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Data Acquisition Hardware

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

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.

　

Data Acquisition Devices: Specifications and Selection Criteria

The following lists contain specification parameters you will find in various DAQ hardware devices.

Main Categories

Bus Type

  • USB
  • PXI
  • PCI
  • PCI Express
  • FireWire
  • PCMCIA
  • ISA
  • CompactFlash

Analog Input Channels

  • At least 8
  • At least 16
  • At least 32

Analog Input Resolution

  • At least 12 bits
  • At least 16 bits
  • At least 20 bits

Analog Input Sample Rate

  • Less than 1 kHz
  • At least 1 kHz
  • At least 100 kHz
  • At least 1 MHz
  • At least 10 MHz

Note: Some devices use the nomeclature S/s (Samples per second) instead of Hz. They both mean the same thing.

Max Analog Input Range

  • 10 mV
  • 80 mV
  • 1 A
  • 3 A
  • 5 V
  • 10 V
  • 20 V
  • 25 V
  • 40 V
  • 42 V
  • 42.4 V
  • 60 V
  • 100 V
  • 250 V
  • 300 V
  • 1000 V

Analog Output Channels

  • At least 2
  • At least 4
  • At least 8

Digital I/O Channels

  • At least 8
  • At least 16
  • At least 32

Triggering

  • Analog
  • Digital

OS Support

  • Windows
  • Linux
  • Real-Time
  • Pocket PC
  • Mac OS
  • RTX

Measurement Types

  • Thermocouple
  • Strain gauges
  • IEPE accelerometer
  • Bridge-based sensors
  • Quadrature encoders

Counter/Timers

  • At least 2
  • At least 4
  • At least 8

Additional Categories

Number of Analog Input Ranges

  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • 14
  • 15

Analog Input Signal Conditioning

  • Low-pass filtering
  • Current excitation
  • Ch-Ch isolation
  • Bridge completion
  • Anti-aliasing filter

Analog Output Resolution

  • 12 bits
  • 13 bits
  • 14 bits
  • 16 bits
  • 24 bits

Analog Output Update Rate

  • Less than 1 kHz
  • At least 1 kHz
  • At least 100 kHz
  • At least 1 MHz
  • At least 10 MHz

Note: Some devices use the nomeclature S/s (Samples per second) instead of Hz. They both mean the same thing.

　

　

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