How online resources can support and simplify the design process

Designers need to perform various calculations when creating a circuit. For everything from simple component selection to designing analogue filters and radio frequency (RF) circuits, tools are available to help. In this article, we will review online calculators and design tools and describe the features of some popular circuit simulation tools.


Even with application circuits and reference designs available from many manufacturers, calculations remain a large part of a designer’s workload. Some may be simple, such as implementing Ohm’s law, while others are more complex, especially those relating to analogue design techniques such as filters, power systems and RF projects.

Mistakes can be made with even the simplest calculations, while complex mathematics can consume significant time. There is also the need to consider real-world effects such as parasitic elements that add more complexity, especially if high degrees of accuracy are required.

Fortunately, there is a wealth of resources online—many of which are free of charge—to assist designers. For example, Texas Instruments’ analog engineer’s calculator (Figure 1) is a Windows®-compatible basic calculator that supports many common tasks, including using passive components, setting op-amp gain, and estimating the parasitic effects of a PCB. It is also useful when designing with analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs) as well as sensors.


With project timescales under pressure and the costs of re-spinning a design high, ‘right first time’ is an essential approach to design. Both analogue and digital simulators are available. Used correctly, these simulation tools can give designers confidence that they are very close to real-world solutions before they spend a penny on hardware.

Simulation Program with Integrated Circuit Emphasis (SPICE) is the industry-standard tool that has been around for half a century, and many manufacturers provide SPICE models of their components. For example, Analog Devices offer its own simulation tool, LTspice® (Figure 2), which includes its own SPICE models.


Once a designer has entered their schematic and defined input signals, the software will provide a detailed simulation and other system analysis. Most analogue techniques, such as RF and power audio, can be simulated.

These days it is even possible to use mobile devices for simulation. While not as sophisticated as its desktop cousins (and lacking some commercial model support), EveryCircuit is a good tool for learning and experimenting. A free trial is available to download.

In fact, there are many calculators available, many of which can be accessed directly from Mouser Electronics’ Conversion Calculators page. While many tools are general purpose, some focus on specific disciplines such as analogue, RF, and power.

Tools for Analogue Filter Design

Analogue filters are a fundamental building block of many systems, whether for processing a signal or another function such as removing unwanted noise.

There are four basic analogue filter configurations:

  • High-pass: Allows higher frequencies through and blocks lower frequencies
  • Low-pass: Allows lower frequencies through while blocking higher frequencies
  • Band-pass: Passes only frequencies within a certain range
  • Band-stop or notch: Cuts out a range of frequencies

A simple filter can comprise just passive (inductance, capacitance, and resistance, or LCR) components, although it is common to include an op-amp to enhance performance and flexibility. When designing a filter, several parameters are important for defining how well the filter will perform its task (Figure 3), including the frequency at which it will begin to operate (cut-off) and the rate at which it blocks unwanted frequencies (slope or roll-off). Another important parameter is the “quality” parameter, called Q, which defines how close the filter is to an ideal (i.e., theoretically perfect) response. 


There are many ‘classic’ filter designs that are taught to engineers. For example, Butterworth filters have a flat frequency response in the passband, while Chebyshev filters offer a faster roll-off at the expense of more ripple in the pass-band or stop-band (Figure 4).


When designing any filter, the transfer function is defined by the L, C, and R—or by the cut-off frequency and Q values. The impedances and the frequencies are complex numbers, thereby entailing the solving of complex polynomials. Furthermore, calculating the phase response requires even more complex calculations.

Support for these calculations is available online from companies such as Analog Devices and Texas Instruments.

The Analog Devices Filter Wizard (Figure 5) allows users to interactively select the filter type and response as well as define the basic parameters. It includes the ability to use and evaluate op-amps and the transfer function; the phase response can also be plotted. The design can be saved in various formats, including as a SPICE model that can then be used in simulations.


Texas Instruments’ filter design tool offers very similar functionality, including the ability to define filter construction and parameters before saving as a SPICE model.

Tools for Power Conversion

There are many techniques available for power conversion, and selection is usually based upon the requirements of the load. The simplest is an unregulated approach where the output voltage may vary in response to changes in the input or the load; often these consist of a transformer, a rectifier, and a capacitor for output smoothing.

In some cases, a regulated supply will be needed to provide a more tightly controlled output. These more sophisticated designs can also include mechanisms that protect the supply and load in the case of a fault. Most often regulated supplies are switched mode but some specialist applications continue to use linear conversion techniques.

If additional voltages are required, then voltage regulators may be used for localised conversion.

Power supplies are relatively complex designs, not least to comply with modern regulations relating to safety and efficiency. At a basic level, a designer will need to know the input voltage type and range as well as the required output voltage and power. The ability of the load to withstand ripple and noise as well as voltage fluctuations is also important information to define a power conversion solution. Given the fact that every design needs some form of power conversion, this is a very common design task, and many tools exist to help designers complete their projects.

Analog Devices provides several tools for power circuits, including LTpowerCAD® for DC-DC power supply design and simulation. This program recommends component values and estimates performance in a specific application. Further analysis and simulation are possible by exporting to LTspice.

Coilcraft provides online tools that guide the choice of transformers and inductors for use in almost any design. They also provide links to useful simulation tools from other sources.

The Analog Devices EE-Sim® Solution Finder online tool enables designers to find devices based on key parameters and features before transferring to the EE-Sim DC-DC Converter Tool to generate and simulate a complete design.

The WebDesigner+™ Power Supply design tool from onsemi lets designers select from a broad range of ICs, MOSFETs, and diodes using sophisticated filters (Figure 6). Suited to AC/DC and DC/DC techniques, the tool also models designs and provides reports. 


STMicrolectronics’ eDesignSuite contains tools for DC/DC and AC/DC power supply design, especially solar-powered battery chargers.

The TI WEBENCH® Power Designer is an online tool that configures AC or DC input power supply circuits based on specific applications.

Further power-related tools are available from Mouser’s Power Management resources.

Tools for RF Design

RF design is fundamental to our modern society as it encompasses technologies such as Bluetooth®, Wi-Fi, and 5G cellular. Simply put, without RF we would not be able to connect or communicate.

However, RF design can be challenging as the high frequencies imply very short wavelengths that bring the physical properties of the PCB and components into play. Therefore, designers will analyse the design in terms of the behaviour of electromagnetic waves rather than simply voltages and currents.

Alongside this, designers must also be mindful of impedance matching, reflections, crosstalk, PCB design, antennas, and a whole variety of other RF-specific phenomena.

While voltages and currents are relevant to RF, additional parameters are normally needed to define circuit behaviour adequately. These include bandwidth, signal power, signal-to-noise ratio (SNR), and RF interference (RFI). In addition, there will be various regulatory requirements depending upon what the design is and where it is to be used.

With the added complexity of RF design, tools are particularly useful for design support as they can help with circuit calculations and PCB layout. RF simulators must model the electromagnetic effects of IC packages and interconnects as well as circuit operation.

Analog Devices has various tools that can help with RF system design, including a PLL synthesiser (ADIsimPLL™) and a 50-stage RF signal chain simulator (ADIsimRF). ADIsimSRD is a design studio that allows designers to create and hone short-range wireless systems.

The Analog Devices website also features a tool which matches complex loads to a line impedance and another to convert between standard units of power measurement and signal strength (e.g., VRMS, dBm, dBu, dBV).

RF component specialist Qorvo offers several tools to assist with RF design, enabling designers to address several previously mentioned issues, such as matching source and load or calculating power transfer with reflected standing waves.


While modern designs can be fraught with complexity and are required to be completed under significant time pressure, getting products to market ‘right first time’ remains important for time and cost savings.

In recent years, a multitude of useful design tools have appeared online that help designers with tasks such as component selection, design calculations, and circuit simulation. By utilising these (often free-of-charge) tools, it is now possible to have a high degree of confidence in any design before committing to (and incurring the expense of) a hardware prototype.

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