eFunda: Data Acquisition Hardware
 Data Acquisition Hardware
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Hardware Objective

The task of data acquisition hardware is to collect incoming analog input signals and convert them to digital signals for further processing and storage. Ideally, it should also be able to convert outgoing digital signals (i.e. instructions) to analog output for the excitation of a target actuator. Additional features such as digital I/O can enable the functions of counter, trigger, or switcher, which are all beneficial to measurement efforts.

Typical analog input specifications of commercially available data acquisition hardware are listed 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 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, for healthy (and 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. However, in that case, 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 smaller than half of the least significant bit, or LSB), relative accuracy, amplifier settling time (if used), and noise.

Analog Output Specifications
Common specifications for analog output of commercially available data acquisition hardware are listed 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.

Besides the above common specifications, some factors should also be considered, such as 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 convertor (DAC) can produce on the output signal.)

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