I made a simple amplifier with a CMOS OPAMP (AD8656 or AD8616). The distortion is very low with AC/DC power (LM317), 0.0005%. However, if I use batteries, the distortion goes up to 0.0025%. Adding a power filtering cap does not make a difference. If I use a bipolar OPAMP (AD8397), then no difference between AC/DC and battery. Why CMOS does not like battery? Any idea?
If the power supply to the op amp sags significantly during parts of your test waveform, then it would increase distortion.
Have you checked for this? Monitoring the power supply voltage with a scope while you drive your op amp may be very educational.
If it's a problem you may need a _big_ power supply cap to smooth the battery voltage rather than the little one necessary to prevent oscillation. You may also want to consider a regulated supply off the battery (perhaps switched to handle battery sag).
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Are the DC voltages identical when you switch from LM317 supply to a battery supply?
Those distortions are really small numbers. If you measure stuff like that day-in and day-out, you can probably ignore the next paragraph :-).
I have had a couple of situations where the ground reference and capacitance changed substantially between line-powered supply and batteries. In every single case where going to batteries increased noise or distortion, it was "user error" for unintentionally relying on a low-impedance ground through the power supply.
Tim.
use
No idea about the prob' but intrigued to know how you're measuring those helluva low distortion numbers. Notch? Notch+spectrum analyser?, Auto notch
-- Posted via a free Usenet account from http://www.teranews.com
I use RMAA with a EMU 1212 audio card.
what are the figures you get when you loop through the soundcard, bypassing the external opamp circuit?
martin
Thanks! . Really nice kit there. Much, much better than my standard sound card. john
I
Any
those
[ As a low end comparison to John's good kit, my cheap 'n' cheerful PC souncard gives a noise floor of about 2uVrms with clipping at about 110mV rms. ]-- Posted via a free Usenet account from http://www.teranews.com
That piece of crap could not legitimately make those measurements if your life depended on it. The data presented is no longer credible. Just try buying real test equipment that is specified to make measurements in that range, the cost will be over 300 times the cost of that card.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
No, not really. Just a few more channels. A fake entry aimed at the ignorant, trying to get to the all together readily accessed semi-pro market. Not much price difference either.
-- JosephKK Gegen dummheit kampfen die Gotter Selbst, vergebens. --Schiller
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