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Op amps implement cheap peak detectors

Posted: 16 Jan 2007     Print Version  Bookmark and Share

Keywords:programmability  digital hardware  analog peak detection  analog circuit 

I recently received an e-mail from a student who needed to design a circuit to detect peaks in a 10MHz analogue signal. His plan was to use an ADC sampling at well beyond the Nyquist frequency of 20MSps, and then pass the successive converter outputs to a fast processor running a tight, high-priority software loop while disallowing interrupts. In short, his approach was expensive and power hungry, and had a large footprint.

While I hesitate to offer free design advice (since free advice is often worth what the recipient pays for it), I did suggest an alternative approach: the classic analogue peak detector. Using a fast, precision op amp, a few low-leakage diodes and a low-leakage capacitor, you can have the core of a very nice peak-detector circuit for about Rs.197.39 ($5). It will be low power and have minimum real-estate needs; it's also fast, cheap, reliable and easy to test. What's not to like?

Unfortunately, the student—a potential practicing EE—simply could not grasp the concept of an analogue-based peak detector. It's not that he wasn't familiar with this particular circuit (I can understand that), but all he could think about was getting the signal into digital format so it could be properly assessed. The idea of computing and analysing in the analogue domain was foreign to him; as they say, for him it "did not compute."

That is too bad, because for many signal-processing needs, analogue is the better—and often the only—way to go. If you need a logarithmic (log) amplifier, an rms-to-DC converter for line power or RF signals, think analogue. You may have no choice.

The peak-detector topology was a popular analogue circuit back in the days before processors ruled our design world. The data sheet for the OPA111 op amp from Burr-Brown (now part of Texas Instruments), for example, showed a possible circuit that I believe was a monolithic version of an older, multichip Burr-Brown device.

There is some irony here, since today's op amps are so much better and cheaper than those of the past that they can be used to implement really high-accuracy, high-speed, inexpensive peak detectors.

- Bill Schweber
Site Editor, Planet Analog

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