CMOS OPAMP and battery = higher distortion?

[John Smith]

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?

 

[Tim Wescott]

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

 

[Tim Shoppa]

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?

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.

 

[John Smith]

Thank you Tim, I tried a 15000uF power bypassing, no difference.

 

[John Smith]

Thanks Tim, yes both LM317 and batteries are set to be 5V.

 

[Eeyore]

I made a simple amplifier with a CMOS OPAMP (AD8656 or AD8616). The
distortion is very low with AC/DC power (LM317), 0.0005%.

At what fequency ?

However, if I use batteries,

The same voltage ?

the distortion goes up to 0.0025%. Adding a power filtering cap

What value ?

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?

More details please. Can you post a schematic in
alt.binaries.schematics.electronic ? What's the spectrum of the distortion
products for example too ?

Graham

 

[Eeyore]

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.

Hmm...... Yup.

Those milliohms do count.

Graham

 

[martin griffith]

Hmm...... Yup.

Those milliohms do count.

Graham

never heard a milliOhm count.......a quantum fx maybe?


martin

 

[john jardine]

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?

No idea about the prob' but intrigued to know how you're measuring those
helluva low distortion numbers. Notch? Notch+spectrum analyser?, Auto notch
+ FFT?. Straight digitise + FFT?.
john

 

[John Smith]

I use RMAA with a EMU 1212 audio card.

 

[martin griffith]

I use RMAA with a EMU 1212 audio card.

No idea about the prob' but intrigued to know how you're measuring those
helluva low distortion numbers. Notch? Notch+spectrum analyser?, Auto
notch
+ FFT?. Straight digitise + FFT?.
john

what are the figures you get when you loop through the soundcard,
bypassing the external opamp circuit?


martin

 

[[email protected]]

I use RMAA with a EMU 1212 audio card.

Thanks! . Really nice kit there.
Much, much better than my standard sound card.
john

 

[john jardine]

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

[ 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. ]

 

[joseph2k]

I use RMAA with a EMU 1212 audio card.

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.

 

[joseph2k]

Thanks! . Really nice kit there.
Much, much better than my standard sound card.
john

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.

 

[Eeyore]

That piece of crap could not legitimately make those measurements if your
life depended on it. The data presented is no longer credible.

Can you elaborate on your thinking behind that statement ?

Graham

 

[joseph2k]

Can you elaborate on your thinking behind that statement ?

Graham

I google'd for precision audio test equipment; no product i found could
reach those measurement levels, and all even close fits were in over 20,000
USD. EMU1212 are about 400 USD IIRC.

I have seen the construction (i have an emu0404), it cannot really produce
the requisite performance.

 

[Eeyore]

I google'd for precision audio test equipment; no product i found could
reach those measurement levels, and all even close fits were in over 20,000
USD. EMU1212 are about 400 USD IIRC.

I have seen the construction (i have an emu0404), it cannot really produce
the requisite performance.

The classic Audio Precision brand test gear can measure THD down to 0.0006% and
that was introduced in the late 1980's.

I'd expect a high end sound card to have little trouble doing that.

All the top audio test gear is now DSP based.

Graham