EMI-Measurements of a Volt-Nut

[Andreas]

Hello,

I did some "nearby" measurements in different configurations.

20191006_LTZ06_EMI_CW_nearby30_K2000.PNG shows the configuration without any external filtering.
When looking at the resonance frequencies we have 12 and 15 MHz + ~44 MHz.
The 44 MHz correspond to a wavelength of 6.8m. With a shortening factor of 0.95 and a quarter wavelength antenna we have to look for a 1.6 m long wire.
The connection between LTZ#6 and K2000 consists of 2 parts: one 0.6 m the other 0.7 m so with some wire length within the K2000 and the LTZ#6 we can assume a 1.6 m total resonant length.
The 12 and 15 MHz resonances could result from the 100nF capacitors which are on this line (they have typical self resonant frequencies of 10-20 MHz). One 100nF WIMA MKS02 foil capacitor within the LTZ (across the output) and one 100nF ceramic which is in the middle of the 2part connection.

Measurement 20191004_LTZ06_EMI_CW_nearby30_K2000filt.PNG shows what happens if we use a EMI core with 3.5 windings near the K2000. The 12 and 15 MHz peaks are dampened and the 44 MHz peak is shifted towards 48 MHz. (so the effective antenna length has been shorted somewhat).

20191007_LTZ06filt_EMI_CW_nearby30_K2000.PNG shows the EMI-core on the LTZ#6 side. There is only some temperature drift during measurement visible. (the two 10Vss measurements are 3 hours apart).

20191007_LTZ06filt_EMI_CW_nearby30_K2000filt.PNG is similar with EMI-cores on both sides over night so less temperature drift on the K2000.

The large EMI effect seems to be really high frequency effect. The early data from the LTZ6 suggest that the that there are resonances and both #7 and #8 seem to have a resonance (just) higher than the frequency range tested. So the difference between the 2 may be just a minimal different resonance frequency.
The interesting part may be a slightly higher frequency (FM radio).

I am still hoping that someone with better equipment (frequency generator up to 80 or even 230 MHz) tries to repeat some of my tests at least for the LTZ1000. Interesting would also be a comparison to a LTFLU based device.
Sorry but I for my part do not want to spend several 100$ for a frequency range beyond 60 MHz.
And I am shure that the measures that are done for 60 MHz also do some improvement above 60 MHz.

with best regards

Andreas

[Andreas]

Hello,

I updated the AD587LW measurements with AD587LW#01 and AD587LW#02 result.
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684097/#msg2684097

It is obvious that the stray of the EMI sensitivity can be rather different from device to device even with same population.

with best regards

Andreas

[Andreas]

Hello,

When comparing the EMI sensitivity of my K2000 with my HP34401A then the K2000 shows factor 3 more sensitivity against EMI. -> it is wise to use the HP34401A for further measurements.

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684091/#msg2684091
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684094/#msg2684094

Interestingly is that both instruments show a increasing sensitivity which is above my 60 MHz measurement limit which can be dampened by additional EMI cores.

with best regards

Andreas

[notfaded1]

This makes me ever more glad I've got a 34401A... 3x less EMI is huge really!  Thanks for the info.  This makes me wonder about other MM now.

Bill

[Andreas]

im curious, are there possible improvements to stacking more filters for the K2000?
sorry wrong question, maybe the right question is, have you tried to stack filters till the K2000 is at the same level as 34401a? and if so what could these filter values be (alot of K2000 owners could be interested  )

no I did not (up to now). When looking on my coil measurements eventually two 47uH chokes could bring a little improvement (7 MHz self resonant frequency filling the gap). But for high ohmic inputs we would need some low ohmic path to the housing to prevent entering the EMI into the device. So I think two Y-capacitors against the housing will give a large improvement. But of course that will reduce the isolation of the floating part of the DMM.

This makes me ever more glad I've got a 34401A... 3x less EMI is huge really!  Thanks for the info.  This makes me wonder about other MM now.

I would not generalize the result. And I would never bet that each device will deliver the same results.
Finally I am only measuring a small window up to 60 MHz. Above 60 MHz I have a blind spot.

On my HP34401A I have added two bypass capacitors on the LM399 reference (to reduce noise).
This could also have some influence on the EMI behaviour.

On the Keithley the soldering side of the PCB is not so easy accessible.
So I did not do this improvement on the K2000.

So for me this only means that I will use the 34401A for trying to improve some other voltage references like AD587, LM399 and LTZ1000.

with best regards

Andreas

[Andreas]

when the 2x 10u are fuzzed, doesnt seem to do much in LTspice but im sure it does EMI things

EMI has much to do with parasitics. (that is shurely one reason why a resistor chain is used).
So how does it look like when you have 2-5 pF input capacitance of the hybrid (for vias, pins,  eventually input protection diodes, etc).

Do your resistor models of the 13 k resistors contain each a ~0.1 pF capacitor across the resistor itself and 0.1pf to some 0.1 pF for the capacitance of the pads on the PCB?

with best regards

Andreas

[Andreas]

but again, on tins picture of 34401a, i could not spot the 6 13k resistors 

According to the Service manual they are 2512 1W types
(CRCW2512 which are specced  pulse proof 1.5W and 500V).
Located left from the front input connector.

I think that your stray capacitances/inductances are a bit on the high side.
Usually I calculate ~1nH / mm so ~5-8nH between each serial resistor.

But of course there is a long ~300 mm input lab chord and the input connector also gives  ~50 mm until the PCB on the 34401A

with best regards

Andreas

[iMo]

I would recommend you to look at the schematics of ie. hamradio transceivers (ICOM, YEASU, KENWOOD) which are easy to find on the web ("service manuals"). You may see there how they do decoupling with 15kHz-60MHz/100W 140-470MHZ/35W inside their boxes.

PS: I can remember an Pi filter ie. Icom used to use at almost any wire inside (except the main signal path and high freq clocks, of course) is 4n7-100uH-4n7.

PPS: there are also other two Pi filter combinations: 47n-100u-47n, 4n7-10u-4n7 (ie in IC-706), most probably adjusted based on required i/o impedance matching..

[iMo]

On my HP34401A I have added two bypass capacitors on the LM399 reference (to reduce noise).
This could also have some influence on the EMI behaviour.

2x100nF ceramics? Did it help somehow?

[Andreas]

Hello,

I cannot tell it for the 34401A since I did no before and after measurements.

But my first references (LM399 based) suffered a lot from EMI by USB-cables nearby.
Here I got a reduction from several uV to below 1 uV change.

with best regards

Andreas

edit:

found a old measurement from 2008:
before it was up to +30/-75uV  (see picture) 
hand sensitivity of difference voltage (169mV) between 2 LM399 references.
(Depending on where you touch the cables or battery packs).

after adding 4.7nF 1206 ceramic capacitors ~ +2/-1uV. (not shure wether it was only noise or real sensitivity).
after changeing to 100nF 1206 Z5U capacitors -> below 1uV (noise limit).

[Andreas]

Hello,

due to the large stray between my samples I decided to build a "EMI-victim" to test what measures give best results against EMI.
So LTZ#9 was born.

In the first step I only populated the components for the "positive reference" according to the LTZ1000 datasheet.
Population is with LTZ1000 (non-A) and therefore the voltage divider is 12K + 1K for the temperature setting as deviation to the data-sheet. R9 (400K) is not populated since I do the T.C. adjustment only if the device survives the "treatment".

There is also no output buffer populated.

with best regards

Andreas

[Andreas]

Hello,

first measurement on LTZ#9 showed a strange behaviour when connecting the CDN.
I first thought there is nearly no capacitive loading because on the DUT side the 10nF capacitors are in series with the 200 Ohms resistors. But I had forgotten the AE side. There we have 2*47nF against RF-Ground so 24nF between the both pins of the unbuffered output. The output voltage reduced from 7177 mV to 7164 mV as soon as the CDN was connected.
See also EMI-Measurement of 17.10.2019.

The scope measurement shows that we have a oscillation with 200 kHz and 100mVpp amplitude.
I never had thought that a slow OP-Amp like the LT1013 can oscillate that fast.
Of course I had to use a floating Oscilloscope for the measurement because the LTZ voltage is centered to the earth ground by the CDN capacitors / resistors. (18.10.2019).

I decided to use my standard buffer cirquit for further EMI measurements with a LTC2057 and capacitive isolating by a 22 Ohms resistor. After removing the power supply from the lap-top connected to the USB scope there was no oscillation visible. (20.10.2019)

The buffered output showed also a very strange effect on the EMI-test. (21.10.2019)
There are very sharp spikes. Even when increasing the number of measurement points from 256 to 2048 per decade (hires measurement of 22.10.2019).

There seems to be some inteference between the chopper and the EMI frequencies.
Fortunately a 100nF capacitor across the output improves the behaviour a lot (25.10.2019).
I tried to reduce the higher frequency part a bit by chosing a smaller capacitor which has a higher self resonant frequency.
100 nf has around 10-20 MHz so 4.7nF should be good for 50-100 MHz. But the result of 27.10.2019 shows only very limited improvement.

So I will have to do further measures to improve the behaviour.

with best regards

Andreas

[Andreas]

Hello,

seeing the "interference" pattern on the response the next logical step is to use a EMI hardened output amplifier like OPA189 or ADA4522-1.
Having the OPA189 in my drawer I tested this one.

First test on 28.10.2019 was with 4.7nF across the output like the previous measurement of the LTC2057.
Output voltage change with same amplitude of 0.45Vss was 5.5 ppm on the LTC2057 and only ~0.7 ppm (with some noise up to 1.2ppm) on the OPA189.

Increasing the amplitude up to 1.78Vss on 29.10.2019 showed a similar frequency behaviour as on the very first unbuffered measurement of 17.10.2019.

Removing the 4.7nF capacitor on 31.10.2019 increases the maximum deviation slightly from 15 ppm to 22 ppm. But also shows some few interference patterns possibly from the chopper frequency.

A 100nF capacitor (01.11.2019) shows similar behaviour (15 ppm) like the 4.7 nF.

So the OPA189 gives a improvement (better isolation of the LTZ1000 against environment) of ~factor 3 to 4 in EMI voltage amplitude than the LTC2057. And is self nearly immune against EMI together with some output capacitor.

I fear I have a new favorite OP-Amp for precision buffers.

with best regards

Andreas

[branadic]

Thanks Andreas, are you planing similar measurements on ADA4522-1 too?

-branadic-

[Andreas]

Hello Branadic,

just digged a bit deeper in my drawer ...
And actually also a few ADA4522-1 showed up.

with best regards

Andreas

[MiDi]

In addition the OPA(2)189 has the nice feature that the inputs do not have back-to-back clamping diodes, which allows to drive the inputs independently between the rails without clamping.

Measurements on ADA4522 appreciated 

[branadic]

Measurements on ADA4522 appreciated   

#Metoo

-branadic-

[Andreas]

Hello,

I also measured a different effect:
During the heater noise measurements (around 4mVpp) I recognized that when someone switched the fluorescent lamp in the bathroom (one floor below my lab) I had some funny spikes on the noise measurement.
And that with having the LTZ#9 within my metal cookies box.

With the fluorescent lamp in my lab I created also several negative jumps up to 1V peak.
Unfortunately these jumps are very different each time. Ranging from "within the heater noise" (below 4uV) then often around 200mVp up to 1Vpeak. So if I want to evaluate this I have to do a "statistical" approach.

I switched 2.5 minutes with a period time of 6 seconds (so 3 seconds between on and off giving 25 switching periods) to catch 20 events above a trigger limit of -20mV.
Average value of the 20 events was -287mV with a standard deviation of 274 mV.

Of course I expect that if the heater power changes the output voltage of the LTZ is also affected.
Especially on the A-type of the reference where the thermal time constant is much lower than on the non-A version.

2 extreme samples with measured heater voltage shown in attached screen shots on LTZ#9 (non-A version).

with best regards

Andreas

[Andreas]

Hello,

yes when I extremely zoom in I see some bursts (in kHz distance) with oscillations in the some MHz range (14 MHz in example).
But I have to admit that I use a 20 MHz bandwidth limiter for these measurements. (so cant see GHz).
The norm pulse for those bursts is ~5ns rise time corresponding to 60 MHz with 5 kHz repetition rate.

with best regards

Andreas

[MiDi]

In the LTZ1000 thread I reported some time back about latch up of 7to10V buffer.
In the mean time I was able to knock down the cause after heavy research:
The switching of old fluorescent tube in our loundry room ~10m away from lab.
This happend only when measuring 10V output and with offline linear PSU for Ref.
On buffered 7V or on batteries this never happened even on forcing it with many cyclings of lamp.
Have to do more investigation...

[3roomlab]

here are 2 fascinating papers

according to this paper
https://www.researchgate.net/publication/266201167_Characterization_of_compact_fluorescent_lamp_RF_emissions_in_the_perspective_of_human_exposure

they have some weird range of EM field going up to 380V/m ? maybe this is tube model specific?
they did not say how they got this peak value. sensor touching lamp? or 10cm?
there is also a plot about the noise surge when turning on

and this paper
https://www.researchgate.net/publication/225303019_Study_of_a_fluorescent_tube_as_plasma_antenna
in this paper they use micro tubes for campers, the resonant for that tube is around 5xxMhz
and there is some kind of resonant voltage gain? +16dB?

[Andreas]

Hello,

just to clarify.

In my case I am using a simple old style 36W fluorescent tube (TL T5-Type) with a inductivity (similar to transformer coil) as ballast. Only the starter has been replaced by a modern quick starter type.
So in my case most of the EMI is generated when switching off the inductivity.

with best regards

Andreas

[MiDi]

Same here, 36W T8 with KVG and electronic quick starter.

[Andreas]

Hello,

I know that you are all waiting for the ADA4522-1 measurements.
Especially as also TIN is using those for his FX-reference.
(The question is: intentionally or not?).

But: after starting the measurements and looking after the data-sheet I saw a PCN regarding the metal mask layer of the ADA4522-1. Since this may massively affect EMI-behaviour I fear that I have to repeat the measurements.

https://www.analog.com/media/en/PCN/ADI_PCN_18_0171_Rev_A_Form.pdf

According to the PCN the "new" devices will be from datecode 22-Aug-2019 on.
My parts from the drawer are significantly older (datecode 643 bought in January 2017).

Does anyone know what datecode DigiKey is currently selling for the ADA4522-1 in SO-8 package?

with best regards

Andreas

[Andreas]

Hello,

first results of ADA4522-1 with datecode 643.

The measurement of 06.11.2019 with no capacitor across the output shows in average a significant improvement against OPA189 (31.10.2019).
Around -13 ppm (ADA) against -22ppm (OPA) measured at 1.78Vpp generator setting.

But there are some sharp spikes.
A zoomed measurement (09.11.2019) reveals that the spikes are in multiples of ~4.8 MHz which is one of the internal chopper frequencies of the ADA4522.

Ok on the OPA189 a 4.7 or 100nF capacitor helped to remove those spikes and reduced EMI influence from -22 to -15 ppm.

But WTF! on the ADA4522 the output capacitor increases EMI to
-57 ppm (4.7nF on 11.11.2019) and
-60 ppm (100nF on 12.11.2019)
at 1.78Vpp.

My estimation is: the isolation through the OP-Amp gets completely lost on the ADA when connecting a output capacitor across X4+X5. So the LTZ itself is affected.

The schematics shows the output buffer (R23 is not populated).
I think we would have a better behaviour when we would split R22 (10K) into two resistors and place one before the non-inverting input.

with best regards

Andreas