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 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.

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