We welcome back guest blogger James Bryant of Analog Devices this week.
You've told us that we need not be too careful when choosing discrete transistors. What about analog ICs?
This is done with a glass slipper. Have your footmen take the slipper to every manufacturer in the kingdom, and hold it by the door. If
is reflected in the glass, you'll be certain of finding your heart's desire within.
More seriously, overspecifying — or calling for arcane and generally unknown devices — can complicate the selection process and increase the cost of ICs, as well as discrete transistors, so you should never demand hard-to-find parts or unnecessary parameters. But ICs are much more complicated than single transistors, making it very important to define all the needed parameters.
This RAQ will talk about general principles, not particular types of ICs, such as amplifiers, references, or converters. The parameters common to many precision analog ICs are absolute maximum ratings, ESD ratings, supply voltages, supply currents, power dissipation, temperature range, temperature coefficients, supply rejection, noise, packages, input impedance, bias current, analog output characteristics, frequency range, and digital interface specifications such as speed, logic type, logic levels, data configuration, and chip enable/shutdown functions. Before working on the parameters for a specific type of device, it is important to decide which of these general parameters are important and to set acceptable limits to their values.
Starting with a list of important parameters, including the device-specific ones that I'll discuss later, arrange them in order of importance. Next, using online parametric search engines from several manufacturers, compile a list of devices that meet all the requirements. Since this requires visits to several websites, and manufacturers will present the data in different formats, it might be nice to use a distributor's website, which should display devices from different manufacturers in a standard format. Unfortunately, parametric comparisons from distributors are rarely adequate. They are improving, but for now it's generally better to use the manufacturers' sites and to compare devices on paper or in a spreadsheet.
If no suitable device can be found, decide which parameters might be relaxed, and by how much, and try again with the new values. Then review all of the device parameters, in case a feature makes one device better or worse for the application.
Finally, list all the devices that will do the job well, and determine which is the least expensive. The cost of using an IC includes the cost of the device itself, and also the cost of any additional components or special power supplies, the cost of any adjustments or calibrations that must be made during manufacture, and the cost of the printed circuit board area that it occupies. Be sure to choose the device that is the least expensive to use, not the cheapest one.
— James Bryant has been a European applications manager with Analog Devices since 1982. He holds a degree in physics and philosophy from the University of Leeds. He is also C.Eng., Eur. Eng., MIEE, and an FBIS. In addition to his passion for engineering, he is a radio ham and holds the call sign G4CLF.
Other articles like this Visit EDN and see How to choose analog ICs, Part 1