET, VB.NET and LabVIEW. The USB-2026 comes with an API library for Windows and Linux; the USB-4018HS uses a virtual COM port and Modbus RTU. Ethernet modules use Modbus TCP, and ET-2200 models add MQTT and a web server. For TCP streaming, the ET-2217H and PET-7H16M have SDKs for Windows (including Python demos and a LabVIEW toolkit) and Linux.

Choosing DAQ hardware: side-by-side comparison

Which ICP DAS product groups fit

• DAQ boards: Universal PCI multifunction boards (PCI-1202, PCI-1602 and PCI-1802 series, PCI-2602U), PCI Express boards (PEX-1202, PCIe-8620, PCIe-8622), the 64-channel PCI-AD64SU and the isolated PISO-813U. Choose these for high sample rates and simultaneous sampling.
• USB I/O modules: the multifunction USB-2026, the USB-4018HS thermocouple modules, digital I/O and relay modules, and USB-87P expansion units for I-87K I/O modules. Choose these for portable rigs and laptops.
• Ethernet I/O modules: the ET-2200 series with high-speed analog (ET-2217H), RTD (ET-2215H) and thermocouple (ET-2218HS-16) inputs, PoE versions with a PET- prefix, and the PET-7H16M and PET-7H24M DAQ modules. Choose these when the measurement must sit close to the test object.

Send your signal list, ranges, sample rates and distances to our technical sales team, and we will suggest a suitable combination.