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.

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

Conversion Scheme

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

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 2 12 , 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.

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.

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

Gain

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

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