Just about every analog IC maker provides
graphical circuit design and analysis tools.
Most are Web-based. Some you download
to your PC. Either way, companies put them
there to harvest sales leads and gain customer
loyalty as you generally have to register to
use them.
In the most advanced cases, you can save
your design on the vendor’s Web site as you iterate it. When
you’re done, you can order a complete evaluation kit, with your
customized bill of materials (BOM) delivered to you anywhere
in the world in a day or so. As a compare and contrast exercise,
I surveyed the tools available and took as many as I could out
for a test drive.
NATIONAL SEMICONDUCTOR
National’s Webench has eight components: Signal Path,
Amplifiers, Audio, Filters, Power, LED, Interface Channels,
and Wireless. In Signal Path, the tool analyzes all the elements
in the design, from the signal being acquired to the analog-todigital
converter (ADC). With the Amplifiers tool, it’s possible
to create and simulate preconfigured operational-amplifier
(op-amp) circuits.
The Audio product search tool calculates thermal and frequency
response. The Filters section lets users design and optimize
an active filter. The Power tool not only provides design
and optimization, it also offers fast turnaround on hardware
prototype kits. Other tools include a parametric search engine
for drivers for high-brightness LEDs and one for phase-locked
loop (PLL) filter design.
Signal Path is the newest Webench tool. Helpfully, it has an
audio-visual step-by-step demo. Naturally, I ignored that and
went straight to the tool itself. It focuses on industrial measurements,
so you’re limited to sample rates of 5 Msamples/s and
below. That signal path you’re designing comprises the input, followed
by an anti-aliasing filter with a gain section. An RC filter
minimizes the effect of the ADC’s capacitance on the sensor.
I started by selecting the ADC101S101, a 3.3-V, 10-bit,
1-Msample/s serial-output ADC with a 62-dB signal-tonoise
ratio (SNR). It comes in an SOT-23 package and costs
$1.14 in sample quantities.
The next step was to specify the anti-aliasing filter. Possible
filter topologies included Bessel, Butterworth, Chebyshev 1.0,
0.5, 0.25, 0.10, and 0.01 dB, EquiRipple 0.5° and 0.05° error
(Linear Phase), and Transitional Gaussian to 12 or 6 dB. I figured
I’d see what the Bessel, Butterworth, and Chebyshev 1 dB
looked like as circuit layouts and Bode plots.
Demonstrating how long it’s been since I was in a classroom,
I naively assumed that while a brick-wall Nyquist filter was out
of the question, it should be easy enough to create an effective
filter with a passband in the low hundreds of megahertz. That
was wrong. Empirically, I determined that I could design a Butterworth for 25 kHz, but I had
to go down to 20 kHz before the
tool would come up with Bessels
and Chebyshevs. But why?
A useful resource is the Web
site’s help link located next to an
input box for intended bandwidth.
It explained that entering a value
for the max frequency for which
gain error is below the gain flatness
error also represents the max
rate of change that one would like
to capture for a sensor or the effective
bandwidth of a channel.
Figure 1 shows gain and phase
response for my design. The tool
depicts group delay and step
response. And, you can zoom into
the graphs to increase resolution.
Regardless of the type of filter you decide
upon, the circuit layout is the same. The
difference is in the component values.
Webench results include a BOM with all
the components identified. Following this
step, you can simulate the actual design.
While Signal Path is the newest tool,
the Webench Power tools are the most
mature. As with Signal Path, you click
on design parameters. After that, the tool
suggests buck, boost, flyback, inverting,
SEPIC, LDO/linear, or switched-capacitor
topologies, and voltage mode, current
mode, emulated current mode, or hysteretic
control methods. Simulations based on
Spice models are provided as Bode plots
and steady-state, input-transient, loadtransient,
and startup simulations.
TEXAS INSTRUMENTS
Search tools for different kinds of ICs dominate
the Texas Instruments Analog eLab
Design Center site. While they’re handy
product selectors, I was more interested in
TI’s Pro series tools for ADCs, digital-toanalog
converter (DAC) buffer amps, active
filters, and switching power supplies.
For example, a new “SwitcherPro tool”
includes a schematic editor and a Spice
simulator with PSpice syntax compatibility.
Optimized convergence algorithms
that have been updated for the current
release accelerate power-management
simulation. The Pro tools supplant some
earlier design software that TI has kept on
its site for the sake of designers who have
used those tools in the past.
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