D/A Conversion
Analog outputs commonly are used to operate valves and motors in industrial environments and to generate inputs for electronic devices under test. Digital-to-analog (D/A) conversion is in many ways the converse of A/D conversion, but tends to be generally more straightforward. Similar to analog input configurations, a common D/A converter often is shared among multiplexed output signals. Standard analog output ranges are essentially the same as analog inputs: ±5 V dc, ±10 V dc, 0-10 V dc, and 4-20 mA dc.
  Essentially, the logic circuitry for an analog voltage output uses a digital word, or series of bits, to drop in (or drop out, depending on whether the bit is 1 or 0) a series of resistors from a circuit driven by a reference voltage. This ladder of resistors can be made of either weighted value resistors or an R-2R network using only two resistor values-one if placed in series (Figure 1-13). While operation of the weighted-value network is more intuitively obvious, the R-2R scheme is more practical. Because only one resistor value need be used, it is easier to match the temperature coefficients of an R-2R ladder than a weighted network, resulting in more accurate outputs. Plus, for high resolution outputs, very high resistor values are needed in the weighted-resistor approach.

Figure 1-13: Weighted Value & Single Value Resistor Networks for D/A Conversion

  Key specifications of an analog output include:     Settling time: Period required for a D/A converter to respond to a full-scale setpoint change.     Linearity: This refers to the device's ability to accurately divide the reference voltage into evenly sized increments.     Range: The reference voltage sets the limit on the output voltage achievable.
  Because most unconditioned analog outputs are limited to 5 mA of current, amplifiers and signal conditioners often are needed to drive a final control element. A low-pass filter may also be used to smooth out the discrete steps in output.

	References and Further Reading
	The Data Acquisition Systems Handbook, Omega Press LLC, 1997.
	New Horizons in Data Acquisition and Computer Interfaces, Omega Press LLC, 1997.
	Omega® Universal Guide to Data Acquisition and Computer Interfaces, Omega Press LLC, 1997.
	Analog I/O Design: Acquisition, Conversion, Recovery, Patrick Garrett, Reston Publishing Co., 1981.
	Analog Signal Processing and Instrumentation, Arie F. Arbel, Cambridge University Press, 1980.
	Analog-To-Digital and Digital-To-Analog Conversion Techniques, David Hoeschele, John Wiley & Sons, 1994.
	Analog-To-Digital Conversion: A Practical Approach, Kevin M. Daughtery, McGraw Hill, 1995.
	Automation Systems for Control and Data Acquisition, Lawrence T. Amy, ISA, 1992.
	Data Acquisition and Control, Microcomputer Applications for Scientists and Engineers, Joseph J. Carr, Tab Books Inc., 1988.
	Data Acquisition and Process Control Using Personal Computers, Tarik Ozkul, Marcel Dekker, 1996.
	Instrument Engineers' Handbook, Third Edition, Bela Liptak, Chilton Book Co., 1995.
	Process/Industrial Instruments & Controls Handbook, Fourth Edition, Douglas M. Considine, McGraw-Hill Inc., 1993.