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Digitization

Digitization. When data acquisition hardware receives an analog signal it converts it to a voltage. An A/D (analog-to-digital) converter then digitizes the signal and makes it ready for transfer to a computer or to a display. Digitization of an analog signal requires two separate operations.

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Digitization

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  1. Digitization • When data acquisition hardware receives an analog signal it converts it to a voltage. An A/D (analog-to-digital) converter then digitizes the signal and makes it ready for transfer to a computer or to a display. • Digitization of an analog signal requires two separate operations. • Define the number of points and the rate at which data are acquired. • Quantization--conversion of data into numerical form.

  2. Signal Inputs into an A/D converter • Single-Ended • All inputs are referenced to a common ground • Adequate for high level signals • Less expensive but problematic if grounding problems exist. (Ground Loop Problems) • Differential • Differences between Hi input and Lo input are measured directly without the influence of ground loop interference. • About 2 times the expense of single ended inputs • Needs 2 times as many wires • Always use for thermocouples and low voltage applications

  3. Conversion Scheme

  4. A/D (analog-to-digital) converters • Many specifications are quoted by hardware manufacturers. Here, we’ll try to explain what some of them mean in practice. For example: • Resolution • Linearity • Throughput • Gain

  5. Resolution • Resolution of an A/D converter is the number of steps into which the input range is divided. Resolution is usually expressed as bits (N) and the number of steps is 2 to the power of N. • Example: A converter with a 12-bit resolution divides the range into 212 , or 4096 steps. • A 0-10 Volt range will be resolved to 10V/4096 or 0.25 mV. • A 0-100 mV range will be resolved to 0.0025 mV. • A -10 to 10 V range is resolved by 20V/4096.

  6. Throughput • Throughput is the maximum rate at which the A/D converter can output data values. • A converter that takes 10 microseconds to acquire and convert will generate about 100,000 samples per second.

  7. Accuracy • % Reading + Count • Look at the scale being used. It will display the value, as given by that scale. • Multiply the readying by the % reading value. • Add the “Counts” x the value of the least digit presented. The example in the manual: V=134.2 mV Accuracy = 134.2 x 0.008 + 2 x 0.1 = 1.3 mV

  8. Gain • On board amplifiers may permit you to reduce the range and thus increase resolution.

  9. GPIB(General Purpose Interface Bus) • Also known as IEEE-488 • Started by HP (HPIB) • 16 line parallel connection • Advantages • Fast data transfer rates • Up to 1 MB/s • Multiple devices (15) on each GPIB • Disadvantages • Limited transmission lengths (2 m to 4 m) between devices • Need GPIB adapter in PC

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