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Sensitivity of opamps to air coupled EM fields, especially of the LM4562/LME497X0 family

pma

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#1
Sensitivity of opamps to air coupled EM fields, especially of the LM4562/LME497XY family

This has been (and will be) a long story that started back in the year 2008 when I started with intensive testing of various opamps concerning their susceptibility to electromagnetic fields in the air or from the inside of the instruments. Quite a long row of opamps was tested, including uA741, uA1458, LM358, OP97, TL071, TL072, AD829, AD844, AD797, AD825, LT1028, LT1122, OPA134, OPA2134, OPA627, OPA211, OPA827, LM6171, LM4562, LME49710, LME49720, NE5532. So they were both BJT and JFET input stage opamps, slow and fast, low supply current or standard supply current. Some of them performed very well and were almost unaffected by a “normal” lab or home environment EM field that surrounded the test PCB or box, some were performing “moderately” and some were performing very poorly, reacting to a slightest change of EM field in their vicinity, like changing their position on a test bench or approaching a hand or just a move of the air or of the body at a 1m distance. In general, JFET input opamps have had no problems and the behavior of BJT input opamps was very different, from excellent immunity to horrible sensitivity to a slightest change in their vicinity. As a conclusion, absolutely worst behavior was that of the LM4562/LME49710/LME49720 family, bought several times in the past 12 years, directly from the manufacturers or authorized distributors. No fakes, no cheap buys. Every time I published some of the results, it started strong reactions and dissatisfaction, especially of the people involved in audio production who have been using those parts that did not perform well.

The parts were measured in the lab where I used to work 12 years ago and then in my home office lab. Various soundcards were used, various PC's both desktop and notebooks battery operated, as well as analog oscilloscopes, DSO stand-alone oscilloscopes and USB scopes. The results were consistent regardless instrumentation used. The best performers re EM immunity were all JFET opamps tested (TL07X, OPA134, OPA627, OPA827, AD825), from BJT opamps excellent were AD797, LT1028, AD844. LM6171 and NE5532 were acceptable, while LM4562 and LME497X0 failed completely.

OK, let's see the story.

1st round, March 2008

Back in 2008 I decided to test open-loop gain linearity according to the circuit published in Analog Devices datasheet of the OP177 precision opamp, page 9. The circuit is shown in Fig.1. It has signal gain of -1 but it releases feedback loop by increasing noise gain (1M/10R resistors in original schematics). I did not go that far with noise gain because I wanted to test opamps with lower OL gain as well, so I used ratio of the noise gain divider as 100k/10ohm. So my circuit has had signal gain of -1 and noise gain of 86dB. As the noise gain is increased, feedback action is suppressed (there is less feedback) and inherent distortion and other inherent linear and non-linear parameters of the opamp become more evident. Input signal gain is -1 (inverting), however the opamp inherent noise is amplified of 86dB, i.e. 20 000x, with respect to opamp frequency response at such high gain. Because soundcard input sensitivity was about 1V, I added the 25dB divider to the output of the test circuit to be able to utilize higher opamp's output voltage.

The 1st test circuit was built on a universal test board, without a ground plane. The board was placed above larger Aluminum sheet metal that was connected with a signal ground in 1 point and provided some electrostatic shielding. The measuring setup was single ended, so currents flowing through signal cables shield unfortunately create small voltage drop that is added to the signal voltage and displayed as lines at mains frequency and its multiples. However, these mains spuriae are constant and same for all parts under test, so the test conditions remain equal for all parts under test.

fig1.png

Fig.1. Opamp test circuit, signal gain -1, noise gain 86dB

LT1028

fig2.png

Fig.2. LT1028 with noise gain 86dB and signal gain -1

LT1028 was an excellent bipolar input opamp with very low noise and very low distortion. We can see some mains system spuriae, low distortion of 0.03% (please take into account that the equivalent gain is 86dB), one of the lowest noise from all parts under test and no unexpected spectral interference lines.

OPA627

fig3.png

Fig.3. OPA with noise gain 86dB and signal gain -1

OPA627 is a very expensive JFET input opamp with low noise and low ditortion. We can see noise level about 10dB higher than with LT1028, low distortion about 0.04% and no unexpected spectral interference lines.

LME49710

fig4.png

Fig.4. LME49710 with noise gain 86dB and signal gain -1

LME49710 from National Semiconductor was a new part at the time and it was introduced with very high expectations and accompanied by high appraisal among audiophiles. So I ordered samples and put them into the test. It was a big disappointment. As we can see from the spectral plot, the distortion is low, however there is a forest of symmetrically distributed spectral lines, every 10Hz. What is that? And why specifically this opamp, all samples?

fig5.png

Fig.5. LME49710 noise gain 86dB, zero line
The measurement was repeated with no input signal, but the forest of spectral lines placed in 10Hz multiples remained there.

fig6.png

Fig.6. AD797 noise gain 86dB, zero line

Another bipolar low noise opamp was tested to compare, the AD797 by @scott wurcer . Zero line is clean, with only some mains lines that are there from the method setup.

I tested much more opamps in the 1st round, but those most interesting I have shown here. I posted those results in 2008 and received doubts about the LME49710 results. Wrong samples, bad PCB bypassing, shielding, you name it. It was difficult for many to digest that LME49710 result was bad, even after the fact that the test conditions were same for all the opamps under test.

So the test was repeated and the test PCB was placed in a completely shielded small metal box. The level of spuriae decreased, however the differences between opamps remained similar, with LME49710 still having forest of spuriae, now mostly at every 100Hz multiples. Excellent result again was that of LT1028.

fig7.png

Fig.7. LT1028 noise gain 86dB, signal gain -1, metal shielded box

2nd round of measurements
was made in 2013, with new opamp samples and almost same results as in 2008. Different soundcard, different measuring place, nothing new, just confirmation of previous results.

3rd round of measurements, January 2016

another samples, now dual opamps, and new measurements, now not only spectrum analysis, but also time domain measurements. Spectra were again similar to those we have already seen, but I have added oscilloscope measurements and I think they were interesting.

NE5532

fig8.png

Fig.8. NE5532 noise gain 86dB signal gain -1, input shorted.
NE5532 was added to the test and we can see spectral lines at every 100Hz, but they are about 20dB lower than that of LM4562 in Fig.9.

LM4562

fig9.png

Fig.9. LM4562 noise gain 86dB signal gain -1, input shorted. High spuriae at every 100Hz and much smaller spuriae at every 10Hz.


LM4562 100Hz sine 8Vp-p

fig10.png

Fig.10. LM4562 86dB noise gain, signal gain -1, 100Hz sine. We can see that every 10ms we have a spike, something triggers the opamp and it looks like if it worked for a while like a comparator without feedback. These sharp and short spikes are responsible for the forest of harmonics at 100Hz multiples that we could see in Fig.9.

fig11.png

Fig.11. LM4562 time record now made by a PicoScope USB oscilloscope. Same spikes, every 10ms, are visible.

fig12.png

Fig.12. LM4562, even the Picoscope low resolution spectral analysis catches the spikes and repetitive spectral lines every 100Hz. Frustrating.

fig13.png

Fig.13. OPA2134 tested clean in the same setup.

fig14.png

Fig.14. Time record of the LM4562 zero line. We can see both spikes with repetition frequency 100Hz and also the wider one with 10Hz repetition.

Battery power

Readers have suggested that the spikes are triggered from the power supply residual ripple. So I supplied the test board from two 9V batteries and helas – the spectrum remained unchanged.

fig15.png

Fig.15. LM4562 battery power supply

OK, the battery supply did not help, so what about further suggestions? They suggested that the problem is in the poor universal PCB and poor supply bypassing.

So I designed, ordered and paid the double-sided PCB with metal through-holes and top shielding ground plane.

fig16.png

Fig.16. New test circuit that comprises 86dB noise gain/-1 signal gain circuit and also 40dB signal gain circuit, for dual opamps.

fig17.jpg

Fig.17. And this is the photo of the new test circuit

The test procedure was repeated, amplitude of multiples of 100Hz spuriae in LM4562 plots was reduced, but they were still there. I was again instructed that the problem is in my PCB design and in the socket that is used for the LM4562 …...... I better add no comments.

4th round of measurements, January 2020

OK, I am stupid and cannot design the PCB to make that poor LM4562 happy enough. So let's move forward, I ordered from experts and from the source, directly from TI, the LME49720NABD Evaluation Board.

fig18.png

Fig.18. TI Evaluation Board schematics (it can be found only in the archive LM4562 datasheet)

fig19.jpg

Fig.19. LME49720NABD Evaluation Board

Nice board, isn't it? Short traces, SMD bypass capacitors right at the opamp pins, this will be a breakthrough, right? (… it was not, as we shall see)

and I have also put my double-sided PCB test board, with the 86dB noise gain circuit, into the test and placed it in a metal sheet box to provide shielding.

fig20.jpg

Fig.20. My 86dB noise gain test PCB in a shielded box

Let's start with the “proper evaluation board” LME49720NABD from the TI manufacturer. As seen in Fig.18, it has gain of mere -1, 10k/10k inverter. So there should be no issues. Nice PCB, proper supply bypassing, what else and better could be done.

So this is the output from the board OUT_1 measured with input of the board shorted by a jumper JP2.

fig21.png

Fig.21. LME49720NABD output with input shorted.

Sometimes it looks like this,
however

fig22.png

Fig.22. LME49720NABD output with input shorted.
Sometimes it looks like this. Depends. Depends on a slightest move of the board on my test bench and also on my hand approaching the board. Translated – the opamp input is catching EM field in its vicinity and triggers on 50Hz mains frequency field zero crossings. Now it depends on EM field intensity near the opamp what happens. So it has nothing in common with supply bypassing, with PSU ripple (we have seen the same issue with battery power in Fig.15) and even placing this board into Cu shielding box did not help. The only cure would probably be the iron box to shield magnetic compound of the EM field that is triggering the LME49720NA like a comparator.

So, no happy fortune with the TI professional evaluation board. Let's go back to my 86dB noise gain double-sided test PCB, placed in a metal sheet box. The similar set of measurements as in the 1st test round, in 2008, was made. The opamps under test were TL072, OPA2134, NE5532 and LM4562. Spectral plots are properly calibrated in dBV, this time. Plot results are following, without further comments, except for the one – LM4562 was the only one to show 100Hz multiples spuriae.

fig23.png

Fig.23. Noise gain 86dB test, TL072 output with input shorted.

fig24.png

Fig.24. Noise gain 86dB test, OPA2134 output with input shorted.

fig25.png

Fig.25. Noise gain 86dB test, NE5532 output with input shorted.

fig26.png

Fig.26. Noise gain 86dB test, LM4562 output with input shorted.

fig27.png

Fig.27. Noise gain 86dB test, TL072 output for 100Hz input.

fig28.png

Fig.28. Noise gain 86dB test, OPA2134 output for 100Hz input.

fig29.png

Fig.29. Noise gain 86dB test, NE5532 output for 100Hz input.

fig30.png

Fig.30. Noise gain 86dB test, LM4562 output for 100Hz input.


Conclusion

Since 2008, I made 4 rounds of tests of opamps in a test circuit with -1 signal gain (inverter) but with 86dB noise gain, to test inherent linearity of opamps. The opamps tested were uA741, uA1458, LM358, OP97, TL071, TL072, AD829, AD844, AD797, AD825, LT1028, LT1122, OPA134, OPA2134, OPA627, OPA211, OPA827, LM6171, LM4562, LME49710, LME49720, NE5532. As a side effect it was found that some of the opamps, especially the LM4562/LME497X0 family were extremely sensitive to EM field of mains frequency in the vicinity of the test board. This was blamed on improper test board design, improper supply bypassing or PSU ripple. All these arguments were proven wrong by a newly designed double-sided PCB with ground plane, battery power instead of DC PSU and professional TI evaluation board purchase. None of this solved the issue of some of the opamps.

As a conclusion, there was never a problem with JFET input opamps, there was never a problem with bipolar input opamps AD797, LT1028, AD844, there were slight issues with LM6171 and NE5532 and there were big issues, always, with the LM4562/LME497X0 family. This family of opamps is extremely sensitive to EM fields in their vicinity, mains frequency 50Hz triggers them during zero crossing and they send narrow spikes to their output, in 100ms distances and of amplitude that depends on shielding. This can be cured extremely well shielded box that shields not only against electric field component, but also against magnetic field component. The problem may remain hidden, sometimes it disappears with respect to momentary EM field conditions, but can be seen also for gains as low as -1 in the professional TI LME49720NABD evaluation board. The problem was observed by various soundcards and PCs in 2 independent measuring places and also by using analog and DSO scopes. It looks like a duck, it quack like a duck, so it would be a duck.

Your comments are welcome, but please let me not to reply the questions that are already covered in this text, it was an exhausting job.
 
Last edited:

solderdude

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#2
How'z about the metal can LM4562 ?
There might be merit to reports of it sounding better when for instance a transformer would be in close proximity ?
In that case one would expect to hear a sharpish mains hum in the background ?
 

pma

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#3
How'z about the metal can LM4562 ?
There might be merit to reports of it sounding better when for instance a transformer would be in close proximity ?
In that case one would expect to hear a sharpish mains hum in the background ?
I do not have the metal can part, unfortunately. If someone is willing to send it me for the the test, I would measure it with pleasure.

Re possible buzz, this depends on actual level of the 100Hz multiples and this again depends on circuit and box construction. I tested 2134 x 4562 in a preamp where some issues were after the box top cover was removed, you could here a slight buzz from speakers with your ear near to midrange and tweeter.

You may find sometimes notes on LM4562 EMI susceptibility by other authors, like
https://www.gearslutz.com/board/showpost.php?p=12990908&postcount=17
 

maxxevv

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#4
Rather than a full box for the circuit box, did you try creating a small "cage"around the opamp in your testing ?

Like say something made of "mu-metal"(its supposedly good for both magnetic as well as low frequency EM waves) plate, folding to form a shield/box encapsulating the opamp? (BTW, high frequency and low frequency EM requires different shielding from what I have read and gathered from field engineers who deal with it on occasion)

It would put to rest what is the possible performance of the opamp if the spurious readings are completely shielded away though.
 

SIY

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#5
Supposedly, the trick to getting the 4562 to work properly was to have a small inductor right at the input pin. I can't personally attest to that, but it's plausible and something I've seen with certain high transconductance discrete parts.
 

March Audio

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#7
Sensitivity of opamps to air coupled EM fields, especially of the LM4562/LME497XY family

This has been (and will be) a long story that started back in the year 2008 when I started with intensive testing of various opamps concerning their susceptibility to electromagnetic fields in the air or from the inside of the instruments. Quite a long row of opamps was tested, including uA741, uA1458, LM358, OP97, TL071, TL072, AD829, AD844, AD797, AD825, LT1028, LT1122, OPA134, OPA2134, OPA627, OPA211, OPA827, LM6171, LM4562, LME49710, LME49720, NE5532. So they were both BJT and JFET input stage opamps, slow and fast, low supply current or standard supply current. Some of them performed very well and were almost unaffected by a “normal” lab or home environment EM field that surrounded the test PCB or box, some were performing “moderately” and some were performing very poorly, reacting to a slightest change of EM field in their vicinity, like changing their position on a test bench or approaching a hand or just a move of the air or of the body at a 1m distance. In general, JFET input opamps have had no problems and the behavior of BJT input opamps was very different, from excellent immunity to horrible sensitivity to a slightest change in their vicinity. As a conclusion, absolutely worst behavior was that of the LM4562/LME49710/LME49720 family, bought several times in the past 12 years, directly from the manufacturers or authorized distributors. No fakes, no cheap buys. Every time I published some of the results, it started strong reactions and dissatisfaction, especially of the people involved in audio production who have been using those parts that did not perform well.

The parts were measured in the lab where I used to work 12 years ago and then in my home office lab. Various soundcards were used, various PC's both desktop and notebooks battery operated, as well as analog oscilloscopes, DSO stand-alone oscilloscopes and USB scopes. The results were consistent regardless instrumentation used. The best performers re EM immunity were all JFET opamps tested (TL07X, OPA134, OPA627, OPA827, AD825), from BJT opamps excellent were AD797, LT1028, AD844. LM6171 and NE5532 were acceptable, while LM4562 and LME497X0 failed completely.

OK, let's see the story.

1st round, March 2008

Back in 2008 I decided to test open-loop gain linearity according to the circuit published in Analog Devices datasheet of the OP177 precision opamp, page 9. The circuit is shown in Fig.1. It has signal gain of -1 but it releases feedback loop by increasing noise gain (1M/10R resistors in original schematics). I did not go that far with noise gain because I wanted to test opamps with lower OL gain as well, so I used ratio of the noise gain divider as 100k/10ohm. So my circuit has had signal gain of -1 and noise gain of 86dB. As the noise gain is increased, feedback action is suppressed (there is less feedback) and inherent distortion and other inherent linear and non-linear parameters of the opamp become more evident. Input signal gain is -1 (inverting), however the opamp inherent noise is amplified of 86dB, i.e. 20 000x, with respect to opamp frequency response at such high gain. Because soundcard input sensitivity was about 1V, I added the 25dB divider to the output of the test circuit to be able to utilize higher opamp's output voltage.

The 1st test circuit was built on a universal test board, without a ground plane. The board was placed above larger Aluminum sheet metal that was connected with a signal ground in 1 point and provided some electrostatic shielding. The measuring setup was single ended, so currents flowing through signal cables shield unfortunately create small voltage drop that is added to the signal voltage and displayed as lines at mains frequency and its multiples. However, these mains spuriae are constant and same for all parts under test, so the test conditions remain equal for all parts under test.

View attachment 44437
Fig.1. Opamp test circuit, signal gain -1, noise gain 86dB

LT1028

View attachment 44438
Fig.2. LT1028 with noise gain 86dB and signal gain -1

LT1028 was an excellent bipolar input opamp with very low noise and very low distortion. We can see some mains system spuriae, low distortion of 0.03% (please take into account that the equivalent gain is 86dB), one of the lowest noise from all parts under test and no unexpected spectral interference lines.

OPA627

View attachment 44439
Fig.3. OPA with noise gain 86dB and signal gain -1

OPA627 is a very expensive JFET input opamp with low noise and low ditortion. We can see noise level about 10dB higher than with LT1028, low distortion about 0.04% and no unexpected spectral interference lines.

LME49710

View attachment 44440
Fig.4. LME49710 with noise gain 86dB and signal gain -1

LME49710 from National Semiconductor was a new part at the time and it was introduced with very high expectations and accompanied by high appraisal among audiophiles. So I ordered samples and put them into the test. It was a big disappointment. As we can see from the spectral plot, the distortion is low, however there is a forest of symmetrically distributed spectral lines, every 10Hz. What is that? And why specifically this opamp, all samples?

View attachment 44441
Fig.5. LME49710 noise gain 86dB, zero line
The measurement was repeated with no input signal, but the forest of spectral lines placed in 10Hz multiples remained there.

View attachment 44442
Fig.6. AD797 noise gain 86dB, zero line

Another bipolar low noise opamp was tested to compare, the AD797 by @scott wurcer . Zero line is clean, with only some mains lines that are there from the method setup.

I tested much more opamps in the 1st round, but those most interesting I have shown here. I posted those results in 2008 and received doubts about the LME49710 results. Wrong samples, bad PCB bypassing, shielding, you name it. It was difficult for many to digest that LME49710 result was bad, even after the fact that the test conditions were same for all the opamps under test.

So the test was repeated and the test PCB was placed in a completely shielded small metal box. The level of spuriae decreased, however the differences between opamps remained similar, with LME49710 still having forest of spuriae, now mostly at every 100Hz multiples. Excellent result again was that of LT1028.

View attachment 44443
Fig.7. LT1028 noise gain 86dB, signal gain -1, metal shielded box

2nd round of measurements
was made in 2013, with new opamp samples and almost same results as in 2008. Different soundcard, different measuring place, nothing new, just confirmation of previous results.

3rd round of measurements, January 2016

another samples, now dual opamps, and new measurements, now not only spectrum analysis, but also time domain measurements. Spectra were again similar to those we have already seen, but I have added oscilloscope measurements and I think they were interesting.

NE5532

View attachment 44444
Fig.8. NE5532 noise gain 86dB signal gain -1, input shorted.
NE5532 was added to the test and we can see spectral lines at every 100Hz, but they are about 20dB lower than that of LM4562 in Fig.9.

LM4562

View attachment 44445
Fig.9. LM4562 noise gain 86dB signal gain -1, input shorted. High spuriae at every 100Hz and much smaller spuriae at every 10Hz.


LM4562 100Hz sine 8Vp-p

View attachment 44446
Fig.10. LM4562 86dB noise gain, signal gain -1, 100Hz sine. We can see that every 10ms we have a spike, something triggers the opamp and it looks like if it worked for a while like a comparator without feedback. These sharp and short spikes are responsible for the forest of harmonics at 100Hz multiples that we could see in Fig.9.

View attachment 44447
Fig.11. LM4562 time record now made by a PicoScope USB oscilloscope. Same spikes, every 10ms, are visible.

View attachment 44448
Fig.12. LM4562, even the Picoscope low resolution spectral analysis catches the spikes and repetitive spectral lines every 100Hz. Frustrating.

View attachment 44449
Fig.13. OPA2134 tested clean in the same setup.

View attachment 44450
Fig.14. Time record of the LM4562 zero line. We can see both spikes with repetition frequency 100Hz and also the wider one with 10Hz repetition.

Battery power

Readers have suggested that the spikes are triggered from the power supply residual ripple. So I supplied the test board from two 9V batteries and helas – the spectrum remained unchanged.

View attachment 44451
Fig.15. LM4562 battery power supply

OK, the battery supply did not help, so what about further suggestions? They suggested that the problem is in the poor universal PCB and poor supply bypassing.

So I designed, ordered and paid the double-sided PCB with metal through-holes and top shielding ground plane.

View attachment 44452
Fig.16. New test circuit that comprises 86dB noise gain/-1 signal gain circuit and also 40dB signal gain circuit, for dual opamps.

View attachment 44453
Fig.17. And this is the photo of the new test circuit

The test procedure was repeated, amplitude of multiples of 100Hz spuriae in LM4562 plots was reduced, but they were still there. I was again instructed that the problem is in my PCB design and in the socket that is used for the LM4562 …...... I better add no comments.

4th round of measurements, January 2020

OK, I am stupid and cannot design the PCB to make that poor LM4562 happy enough. So let's move forward, I ordered from experts and from the source, directly from TI, the LME49720NABD Evaluation Board.

View attachment 44454
Fig.18. TI Evaluation Board schematics (it can be found only in the archive LM4562 datasheet)

View attachment 44455
Fig.19. LME49720NABD Evaluation Board

Nice board, isn't it? Short traces, SMD bypass capacitors right at the opamp pins, this will be a breakthrough, right? (… it was not, as we shall see)

and I have also put my double-sided PCB test board, with the 86dB noise gain circuit, into the test and placed it in a metal sheet box to provide shielding.

View attachment 44456
Fig.20. My 86dB noise gain test PCB in a shielded box

Let's start with the “proper evaluation board” LME49720NABD from the TI manufacturer. As seen in Fig.18, it has gain of mere -1, 10k/10k inverter. So there should be no issues. Nice PCB, proper supply bypassing, what else and better could be done.

So this is the output from the board OUT_1 measured with input of the board shorted by a jumper JP2.

View attachment 44457
Fig.21. LME49720NABD output with input shorted.

Sometimes it looks like this,
however

View attachment 44458
Fig.22. LME49720NABD output with input shorted.
Sometimes it looks like this. Depends. Depends on a slightest move of the board on my test bench and also on my hand approaching the board. Translated – the opamp input is catching EM field in its vicinity and triggers on 50Hz mains frequency field zero crossings. Now it depends on EM field intensity near the opamp what happens. So it has nothing in common with supply bypassing, with PSU ripple (we have seen the same issue with battery power in Fig.15) and even placing this board into Cu shielding box did not help. The only cure would probably be the iron box to shield magnetic compound of the EM field that is triggering the LME49720NA like a comparator.

So, no happy fortune with the TI professional evaluation board. Let's go back to my 86dB noise gain double-sided test PCB, placed in a metal sheet box. The similar set of measurements as in the 1st test round, in 2008, was made. The opamps under test were TL072, OPA2134, NE5532 and LM4562. Spectral plots are properly calibrated in dBV, this time. Plot results are following, without further comments, except for the one – LM4562 was the only one to show 100Hz multiples spuriae.

View attachment 44459
Fig.23. Noise gain 86dB test, TL072 output with input shorted.

View attachment 44461
Fig.24. Noise gain 86dB test, OPA2134 output with input shorted.

View attachment 44462
Fig.25. Noise gain 86dB test, NE5532 output with input shorted.

View attachment 44463
Fig.26. Noise gain 86dB test, LM4562 output with input shorted.

View attachment 44464
Fig.27. Noise gain 86dB test, TL072 output for 100Hz input.

View attachment 44465
Fig.28. Noise gain 86dB test, OPA2134 output for 100Hz input.

View attachment 44466
Fig.29. Noise gain 86dB test, NE5532 output for 100Hz input.

View attachment 44467
Fig.30. Noise gain 86dB test, LM4562 output for 100Hz input.


Conclusion

Since 2008, I made 4 rounds of tests of opamps in a test circuit with -1 signal gain (inverter) but with 86dB noise gain, to test inherent linearity of opamps. The opamps tested were uA741, uA1458, LM358, OP97, TL071, TL072, AD829, AD844, AD797, AD825, LT1028, LT1122, OPA134, OPA2134, OPA627, OPA211, OPA827, LM6171, LM4562, LME49710, LME49720, NE5532. As a side effect it was found that some of the opamps, especially the LM4562/LME497X0 family were extremely sensitive to EM field of mains frequency in the vicinity of the test board. This was blamed on improper test board design, improper supply bypassing or PSU ripple. All these arguments were proven wrong by a newly designed double-sided PCB with ground plane, battery power instead of DC PSU and professional TI evaluation board purchase. None of this solved the issue of some of the opamps.

As a conclusion, there was never a problem with JFET input opamps, there was never a problem with bipolar input opamps AD797, LT1028, AD844, there were slight issues with LM6171 and NE5532 and there were big issues, always, with the LM4562/LME497X0 family. This family of opamps is extremely sensitive to EM fields in their vicinity, mains frequency 50Hz triggers them during zero crossing and they send narrow spikes to their output, in 100ms distances and of amplitude that depends on shielding. This can be cured extremely well shielded box that shields not only against electric field component, but also against magnetic field component. The problem may remain hidden, sometimes it disappears with respect to momentary EM field conditions, but can be seen also for gains as low as -1 in the professional TI LME49720NABD evaluation board. The problem was observed by various soundcards and PCs in 2 independent measuring places and also by using analog and DSO scopes. It looks like a duck, it quack like a duck, so it would be a duck.

Your comments are welcome, but please let me not to reply the questions that are already covered in this text, it was an exhausting job.
What I don't see here is how you have actually provided correlation to "EM fields".

You have odd behaviour going on but you have produced no evidence to correlate it with the alleged cause.

How did you generate/measure/quantify the "EM fields"?

Just looking at the AD627 plot, which you say was a good performer, it has significant 50Hz pick up. Your 5532 plot, well something is obviously broken. People have been using that part for decades without issue.

I think I would be looking at other causes, test set up/instrumentation/circuit design/pcb layout etc.
 
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March Audio

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#9
This is an LM4562 buffer in a Hypex amp. Loads of EM inside there from the amp and the PSU.

I quickly threw this together on the bench with some trailing unshielded cabling, and measuring the power amp output with several metres of cable connecting the dummy load. So yes some 50Hz pick up, but not the odd things you have plotted above.

1578062244093.png
 
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pma

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#11
Of course the board with LM4562 may be put in a well shielded box and the problem seemingly disappears. The case is that the relative differences in opamps susceptibility to EM fields remain unchanged, LM4562 remains on the tail. The problem is only suppressed and not solved.

I have had an “A-B box” to test 2 audio paths by immediate switching from one to another and the box allowed for level matching. It was built both with LM4562 and OPA2134. When closed with top cover firmly, both versions behaved almost same. When the top cover was removed, the OPA2134 version was unaffected and the LM4562 version start to trigger at 100Hz multiples in the same way I have already shown.

Attached are noise bottom (noise spectral density) plots of both versions, dB relative with 0dB = 168mV. So yes, so far so good, but the problem is hidden there. It is up to everyone's decision, I want to have high EMF immunity margin, so I use OPA2134 and avoid LM4562/LM49720.

AB_box_2134_noise.png


AB_box_4562_noise.png


AB_box_inside.jpg


AB_box.jpg
 
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#12
EMS (Electromagnetic Magnetic Susceptibility) is probable but you are not providing any characterisation of the supposed Interferer(s) (EMI).

Under EM fields an electronic component may have different operational states:
  • not degraded
  • degraded but back to normal status after EMI' s removal
  • degraded with no return to initial status/performance after EMI's removal
  • destroyed.
I have no idea about your op amps' cases, many years ago I 'played' with As GA transistors susceptibility vs millimeter waves.
In principle you need to have a quite fair amount of EM fields for such behaviour...
 

pma

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#13
EMS (Electromagnetic Magnetic Susceptibility) is probable but you are not providing any characterisation of the supposed Interferer(s) (EMI).

Under EM fields an electronic component may have different operational states:
  • not degraded
  • degraded but back to normal status after EMI' s removal
  • degraded with no return to initial status/performance after EMI's removal
  • destroyed.
I have no idea about your op amps' cases, many years ago I 'played' with As GA transistors susceptibility vs millimeter waves.
In principle you need to have a quite fair amount of EM fields for such behaviour...
There is no component degradation in my case. The test board is freely placed on a test bench placed in a standard room or is eventually put in a shielding box. The only EM fields are generated by standard home electric mains wiring in a 230V/50Hz network. Nothing special. The B^, H^ and E^ all would be low and normal as in any home here. Just turn on your scope, put the tip of its probe into the air and that's it. Nothing else, nothing more. We may differ in a distribution network an grounding and protecting system, nothing else.

We are not talking any severe EMC tests or high dV/dt, dI/dt discharges.

P.S.: like this. The scope probe tip reads 16mVp-p/50Hz at 1Mohm impedance. The scope chassis is grounded.

1578071447491.png
 
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#14
Obviously I am not expecting you to be listening inside the plane in the picture under ;)
Since you wrote air coupled EM fields in your title I am just curious what it can be nothing more.
With a probe in the air connected to scope or better a spectrum analyzer I can catch lots of external/internal RF...

Q320.jpg
 
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#15
In case it helps.
You can use your probe for EMC/EMI purposes. Use a small DIY loop at the tip and do not loop the BNC cable for better efficiency
The gearslutz' link, you provided, on LM4562 EMI susceptibility is relating sensitivity with cordless base station DECT phones. Those are operating around 1.9 GHz.
 

March Audio

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#16
Of course the board with LM4562 may be put in a well shielded box and the problem seemingly disappears. The case is that the relative differences in opamps susceptibility to EM fields remain unchanged, LM4562 remains on the tail. The problem is only suppressed and not solved.

I have had an “A-B box” to test 2 audio paths by immediate switching from one to another and the box allowed for level matching. It was built both with LM4562 and OPA2134. When closed with top cover firmly, both versions behaved almost same. When the top cover was removed, the OPA2134 version was unaffected and the LM4562 version start to trigger at 100Hz multiples in the same way I have already shown.

Attached are noise bottom (noise spectral density) plots of both versions, dB relative with 0dB = 168mV. So yes, so far so good, but the problem is hidden there. It is up to everyone's decision, I want to have high EMF immunity margin, so I use OPA2134 and avoid LM4562/LM49720.

View attachment 44504

View attachment 44505

View attachment 44506

View attachment 44507
Nether the above examples, mine or nw were in "well shielded" boxes. The Hypex amp the case was open. Also In the case of the Hypex amp the major source of EM noise is generated inside the box within a few cm of the OP amp from the switching psu and the amp switching.

If you are getting the results below with a ne5532 then there is something wrong with the circuit, pcb layout, instrumentation or test set up. People have been designing literally for decades with this component and don't get problems like this.

fig8.png
 
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restorer-john

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#17
Just turn on your scope, put the tip of its probe into the air and that's it. Nothing else, nothing more.
Yes, here is my identical signal, in Australia, at my bench, same probe in the air...

1578097441189.jpeg


RIGOL Print Screen4-01-2020 10_20_46 AM.233.jpeg
 

pma

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#18
Yes, here is my identical signal, in Australia, at my bench, same probe in the air...
Thank you @restorer-john , below is an explanation, the signal is a voltage across probe resistance, this induced by capacitive coupling to mains voltage in the air, capacitive current is a capacitance times derivative of voltage, this capacitive current makes voltage drop on probe resistance. As mains voltage usually has flat top and bottom of the voltage wave, this reflects as straight line near zero crossing of the voltage derivative. This all is only a nice view of college physics in a real world. And this dv/dt is triggering the opamp family mentioned.

My 1st job position - a researcher in High Power testing laboratory - taught me to find answers in application of basic physics/electricity law and humbleness to laws of physics. Something that is difficult to get if one works with 5V circuits for the whole career.

1578127526361.png
 

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#19
Pavel, we are all fully aware of this phenomenon and have all put our finger on an RCA lead and got mains buzz out of an amp. This isn't news.

However there is no doubt something wrong in your test set up to obtain such dire results out of a ne5532 circuit.
 

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#20
Pavel, we are all fully aware of this phenomenon and have all put our finger on an RCA lead and got mains buzz out of an amp. This isn't news.

However there is no doubt something wrong in your test set up to obtain such dire results out of a ne5532 circuit.

Experienced suggestions?
 

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