It may seem strange that in a digital world, some of the most expensive watches are analog. This is because it takes a lot of skill to create an instrument that continuously displays the exact time of day. Analog watches remind us that the natural world is not digital and hence there will always be a need for engineers with analog design skills. Unfortunately, these skills are becoming increasingly scarce due to a misperception that analog design is ‘outdated’ and ‘difficult’.
While in the past, it may have been portrayed as a ‘dark art’ requiring a combination of tedious hand calculations, cumbersome laboratory equipment and rules of thumb, analog design has changed considerably. Nowadays, analog designers have access to the kind of innovative hardware and software tools that were previously only available to their digital counterparts. In this article we will review the traditional approach used by analog design engineers when designing circuits. We then highlight the extraordinary impact that new design tools can have when performing these same tasks – helping to make analog circuit design more attractive to the next generation of engineers.
The analog signal chain As with time, natural phenomena such as light, heat and pressure are analog quantities, meaning they vary continuously. Sensors (also called transducers) convert these signals into an analog voltage which forms the input to an electronic control system (e.g. central heating). This voltage signal is usually tiny and often contaminated by other signals unintentionally picked by the sensor, making its raw form unsuitable for use by the digital microcontroller in the system. To overcome this, analog engineers develop a series of circuits known as the analog signal chain, which modifies the sensor output, making it useful to the system (figure 1).
In the above block diagram, the amplifier increases the amplitude of the sensor signal while the filter circuit removes unwanted signals and noise (of higher or lower frequencies). The amplified and filtered sensor output is then fed to an analog-to-digital converter (ADC), where it is converted to a digital format suitable for processing by the system microcontroller. While this task may appear to be straightforward, the combined variety of sensor types and operating conditions make the design of the analog signal chain challenging. In the next section, we will explore why the design of analog filter circuits was traditionally time consuming.
Old-school filter design
An analog filter circuit typically consists of several active and passive components (operational amplifiers, resistors, capacitors and sometimes inductors). The design task is further complicated by the variety of filter types available, and their associated specifications. A methodical approach to design is required in which the first step is to decide the type of filter required for the application. Options available include:
- Low-pass (or LPF to remove high-frequency signals)
- High-pass (or HPF to remove low-frequency signals)
- Band-pass (or BPF that only allows signals within a defined frequency range to pass through)
The next step is to select a filter transfer function to meet the filter specifications, including:
- Bandwidth, which describes the range of frequencies that a circuit allows to pass through unattenuated (little or no reduction in amplitude).
- ‘Roll-Off,’ which describes the rate of attenuation or how sharply the filter begins to remove unwanted frequency components in the sensor signal.
- Phase, referring to the relative delay between the input and output signals. This is important if a feedback loop is being used in the signal chain, as it can affect loop stability.
A transfer function is a complex mathematical formula that describes the frequency response of the filter (the relationship between the input and output signals). Analog designers must match the desired filter frequency response as closely as possible to pre-calculated frequency responses for different filter types, as described in filter tables (Butterworth, Bessel, Chebyshev and others). Having selected the filter type that best matches the desired performance, the designer must next calculate the component values so that they can build (or simulate) a real circuit. Once that stage is completed, the filter performance can be evaluated to check that it meets the required specifications. This can be a time consuming and sometimes frustrating process, often needing to be repeated several times until the best compromise is found.
New age filter design While the analog design procedure described previously may be familiar to many, the modern-day reality is quite different thanks to the availability of tools like ’Analog Filter Wizard’ from Analog Devices. This piece of software fully automates the filter design flow from initial filter selection through to real-world prototyping. After making an initial filter selection (LPF, HPF or BPF), the designer simply enters the filter specifications into a user-friendly GUI which displays and dynamically adjusts a visual representation of the filter frequency response (figure 2).
Making further adjustments is as easy as re-entering specification values and/or moving a slider. After the desired frequency response has been specified, the tool automatically displays the circuit that will deliver it (figure 3). There is no need for the designer to manually match transfer functions to tables of different filter types. Even the circuit component values are provided, with no need for mathematical calculations. Far from simply providing only an ‘ideal’ circuit model, the tool also allows the designer to specify component tolerances, so they can get a feel for the ‘real-world’ performance of the circuit. It also provides fully functional SPICE files (no debugging required) to allow quick and easy simulation of the effects of temperature and voltage.
Next generation hardware
Apart from the significant improvement in software, the advance in available hardware tools is equally impressive. Traditionally, analog engineers set up their circuits in a lab because the equipment they needed – a separate power supply, oscilloscope, and signal generator – was cumbersome and had limited portability. Connecting each of these devices to a circuit quickly leads to an unwieldy tangle of wires and probes that can make fault-finding tricky. Thankfully, this complicated setup can now be replaced using a single USB-powered device, the Digilent Analog Discovery 2, which combines the features of an oscilloscope, signal generator and power supply into a pocket-sized device. As a result, analog engineers now only need a laptop to quickly set up and evaluate circuit performance, with potentially no need to visit the lab at all. More specialist designs may still need the capability of a purpose-built lab but for many general purpose applications, the Digilent approach is ideal.
Without analog circuits, the digital world that we take for granted could not exist. Analog signals can have any value at any time, making the design of analog circuits challenging. However, the development of sophisticated design tools (both software and hardware) has automated many of the more difficult design tasks. Analog design engineers will always be in demand and the opportunity to work with these leading-edge tools is certain to stimulate a renewed interest in this profession among young engineers.
Mark Patrick is Mouser Electronics’ technical marketing manager for EMEA, responsible for the creation and circulation of technical content within the region to support, inform and inspire its engineering audience. Prior to this, Patrick was part of the EMEA supplier marketing team and played a vital role in establishing and developing relationships with key manufacturing partners. His previous roles include eight years at Texas Instruments in applications support and technical sales. A “hands-on” engineer at heart, with a passion for vintage synthesizers and motorcycles, he thinks nothing of carrying out repairs on either. Patrick holds a first-class honors degree in electronics engineering from Coventry University in the U.K.
- Pulsic offers real-time analog chip layout preview in schematic editor
- Selecting the right software defined radio solution for your application
- How to generate a Bode plot with LTspice