AN_7370 pages

AN-737
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com

How ADIsimADC Models an ADC
by Brad Brannon and Tom MacLeod

CONVERTER MODELING
Converter modeling has often been overlooked, omitted,
or accomplished using an ideal data converter model. With
more and more systems using mixed-signal technology, the
importance of system modeling is ever increasing. Coupled
with shortened design cycles and pressure for first pass success,
this drives the continuing importance of complete system
modeling. ADIsimADC™ has been developed to answer this
growing need.
Often ideal converter models are used for functional modeling,
but these fail to give the required details of performance to
determine if a particular device meets the desired goals of the
system. This is why ADIsimADC has been developed. For the
first time, ADIsimADC provides a means for users to validate
performance of a particular converter in their system, using
their conditions to determine the applicability of a selected
device. While ADIsimADC does not emulate every characteristic of an ADC, it goes a long way towards achieving the goal
of allowing users to model real converters in their system
simulations.

BIT EXACT vs. BEHAVIORAL
A bit exact model is a model that, given a known stimulus,
provides a known and predictable output. ADIsimADC is not a
bit exact model. These types of models are often found in digital
systems. In dealing with analog functions, there is never a known
response for a given input because of noise, distortion, and
other nonlinearities. While some portion of the response may
be predictable, much of the remainder is subject to distortion,
noise, and even part-to-part variation. Additionally, to provide
a bit exact model requires providing circuit simulation files,
such as SPICE models, that process transient response. However,
these models are large, complex, very slow and, in the end,
provide limited accuracy. A reduced or equivalent SPICE model
reduces the complexity, but is not able to provide adequate
modeling of fine details of static and dynamic performance.
A behavior model eliminates the complexity and, at the same
time, allows modeling of fine performance details not possible
to attain with a circuit file. ADIsimADC in conjunction with
VisualAnalog™ acts as a standalone converter evaluation tool.

ADIsimADC™ can also be used with many other third party
simulation tools, including ADS from Agilent Technologies,
VSS from Applied Wave Research, Inc, National Instrument
tools, as well as MATLAB® and C++. Usage information with
these tools can be found at www.analog.com/ADIsimADC.

MODEL vs. HARDWARE
Modeling a system or an ADC should never be a substitute for
building and characterizing a real system. It is one thing to model a
circuit, but it is another matter to actually build it and test it.
As with any analog or mixed-signal device, proper layout and
configuration is required to achieve the performance shown in
simulation. Therefore, it is important that all layout rules and
guidelines be followed as shown in the product data sheet (see
Figure 4). An example is the importance of providing adequate
power supply bypass capacitors. Because mixed-signal devices
include some amount of digital circuitry, digital switching noise
is often a problem, and failure to provide capacitors to moderate
these switching currents can significantly reduce performance
of even the best devices. Other support devices are often required
around the converter, including additional capacitors, inductors,
and resistors. The best way to know what is required is to consult
the product data sheet and the evaluation board schematic.

WHICH SPECIFICATIONS ARE IMPORTANT TO
MODEL?
ADIsimADC is targeted to provide realistic performance of real
devices. Which specification is important to model depends on
what kind of analysis the user is trying to perform. For example,
control loops need accurate transfer function and delay information, while radio systems may require an accurate representation of noise and distortion. ADIsimADC models many of the
critical specifications of data converters, including: offset, gain,
sample rate, bandwidth, jitter, latency, and both ac and dc linearity.
(See the AN-835 Application Note Understanding High Speed
ADC Testing and Evaluation, for additional information on ac
linearity.)
This application note describes these specifications in detail and
how ADIsimADC treats them.

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