Data Acquisition Hardware

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Q: What does Data Acquisition (DAQ) hardware do?

A: DAQ hardware acts as the interface between the computer and the environment being sampled or tested. It mostly functions as a device that converts incoming analog signals into digital signals so that a computer (PC) can interpret them. Other data acquisition functionality includes:

  • Analog Input/Output
  • Digital Input/Output
  • Counter/Timers
  • Multifunction – a mixture of analog, digital, and counter operations on a single device

Q: What kind of DAQ hardware components comprise myriad DAQ systems?

A: Many DAQ hardware/software manufacturers consider the analog-to-digital (A/D) converter (aka ADC) to be the core component of a DAQ hardware system. A data acquisition card can offer measurements of up to 64 channels at a resolution of 16 bits, (one part in 65,536) with data throughput rates up to 20 million samples per second. Often, the same card can include discrete digital bi-directional I/O lines, counter timers, and D/A converters for outputting analog signals for control applications. The A/D convert or can be a stand-alone device, such as this USB-based module from National Instruments and this USB Data Acquisition Module form IOTech, or it may be a PCI card that is installed in a common personal computer (here are some examples of such cards from IOTech), or a PXI (PCI eXtensions for Instrumentation) interface that install into dedicated chassis for portable, bench-top, rack-mount, and embedded applications. Data acquisition systems can easily be incorporated into common networks; here is an example of an Ethernet-based solution. If portability in a DAQ systems is required, look for a PCMCIA DAQ card for notebooks and laptops.

The A/D converter may also be found in an integrated component that offers features such as additional amplification and signal conditioning (examples: SCC and SCXI from National Instruments).

Other stand-alone DAQ hardware includes:

Q: What are the typical analog input specifications for commercially-available data acquisition hardware?

A: See table below:

Number of Input Channels: 2 ~ 16, specified for both single-ended (i.e. common grounded, high signal to noise ratio, short lead wires) and differential inputs.
Sampling Rate: 10 kHz ~ 1 MHz. According to the Shannon sampling theorem (Nyquist sampling rate) the sampling rate be should no less than twice the highest frequency in the targeted signals. For instance, the sampling rate used to collect audio signals should not be less than 40 kHz (CD is 44.1 kHz), since healthy young human ears can detect acoustic signals up to 20 kHz.
Multiplexing: Defined as the real maximum sampling rate = listed sampling rate / number of channels.
Buffer Size: 512 ~ 2048 samples
Range: ±10 V peak for the signal entering the DAQ hardware. If the signal is amplified prior to the DAQ, then the effective range, with respect to the target signal, is hardware range / amplifier gain.
Resolution: 8 ~ 24 bits. The maximum range of the DAQ is divided into
(28 - 1) ~ (224 - 1) equal spacing slots. Signal voltages that fall within a given slot will be interpreted (digitized) as the voltage representing that slot. Combined with selectable gain, the smallest voltage DAQ hardware can resolve is given by,
(Maximum Range / Gain) / (2Resolution - 1)
For a 16-bit, -10 ~ 10 Volt range, 40 dB gain DAQ card, the voltage resolution could be as small as (10 / 100) / (216 - 1) = 1.5 µV and the maximum allowable input should not be larger than (10 / 100) = 0.1 V.

Some other factors to be considered include differential non-linearity (DNL, which should be less than half of the least significant bit (LSB)), relative accuracy, amplifier settling time (if used), and noise.

Q: What are the typical analog output specifications for commercially-available data acquisition hardware?

A: See table below:

Number of Output Channels: 1 ~ 2
Sampling Rate: 5Hz ~ 500 kHz
Resolution: 8 ~ 16 bits
Range: ±10 V peak to peak or 0 ~ 10 V

Other than the above common specifications, some other factors should also be considered, including: Settling Time (the time required for the output to settle to the specified accuracy), Slew Rate (the maximum rate of change that the digital to analog converter (DAC) can produce on the output signal.

Q: What is "Form Factor"?

A: The type of hardware bus(es) used in DAQ equipment. These types may include: Compact FieldPoint, CompactDAQ, CompactFlash, CompactRIO, Ethernet, FireWire, ISA, PCI, PCI Express, PCMCIA, PXI, PXI Express, SCC, SCXI, USB

Q: What are common operating systems (OS) encountered in data-acquisition hardware?

A: Windows, Linux, Real-Time, Pocket PC, Mac OS, RTX

Q: What is "Isolation" and what are common isolation types found in data-acquisition equipment?

A: Isolation electrically separates the sensor signals, which may be exposed to hazardous voltages, from the measurement system’s low-voltage backplane. . Types include: Bank Isolation, Ch-Ch Isolation, Ch-Earth Ground Isolation.

Q: What are common instrument or measurement types used in measurement found in data-acquisition equipment?

A: Bridge-based sensor, Current, Digital, Filters, Frequency, IEPE accelerometer, LVDT, Quadrature encoder, RTD, RVDT, Resistance, Strain gauge, Thermistor, Thermocouple, Voltage.

Q: What are the common Analog Input types found in data-acquisition equipment?

A: None, 1, 8, 16, 32

Q: What are the common Analog Output types found in data-acquisition equipment?

A: one, 1, 2, 4, 8

Q: What are the common Digital I/O Channels found in data-acquisition equipment?

A: None, 1, 8, 16, 32

Q: What are the common Counter/Timers found in data-acquisition equipment?

A: None, 1, 2, 4, 8

Q: What are the various Triggering methods used in data-acquisition equipment?

A: Analog, Digital, Video

NEXT: Data Acquisition Devices: Specifications and Selection Criteria

Updated: Tuesday, March 3, 2020 22:55 PST