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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