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Know your duty when duty cycling

Posted: 22 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:SAR ADC? PSoC 4? voltage? switched-capacitor? input buffer amplifier?

They say that power corrupts. I don't know about that; maybe I'll let you know when I actually have some. But one thing I have found in electronic system design is that the search for low power corrupts. Corrupts signals, that is. And the scary part is that the people doing the searching often don't realize it.

With all this talk about the Internet of Things (IoT) and cute little widgets sipping energy frugally from their tiny batteries, everyone is trying to minimize power consumption in electronic systems. The standing current taken by circuit blocks such as amplifiers and converters, for any given level of performance, has fallen steadily over the many years that I've been plying the electronics trade. However, it hasn't fallen fast enough to keep pace with our requirements for long battery life.

And that means that, more and more, system designers are resorting to the ultimate sanction: Duty-cycling.

Now, you probably know what duty-cycling is. If I had my shiny new copy of The 3rd Edition of The Art Of Electronics in front of me C by the way, when you have finished reading this, order yourself a copy of this, it is the most important book in the electronics pantheon, lexicon, bibliodrome, whatever C I'd look up what H&H have to say about it. But I don't, so I'll just tell you what I think, and what I've found recently.

My employer (full disclosure: Cypress Semiconductor) recently launched a great crossover product C a mixed-signal system-on-chip with an on-chip Bluetooth Smart radio. It's exciting, because all the cool analog sensor interfacing stuff I've done with colleagues over the past few years can now be bolted seamlessly to your smartphone and thence to the web. Duty-cycling at the radio and digital processing level is intrinsic to the achievement of low power in these modern radio systems; no mystery. So some of our guys wanted to build a little demo system that integrated the radio with some galvanic heart-rate monitoring stuff I'd done a while back.

Hmm, I said, I'm not sure the approach I took was optimum for power consumption. Let me go and think about it, see if I can get the front-end current consumption down.

"Oh, don't worry, we're going to duty-cycle the analog too."

As you might expect, this triggered my "let me explain why you might want to do things differently" mode.

Some kinds of analog circuits can be switched off and back on again with impunity. So-called 'memory-less' circuits have a theoretical transfer function that doesn't refer to previous states. An ideal amplifier is an example. Its output now is a simple linear function of its input now. It's not a function of its input then, and it's not a function of its power supply voltage then.

A 'real' amplifier will have a finite bandwidth and, as all you Fourier fanatics know, defining a frequency range inherently defines a time interval. Still, most op-amps are up and rarin' to go not that long after you give them some power, or pull them out of shutdown with a dedicated pin.

What's an example of a circuit block with memory? (Waits expectantly for many hands in the audience to go up). Well, quelle surprise, a filter. Any circuit that has a response to a sinewave excitation that depends on the frequency of that excitation has a signature in the time domain. Its output is partly dependent on the history of its input.

And that means that it will remember if you, um, mucked around with it a little while ago. This in turn means that you have to be very careful if you decide you are going to duty-cycle it, in your search for reduced power.

The proposal I have seen all too often involves a sensor-processing front end that involves some filtering. It could be high-pass filtering, to trim off DC offsets and low frequency disturbances or low-pass filtering, to suppress untoward goings-on that might occur if nasty high frequency signals get into your low-sample-rate converter. Especially in the latter case, this will often involve an op-amp or two in some sort of active filter configuration. "When we want to sample the filtered signal, we'll turn the filter back on." I'm pretty sure I have heard those exact words.

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