Nuts and Volts - September 2007

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READER FEEDBACK

Continued from page 8

Fonte mentioned in the article that a more sophisticated filtering circuit would be better. I agree but am wondering if he has tried anything that he is happier with.

Thanks for any help you can provide. The ARB looks very interesting and I want to build it as soon as possible. Keep up the good work!

Jim Rybak Grand Junction, CO

Response: The important characteristics of the op-amp in question are: 1) operate from a single supply; and 2) be fast enough ( gain-bandwidth product >1 MHz). The LMC6042 has a GBW of only 0.1 MHz and is too slow. If used, there can be additional problems at the high frequency end. The LMC6442 has a GBW of only 9. 5 kHz and cannot be used. (I used the LMC6082 because I had it on hand.)

The LM6142 is a good substitute. It operates from a single supply and has a GBW of 17 MHz. It is available from Jameco and other suppliers in the eight-pin DIP package (check the website at www.jameco.com). Note: The National Semiconductor website (and most other IC manufacturers) will often provide samples for free and/or provide access to small quantities for evaluation. The important requirements are that you have to provide a business address and a proper engineering job description. (A school address will probably work, too.)

In regards to the filtering question, this application seems to be an ideal project to incorporate a tunable switched-capacitor filter. There is already a variable high-speed clock available that is 25 times faster than the output. The July 2007 issue of N&V (Making Waves) discusses SC filters.