MT-019 Datasheet, PDF (1/14 Pages) Analog Devices – DAC Interface Fundamentals

DAC Interface Fundamentals
by Walt Kester
MT-019
TUTORIAL
INTRODUCTION
This tutorial outlines some important issues regarding DAC interface circuitry including the
voltage reference, analog output, data input, and clock driver. Because ADCs require references
and clocks also, most of the concepts presented in this tutorial regarding these subjects apply
equally to ADCs.
DAC REFERENCE VOLTAGE
There is a tendency to regard DACs simply as devices with digital inputs and an analog output.
But the analog output depends on the presence of that analog input known as the reference, and
the accuracy of the reference is almost always the limiting factor on the absolute accuracy of a
DAC. Design tools such as the Voltage Reference Wizard are useful is matching references to
data converters. These tools and others are available on the Design Center portion of the Analog
Devices' website.
Some ADCs and DACs have internal references, while others do not. Some ADCs use the power
supply as a reference. Unfortunately, there is little standardization with respect to ADC/DAC
voltage references. In some cases, the dc accuracy of a converter with an internal reference can
often be improved by overriding or replacing the internal reference with a more accurate and
stable external one. In other cases, the use of an external low-noise reference will also increase
the noise-free code resolution of a high-resolution ADC.
Various ADCs and DACs provide the capability to use external references in lieu of internal
ones in various ways. Figure 1 shows some of the popular configurations (but certainly not all).
Figure 1A shows a converter which requires an external reference. It is generally recommended
that a suitable decoupling capacitor be added close to the ADC/DAC REF IN pin. The
appropriate value is usually specified in the voltage reference data sheet. It is also important that
the reference be stable with the required capacitive load (more on this to come).
Figure 1B shows a converter that has an internal reference, where the reference is also brought
out to a pin on the device. This allows it to be used other places in the circuit, provided the
loading does not exceed the rated value. Again, it is important to place the capacitor close to the
converter pin. If the internal reference is pinned out for external use, its accuracy, stability, and
temperature coefficient is usually specified on the ADC or DAC data sheet.
Rev.A, 10/08, WK
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