Electronic Design

Online Design Boosts Analog Engineer Productivity

Why fuss around on your workbench? A little web surfing will provide all the tools you need to streamline your design.

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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For my test I selected SwitcherPro. I elected to design a dc-dc buck converter for 3.3 V. Of the IC choices presented, I opted for TI’s TPS54610, which integrates the switching MOSFETs and can handle up to 6 A. The tool immediately produced the schematic shown in Figure 2. (This screen capture ignored my mouse arrow, which was pointed at capacitor C9.) The power of SwitcherPro is in its analysis capabilities (Fig. 3). The tool immediately produced a schematic. Actual BOM data pops up when you roll your cursor over any of the components in the schematic.

ANALOG DEVICES
Analog Devices’ design tools include an ADC simulator called ADIsim- ADC. They also feature a number of design assistants, an error-budget analysis tool, and a downloadable version of the company’s Multisim Spice Program.

Designed for op amps, one of those design assistants lets users select an amplifier, configure a circuit, apply a signal, and evaluate bandwidth, slew rate, input/output range, gain error, and load current while assessing stability issues and dc errors in that circuit. There is a similar tool for differential amplifiers, as well as others expressly for amplifiers used with photodiodes and bridge sensors.

A filter wizard automates the design of Bessel, EquiRipple, Chebyshev, Gaussian, and Butterworth active filters, based on the op amp selected. ADI’s OpAmp Range/ Gain/Error Calculator calculates the error budget for selected op amps based on operating temperature, topology, gain, and resistor tolerance.

ADI’s site also offers harmonic imager tools that display harmonic images and spurs in DAC outputs and helps designers create post-DAC filters to suppress the images. The other companies’ sites didn’t have this feature.

Finally, a voltage-regulator design tool helps optimize buck regulators for efficiency, printed-circuit board (PCB) space, cost, or part count. Users enter five design parameters and select a design goal. The tool then calculates optimal values to produce a complete schematic, BOM, efficiency plot, and performance summary.

For a test drive, I chose the Web version of the ADIsimADC tool. It uses the “typ” datasheet values to model the general behavior of a particular chip in response to selected input signals and sample rates. It generates time and frequency-domain fast Fourier transforms (FFTs) and shows values for SNR, spurious-free dynamic range (SFDR), signal, noise, and distortion (SINAD), total harmonic distortion (THD), and effective number of bits (ENOB). It’s noteworthy to mention that a more comprehensive version is available for downloading

As with the other tools, users start by selecting a device, either from a parts list or by identifying required performance parameters. I chose the 12-bit, 250-Msample/s AD9626. Types of simulation include single-tone and two-tone, along with frequency and amplitude sweeps. The tool helps inexperienced users understand practical design limitations.

For example, I started by specifying a two-tone input with the ADC operating at 250 Msamples/s and one of the inputs at 50 MHz. I immediately got a warning indicating that the specified encode rate was an integer multiple of my signal frequency and that this can lead to degraded performance. Further, quantization noise can be concentrated at harmonics of the fundamental, reducing SFDR dramatically. I had not realized that.

A two-tone simulation naturally produces a frequency-domain FFT plot (Fig. 4). A frequency sweep produces a continuous plot of SNR versus frequency, and an amplitude sweep yields a plot of spur-free dynamic range versus frequency.

LINEAR TECHNOLOGY
Linear Technology’s tools are all downloadable packages that users can run on their own PCs. The tools consist of SwitcherCAD, BodeCAD, and FilterCAD, plus a program that lets designers get a handle on noise in op-amp circuits. Linear also has some evaluation boards that interface to a common USB-based interface.

On the power side of design, SwitcherCAD III is a Spice III simulator, schematic capture, and waveform viewer with enhancements and models that make simulating switching regulators faster and easier than other Spice simulators. BodeCAD works together with SwitcherCAD to perform large signal ac analysis automatically and extract the Bode plot of the power system.

For amplifier design, Linear’s “Configurator” generates schematics for amplifier stages built around the company’s gainselectable amplifiers. FilterCAD does what the name implies for Linear’s proprietary active-filter ICs, such as the LT1568, which incorporates circuitry that makes it simple to build fourth-order filters from a handful of parts.


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MICROCHIP
All of these tools invite you to jump in and see what happens. If you make an entry that doesn’t conform to good design practices, they may even warn you and suggest an alternative. Microchip’s MINDI is different in several ways.

Like Linear Technology’s tools, it runs on your machine rather than in a browser window. But that’s not the big difference. Smart users approach MINDI with its manual in their hands. The reward for this is greater design versatility for experienced engineers and greater depth of explanation for novices.

To get the feel of the tool, download the documentation from the Microchip site at www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2534&pageId=64&redirects=mindi. Each of its three manuals is a 2- to 4-Mbyte Adobe file. They’re comprehensive enough for basic instruction for sophomore EE candidates, making them great just to refresh your memory. But depending on how familiar you are with the material, you’ll probably want to print out selected pages and put them in a binder.

Downloading and installing the actual software on a Windows XP machine is quick. Launching the program starts a command shell. If you experiment, you can find demonstrations. MINDI provides more flexibility than the other tools discussed here, including actual schematic capture and more extensive simulation. I could have played with MINDI’s capabilities for several days, if I didn’t have deadlines for this story.

It’s always rewarding to produce a design from the sweat of one’s brow and an intuitive understanding of fundamentals. But sometimes design time is short and it’s been a long time since you’ve thought about those fundamentals. When the chips are down, it’s handy to be able to access the experience of engineers who live and breathe particular application areas. And most importantly, it’s wise to know what’s out there.

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