Very low distortion

I need to make a test source with THD ~ 0.0001% at the frequencies below

1kHz. Leaving many details aside, the biggest problem seems to be an opamp.

The typical THD vs frequency plot for a good opamp looks like this:

ADTL082

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It saturates at ~0.001% at the frequency ~2 kHz. Why is that? According to the GBV = 5MHz and G = 2e5, the corner should be at 25 Hz.

Perhaps the limit is set by Audio Precision; it can't measure THD below ~0.008% in direct way. But, even if 0.001% is the artifact of the measurement, there is still not enough of open loop gain to get to

0.0001% at 1kHz. Would it be possible to make a cascaded amp to boost the open loop gain and improve distortion?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky
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This might be better:

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Reply to
David Eather

p.

4.pdf

How about a different opamp? The opa134 seems to have enough zero's in the THD+noise spec. (0.00008%) Though my eyes start to boggle with so many zeros in a row.

George H.

Reply to
George Herold

Follow Jim Williams' rule: always invert. Add JL's rule: use really good resistors.

John

Reply to
John Larkin

Yes. See AN67, Linear Technology Magazine Circuit Collection, Volume III

An Ultrapure Oscillator By Dale Eagar.

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Article starts on page 62

Schematic is Figure 96 on page 65

Reply to
Mike

I wonder how one could ever verify low distortion. I guess you could trust Audio Precision or Agilent or somebody, at their guaranteed THD floor. But is there any fundamental way to demonstrate the THD of a source or analyzer, or to make a sinewave source that's known to have below, say, 1 PPM distortion?

How do you build a lowpass or highpass or bandpass filter that's known to have < 1 PPM distortion? Vacuum caps and toroidal inductors wound on quartz forms? Would there be mechanical issues that create distortion?

An RC highpass filter could be switched into the signal chain and its effect on distortion noted. If it were a very good filter, and the switch/connectors were to be trusted.

John

Reply to
John Larkin

The direct measurement with AP provides for the THD floor slightly below

0.001%. I don't know if it is limited by generator or analyser, but this is only -100 dB.

You just have to operate at small voltages and currents. Noise can be eliminated by sufficient averaging.

Unfortunately, there is very little data on distortion in the passive components. I did some experimentation with high voltage film capacitors and good quality resistors; it is feasible to make an active filter with

0.0001% distortion provided the opamp is good enough.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

That's what I need. Thank you, Mike.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

I was thinking of using a rigid vacuum capacitor into a TIA as a differentiator/high pass filter. If some sort of analyzer reports the amplitudes of the fundamental and the harmonics, then you boost the harmonic content with a known-good highpass, that sort of crosschecks the analyzer. Or make a really good lowpass, ditto.

We've occasionally used a passive notch filter to remove the fundamental of a sine wave before going into a spectrum analyzer. If the reported harmonics change with/without the notch, one suspects the analyzer.

Reply to
John Larkin

That's what I planned > This might be better:

I thought about bipolar opamps from National also. One drawback is high power consumption, the other problem is high noise current. I am not sure if it would be possible to make them work right with ~100k impedances in the circuit.

VLV

Reply to
Vladimir Vassilevsky

John Larkin a écrit :

I just have to design some mid power amplifier for 50kHz with 1 ppm distortion.

The first task was to set up the generator. Getting bellow ppm is easy.

Sort of.

Start from an already good source (about 50ppm) then set up a passive filter which first trap the unwanted harmonics (in my case I want bellow ppm H2). Two cells in series gives about 60dB rejection. Follow that with a parallel LC to reject the fundamental before getting to the SA and I get... 5-7ppm H2. Ahem... Even with small pulley inductors I should only have odd harmonics...

Eureka! Playing with a small toy magnet to cancel the inductors residual magnetization get me to the bellow ppm level.

Now the surprise is that that small (4mm dia x 2mm thick) magnet easily gets to the 10s of ppm level distortion from a two feet distance.

Time to go for an active filter...

--
Thanks,
Fred.
Reply to
Fred Bartoli

w

amp.

" That's what I planned on using initially. Unfortunately, the availability is an issue."

Geesh, you scared me there for a moment, (All my favorite TI/BB opamps seem to have 6 month delivery times these days.) but TI reports stock almost everywhere. Still I'd better order a couple of tubes.

George H.

Reply to
George Herold

mp.

I'm not sure why Jim Williams gets credit for "always invert." It is well known that op amp have less distortion in inverting comfiguration. For non-inverting, especially a follower, you are at the mercy of the common mode rejection of the differential pair.

Reply to
miso

ow

pamp.

f
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oted text -

Well, I first read it in either one of his app notes, or his reprints of app notes in one of his books. And doesn't the whole 'rule' go something like, "Always invert, except when you can't." Those are certainly words to try and design by.

George H.

Reply to
George Herold

You are welcome. Let us know when you have measured the distortion. :)

Reply to
Mike

Well, at minimum without filter, one can suspect overload distortion in the front end (if difference is seen). Noting that, there is no guarantee that all is OK if the filter is used.. ..only that things are a bit better..

Reply to
Robert Baer

  • Now we know why there is a stocking problem..
Reply to
Robert Baer

p.

At the lowest frequencies, the op amp temperature is an issue, but at

1 kHz the big problem will be in the output stage (usually class AB). Crossover distortion cannot be eliminated by feedback to the desired extent. You can apply a bias current to the output pin to move the bias to class A, which should improve most op amps. Preamp on one chip and power stage on a separate chip is a possible strategy, and heatsinking the power stage might help a lot.

Transformers and emitter followers are fairly safe, but resistors aren't as good as you'd want. Self-heating is completely nonnegligible at low frequencies, and the best resistors aren't specified for their thermocouple voltages AT ALL. Resistors will make second-harmonic distortion.

Lotsa luck verifying your distortion figures. I'm thinking there will need to be some tuned amplification.

Reply to
whit3rd

Tektronix called this effect "thermals", although that was regarding transistor self-heating specifically.

Curiously, this effect has been recognized since the vacuum tube days. Wonder if that's due to plate/grid heating, exapnding and distorting the electrode structure.

Of course, a resistor with AC across it will cause zero 2nd, but it will cause 3rd and other odds.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

This is just my curiosity..

Having a look at this circuit they are using an lt1228 OTA for amplitude control. Does anyone have any experience in using a light dependent resistor in this role and what sort of distortion did they experience?

Reply to
David Eather

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