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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!
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.
Hope this helps. It’s always good to get feedback from readers.
— Gerard Fonte
September 2007 97