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Old 14th March 2012, 09:07 AM   #101
PChi is offline PChi  United Kingdom
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@richiem -- Thanks for your feedback and answers.
I'll not fit the AC coupling capacitor or the 49600 / 49710 combo.
I'll stay with the single ended output.

If someone designs a 16-bit ADC with a bandwidth of 500 kHz I would build one as well.

I have investigated the rapid rise in THD with frequency by both changing the level and using a passive Twin-T filter and I agree that it is caused by GBW limitation. The OPA134 operational amplifiers data sheet features list imply that they are good for 600 ohm loads but the typical performance curves show a marked increase in distortion when the load is reduced from 2 kohm to 600 ohm. Douglas Self's measurements in 'Small Signal Audio Design' confirm this. I kept the impedances low in the filter for low noise and dip a little below 600 ohms.
I built the Distortion Analyzer back in 1999 and used the OPA134 because they were in the original schematic from the Electronics and Wireless World article. I think that it's time to change them for some LME49710s which can drive 600 ohm with low distortion (according to the data sheet).
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Old 14th March 2012, 09:24 AM   #102
coluke is offline coluke  Italy
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Quote:
Originally Posted by PChi View Post
I think that it's time to change them for some LME49710s which can drive 600 ohm with low distortion (according to the data sheet).
Was it a design by Ian Hickman? If you are going to replace the OPA2134s with LME49710s then you will probably need to lower quite a bit resistor values both in the inverter stage and in the integrators - 2134's current noise density is about 3 orders of magnitude lower than the 49710's.

L.
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Old 14th March 2012, 09:50 AM   #103
PChi is offline PChi  United Kingdom
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Originally Posted by coluke View Post
Was it a design by Ian Hickman? If you are going to replace the OPA2134s with LME49710s then you will probably need to lower quite a bit resistor values both in the inverter stage and in the integrators - 2134's current noise density is about 3 orders of magnitude lower than the 49710's.

L.
Thanks for your input. Yes is was a design by Ian Hickman published in the August 1999 edition of Electronics World. I modified the design but my memory is a little vague and my documentation is incomplete. The changes were:-
I didn't include the Low Pass Filter. I was going to exceed the pin limit of the Vutrax PCB design software version I was using. Also the roll off is set for 20 kHz. I guess OK for measuring distortion at 1 kHz but not that much above.
I didn't use the residual amplifier design as I thought that it was lacking in bandwidth so I used 4 stages each using an OPA134 with switchable gain x1 or x10.
I used an Analog Devices AD536A rms to DC converter instead of the full wave rectifier and averaging circuit. I had the Analog Devices part on a board and I don't like just measuring the average of noise or distortion residual. Also full wave rectifier circuits can have bandwidth limitations.
I changed all the Notch Filter component values. I believe that there are some errors in the original schematic. R19 needs to be reduced for the notch filter to have unity gain for the residual. I believe that the values of R28 and R30 are transposed.
I reduced all the impedances by about 100 so the current noise density should be less of a problem but the OPA134 output drive capability is.
I also added a notch bypass switch, buffer and level set control.
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Old 14th March 2012, 10:57 AM   #104
coluke is offline coluke  Italy
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I reduced all the impedances by about 100 so the current noise density should be less of a problem but the OPA134 output drive capability is.
I agree - if the driving impedances of the three stages are already in the kohm range the 49710's higher current noise density should not be a problem. I don't have Hickman's schematic at hand at the moment, but if I remember well it was a simple and clever design, with steep response despite of the pretty low filter Q.

L.

Last edited by coluke; 14th March 2012 at 11:19 AM.
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Old 14th March 2012, 05:09 PM   #105
richiem is offline richiem  United States
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But the lowered Zs cause other, possibly serious, problems. Davada has observed the significant loss of open-loop gain due to even moderate output loading. A spec sheet statement about driving low-Z loads never points out this type of loss, which has big impacts on THD. Also, although the LME79XXX parts have many virtues, high input Z is not one of them, so that has to be taken into account, too, especially for integrators.

I think some form of compound circuit, or perhaps better, paralleling amps to increase drive and preserve open-loop gain, will turn out to be important for performance improvements.

I've not seen the Hickman analyzer article(s) -- are they available somewhere?
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Old 14th March 2012, 06:55 PM   #106
coluke is offline coluke  Italy
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Could be a problem if the load was * very * low and you are looking for -120dB or lower THD throughout the whole audio band, of course, but - as far as i remember - hi-perf opamps are usually specified @ 2kohm load for THD, and @ 600 ohm, 2kohm and 10kohm load for OLG; Self in his 'Small signal audio design' checked a bunch of opamps for THD down to 500 ohm load @ 10Vrms out (i.e. true border-line operating conditions), and found very little (if any) loading-induced THD for quite a few of them (LM4562/LME49710, the venerable 5534 itself, AD 797, OPA627/637).

Anyway, going for such a low value of feedback resistors is pointless - I think something in the 10k range for the input inverter of an SVF notch, although not yet optimal from the noise point of view, could be a good trade-off between input impedance and noise itself; the 100 kohm Hickman used throughout his design are certainly appropriate for an OPA2134 (In ~ 3 fA/sqrt(Hz) @ 1kHz), but are way too high for a LME497XX (In ~ 1.6 pA/sqrt(Hz) @ 1 kHz).

Hickman's notch: I know I do have somewhere in my bookshelf a copy of the August 1999 EW issue containing the design, but I need some time to look for it - I'll let you know.

L.
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Old 14th March 2012, 09:07 PM   #107
coluke is offline coluke  Italy
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Davada has observed the significant loss of open-loop gain due to even moderate output loading.
Oh, I missed this line - seems an interesting point: what do you mean by 'moderate'? And at what frequencies? I have a couple of experimental SVF generator based on LM318s and NE5534s working with 3.3 kohm resistors both in the feedback of the inverter stage and in the overall feedback from LP out to HP virtual ground, and this doesn't seem to have any sensible impact on THD even at 20Vpp out - at moderate frequencies, at least (THD is in the -120dBc); the SG505 by Tek (5534, 10Vpp or so max out, if I remember well, but it employs an output buffer) uses 5k42 and 2k15 in the same places, while AP1 (5534) goes for a little more confortable 5k and ~5k - it seems that output loads in khom order of magnitude (let's say higher than 2-3 kohms) aren't really going to affect THD performances of good opamps up to 20Vpp out or so (and LME497XX are way better than the good ol' 5534s...)

Ciao,

L.
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Old 14th March 2012, 11:25 PM   #108
richiem is offline richiem  United States
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The ability to characterize the point at which distortion increase is significant obviously depends on the ability to measure distortion accurately at extremely low levels. Pease's noise-gain method is the easiest to do; but "significance" is in the eye of the beholder. Is a distortion increase of 6dB above a very small amount significant?

I'm going to go out on a limb, since I haven't actually been making these measurements, and say that a reasonable loading where OLG starts to be affected in most amps is around 5kohm -- I'm just guessing; but the OLG decrease with frequency makes a single number impractical in any case. So, I guess I'm leaning toward techniques to isolate amps from loading effects and for extending GBW.

Why bother? Well, Cordell has shown that a lot can be learned about amplifiers by looking at their THD at 1kHz and at some higher frequency, like 20kHz. So having unimpeachable sources for 1kHz and higher -- for practical reasons I like 10kHz -- seems desirable. The old conventional wisdom about having measurement instruments 10X better than what's being measured may be too stringent, but it's a nice and convenient benchmark.
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Old 15th March 2012, 02:04 AM   #109
davada is offline davada  Canada
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"Davada has observed the significant loss of open-loop gain due to even moderate output loading."

We should probably put this statement in contest here. It would be better if this read,
Davada has observed the significant loss of open loop gain due to even moderate loading in an undamped state variable filter/ocillator from a small number of op amp specimens.

Most of the datasheets I've seen that give distortion specs use a 1KHz standard and perform the distortion analysis using a unity gain inverter or unity gain follower circuit with exception to Nationals LME seires which used a special technique. But even with the special technique the test is normalized to unity and the distortion is inferred. Some manufactures are more honest then others so I take it all with a grain of salt.

In the case of an undamped SVO, with the damping components removed the Q and Fr gain becomes mathematically undefined and is limited by available open loop gain, amplifier bandwidth, component and board leakage. In terms of significance probably in that order.

Continued on next post.
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Old 15th March 2012, 02:50 AM   #110
davada is offline davada  Canada
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Available open loop gain and bandwidth is not only required for the sections function but also for the filter,s loop stability.

With op amps under these conditions I have observed a significant rise in distortion from either internal or external loading of the op amps as well as lose of gain. even with moderate values of resistance in the the range of 1 to 5K ohms. But this must be taken into consideration with the existing loading from circuit components and operating conditions. In an undamped SV the Fr gain can be as high as 60dB or greater. That means a significant amount of available gain is used and there may not be a lot left to drive distortion down. So loading can become an issue.

There is more to this which I will get into at a later time.

What I said to Richiem was meant to be 'just something to watch out for' when designing and constructing oscillators, filters or other ultra low distortion op amp based instrumentation.


David.
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Last edited by davada; 15th March 2012 at 03:17 AM.
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