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 mV range will be resolved to 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 = x 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