Introduction to Data Acquisition
Data acquisition systems, as the
name implies, are products and/or
processes used to collect
information to document or analyze
some phenomenon. In the simplest
form, a technician logging the
temperature of an oven on a piece of
paper is performing data acquisition.
As technology has progressed, this
type of process has been simplified
and made more accurate, versatile,
and reliable through electronic
equipment. Equipment ranges from
simple recorders to sophisticated
computer systems. Data acquisition
products serve as a focal point in a
system, tying together a wide
variety of products, such as sensors
that indicate temperature, flow,
level, or pressure. Some common data acquistion terms are shown below:
- Analog-to-digital converter (ADC)
An electronic device that converts analog signals to an equivalent digital form.
The analog-to-digital converter is the heart of most data acquisition systems.
- Digital-to-Analog Converter (D/A)
An electronic component found in many data acquistion devices that produce an analog output signal.
- Digital Input/Output (DIO)
Refers to a type of data acquistion signal. Digital I/O are discrete signals which are either one of two states. These states may be on/off, high/low, 1/0, etc.
Digital I/O are also referred to as binary I/O.
- Differential Input
Refers to the way a signal is wired to a data acquisition device.
Differential inputs have a unique high and unique low connection for each channel.
Data acquisition devices have either single-ended or differential inputs, many devices support both configurations.
- General Purpose Interface Bus (GPIB)
Synonymous with HPIB (for Hewlett-Packard), the standard bus used for controlling electronic instruments with a computer.
Also called IEEE 488 in reference to defining ANSI/IEEE standards.
The smallest signal increment that can be detected by a data acquisition system.
Resolution can be expressed in bits, in proportions, or in percent of full scale.
For example, a system has 12-bit resolution, one part in 4,096 resolution, and 0.0244 percent of full scale.
A standard for serial communications found in many data acquistion systems. RS232 is the most common serial communication, however, it is somewhat limited in
that it only supports communication to one device connected to the bus at a time and it only supports transmission distances up to 50 feet.
A standard for serial communications found in many data acquistion systems. RS485 is not as popular as RS232, however, it is more flexible in
that it supports communication to more than one device on the bus at a time and supports transmission distances of approximately 5,000 feet.
- Sample Rate
The speed at which a data acquisition system collects data. The speed is normally expressed in samples per second. For multi-channel data acquisition devices
the sample rate is typically given as the speed of the analog-to-digital converter(A/D). To obtain individual channel sample rate, you need to divide the speed of the A/D by
the number of channels being sampled.
- Single-ended Input (SE):
Refers to the way a signal is wired to a data acquisition device. In single-ended wiring, each analog input has a unique high connection but all channels share a common ground connection. Data acquisition devices
have either single-ended or differential inputs. Many support both configurations.
Wireless Data Aquisition Systems
Wireless data acquisition systems can eliminate costly and time consuming field wiring of process sensors. These systems consist of one or more wireless transmitters sending data back to a wireless receiver connected to a remote computer. Wireless transmitters are available for ambient temperature and relative humidity, thermocouples, RTDs, pulse output sensors, 4 to 20 mA transmitters and voltage output transducers. Receivers can be connected to the USB or Ethernet port on the PC.
Serial Communication Data Acquistion Systems
Serial communcation data acquistion systems are a good choice when the measurement needs to be made at a location which is
distant from the computer. There are several different communication standards, RS232 is the most common but only supports tranmission
distances up to 50 feet. RS485 is superior to RS485 and supports transmission distances to 5,000 feet.
USB Data Acquistion Systems
The Universal Serial Bus (USB) is a
new standard for connecting PCs to
peripheral devices such as printers,
monitors, modems and data acquistion devices. USB offers
several advantages over conventional
serial and parallel connections,
including higher bandwidth (up to
12 Mbits/s) and the ability to provide
power to the peripheral device.
USB is ideal for data acquisition
applications. Since USB connections
supply power, only one cable is
required to link the data acquisition
device to the PC, which most likely
has at least one USB port.
Data Acquisition Plug-in Boards
acquisition boards plug
directly into the
computer bus. Advantages of using boards
are speed (because they are
connected directly to the bus) and
cost (because the overhead of
packaging and power is provided by
the computer). Boards offered are
primarily for IBM PC and compatible
computers. Features provided by the
cards can vary due to number and
type of inputs (voltage,
thermocouple, on/off), outputs,
speed and other functions provided.
Each board installed in the computer
is addressed at a unique
Input/Output map location.
The I/O map in the computer
provides the address locations the
processor uses to gain access to
the specific device as required by