EMI-Measurements of a Volt-Nut

[Andreas]

Hello,

I fear this will end in a metal housing with feed through capacitors for all in/outputs.

with 4.7nF: what did you do so that the Op-Amp does not oscillate with capacitive loading?

with best regards

Andreas

[iMo]

with 4.7nF: what did you do so that the Op-Amp does not oscillate with capacitive loading?

There is the 25ohm resistor at the opamp's output (then there is 10nF foil and 4n7 ceramics).

[dietert1]

You could try resistors on the supply side, too. Sometimes resistors make better filters than inductors. There should be enough headroom to loose some 0.5 V DC on the resistors.

Regards, Dieter

[iMo]

Is it only me affected by the wifi virus ?? Could it be people do not see it because a) they do not switch their routers on/off during such a measurements, b) their measurement resolution is not enough to see the jump?

[MegaVolt]

Is it only me affected by the wifi virus ?? Could it be people do not see it because a) they do not switch their routers on/off during such a measurements, b) their measurement resolution is not enough to see the jump?

I checked the Fluke 732a along with the Keithley 7510. Turning the Wi-Fi or computer on or off did not give noticeable steps. But the inclusion of an ancient refrigerator, I see But the connection was made quickly and the wires were not shielded.

[SilverSolder]

Is it only me affected by the wifi virus ?? Could it be people do not see it because a) they do not switch their routers on/off during such a measurements, b) their measurement resolution is not enough to see the jump?

Router 2 floors away from the lab?

[Andreas]

Hello

some further measurements:

LTZ#9 populated with ADA4522 as output buffer.
Capacitor 100nF across the output.
And a 100nF between Output Ground and ADA4522 (power) ground which fixes the layout error.
On the input of the ADA4522 there is a 4K7+100nF low pass filter.
And C9+C11 of my LTZ1047B schematic are also populated.

Measurement of 04.04.2020 shows -240 ppm deviation @ 60 MHz with 5Vpp Output level on the FY6800.

Measurement of 03.04.2020 shows improvement by the 3.5 windings on a EMI core suggested by Jason.
so -7.4 ppm @ 24 MHz as max deviation with 5Vpp output level.
At 20 MHz the ouput amplifier is switched so that the level is slightly different after switching.

And finally Measurement of 02.04.2020 shows best result with the Epcos 51 uH common mode choke for signal lines (CAN bus choke)
This choke already looked very promising when being measured here: (3rd last section)
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684085/#msg2684085

Result: -1.4 ppm @ 60 MHz with 5Vpp: so my personal target for EMI reduction is reached here.
Since the choke is now outside the LTZ#9 housing I will have a slightly worse result with the choke inside the housing due to parasitic capacities increasing.

With best regards

Andreas

Edit: picture of external CAN coil attached.
picture of output buffer ADA4522 with capacitors attached

By the way PCN 18_0171 has been updated on the Analog Web site: the new parts with the mask change will be available earliest mid of this year.

https://media.digikey.com/pdf/PCNs/Analog%20Devices/PCN_18_0171_RevB.pdf

[Andreas]

Hello,

I have updated first results of LM399 here:

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684100/#msg2684100

with best regards

Andreas

[notfaded1]

Is it only me affected by the wifi virus ?? Could it be people do not see it because a) they do not switch their routers on/off during such a measurements, b) their measurement resolution is not enough to see the jump?

I should really check this... I have a wifi router on my bench.  I do see some jumps I had though might have been from AC mains but with lot of filtering still seem to be there.

[Andreas]

Is it only me affected by the wifi virus ?? Could it be people do not see it because a) they do not switch their routers on/off during such a measurements, b) their measurement resolution is not enough to see the jump?

I should really check this... I have a wifi router on my bench.  I do see some jumps I had though might have been from AC mains but with lot of filtering still seem to be there.

Hello,

You might have more EMI sources than that what you think:

It took a while until I noticed that my network printer connected by ethernet also has a WIFI and additionally a NFC source.

with best regards

Andreas

[Andreas]

Hello,

a further update on the LM399 here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684100/#msg2684100

with best regards

Andreas

[dietert1]

Thanks Andreas, since i am trying to "tune" our HP 3456As.

I was wondering though, how much might be the effect of the bypass capacitor leakage current. For example if leakage gets increased somewhat due to heating to let's say 1 uA, times zener differential resistance would give us maybe 10 uV, so that would be 1.5 ppm - as an upper limit of an unstable contribution. Leakage of MLCC caps seems to be of similar magnitude, so it gets a bit nasty, if one needs to use foil caps.

Also i remember a claim (MisterDiodes?) that a direct bypass would accelerate aging due to increased noise current. So the bypass could include a series resistor, for example 5 or 10 Ohms, with the output taken from the capacitor. I remember from my LTFLU experiments that a direct 100 uF bypass on the 10 V output caused a lot of noise current inside the 7 -> 10 V loop. Improved with 100uF + 3 Ohms, but i don't have numbers for that.

Regards, Dieter

[Kleinstein]

For a voltage regulator of any kind a large low ESR capacitance is difficult. This is because for principle reasons the output impedance will be inductive over a large frequency range and often very close to a low loss inductor. In this case a low ESR capacitor promotes ringing and only helps for frequencies well above the resonance. A capacitor with some extra resistance can help.

For just RF interference one should not a really large cap, at higher frequency the LM399 is no longer so low in impedance. So chances are one could get away with some 100 nF or 10 nF. 10 nF are even available as C0G with really low leakage.

[Andreas]

For example if leakage gets increased somewhat due to heating to let's say 1 uA, times zener differential resistance would give us maybe 10 uV, so that would be 1.5 ppm - as an upper limit of an unstable contribution.

So the bypass could include a series resistor, for example 5 or 10 Ohms, with the output taken from the capacitor.

Zener differential resistance of a LM399 is below 1 ohm -> 1 uA will give a 1uV change.
But from my LNA I know that if you constantly keep the capacitor charged at room temperature the leakage will be much lower than 1uA.

With a series resistor you will increase the differential resistance so leakage has a increased effect.
And remember: most of the current on a LM399 Zener is shunted with a parallel regulator.

And what is still open is what happens when you use a buffer amplifier. It might keep the low frequency peak away (if the EMI does not travel by the ground pin). So a 100nF would do the job for the higer frequencies.

with best regards

Andreas

[dietert1]

Thanks, so the LM399 includes a buffer loop and i would try to avoid a large capacitor over the output, but put a small resistor in between, maybe with a 1 uF PP capacitor to Gnd. I also have 10 uF MKT that would fit. For the heater in the HP3456A reference it's probably two capacitors, since the heater gets fed from +/- 15 V. And maybe again two small resistors in series with the supply.

Regards, Dieter

[Kleinstein]

The capacitor directly at the LM399 reference section would be more about keeping RF out, so no need for really large capacitance. Some 100 nF should be large enough. Because of the internal control loop some series resistance is probably a good idea.  Due to the low output resistance even 10 µF would not give much filtering action. In the 3456 filtering would be more effective at the other side of R502. One may just see the effect of reduced noise in the 5-30 kHz range. The next frequency range of interest would than be at some 25 Hz (1 PLC mode) or 2.5 Hz (10 PLC mode).

[Andreas]

Thanks, so the LM399 includes a buffer loop

For the heater in the HP3456A reference it's probably two capacitors, since the heater gets fed from +/- 15 V. And maybe again two small resistors in series with the supply.

Hello,

not yet: it is still on my todo list wether a buffer can avoid a large capacitor directly on the zener output.

On the heater side: I am not shure wether the 47uF capacitors are really needed there since I usually put those anyway at the heater. In any case I would not put series resistors on the heater side unless I really need to limit inrush current. The datasheet already mentions stability issues with series resistors. (a > 2uF Tantalum is needed).
And yes: if you want to use larger capacitors with a split supply you might need 2 capacitors depending on the voltage regulators. (some regulators might not start up without protection diodes across the output when there is a larger capacitor across +/-15 V).

The capacitor directly at the LM399 reference section would be more about keeping RF out,

unfortunately the LM399 is very sensitive for lower frequencies too. So you need some measure which also works at 50-100kHz where many switchmode supplies are working.

with best regards

Andreas

[dietert1]

As mentioned before, a good measure at lower frequencies may be an external buffer. The HP 3456A has on its reference module the 7 V to 12 V amplifier, so the LM399 output signal stays inside that module. Maybe a metallic shield around the whole reference module can still improve isolation from external fields.

Regards, Dieter

[Andreas]

Hello,

I have updated the LM399 measurements with a ADA4522 (EMI hardened) buffer
So this one actually filters the low frequencies:

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684100/#msg2684100

with best regards

Andreas

[MegaVolt]

Can add a little more schemes? A verbal description leaves room for misunderstandings.

[Andreas]

Hello,

got some ADA4522-1 from Mouser with Datecode #A946
so shurely after PCN change.

Lets look wether the EMI behaviour has changed or not.

with best regards

Andreas

[Andreas]

Hello,

Measurement of 12.04.2020 shows LTZ#9 with already a "newer" datecode DC751 against the measurement on 04.04.2020
(additionally the 100nF on +input of the OP was removed).
Result: -219 ppm @ 60 MHz and 5Vpp

On 25.08.2020 I did a repeated measurement to see the "stray" of the measurements.
Result: -259 ppm @ 60 MHz and 5Vpp

So as expected there is some stray between the measurements (Temperature, slightly different parasitics due to slight position changes).

Measurement of 26.08.2020 shows LTZ#9 with the Datecode DC946 (after PCN)
Result: -238 ppm @ 60 MHz and 5Vpp

So the EMI behaviour of ADA4522-1 obviously has not changed by the mask change.

with best regards

Andreas

[dietert1]

This may be a strange proposal, anyway: Do you have a spectrum analyzer? Did you ever check your references for RF emission? A LTZ1000 probably has enough gain inside to be a RF oscillator. Frequency would depend on length of signal tracks, probably in the 100 MHz - 500 MHz range. I am asking this because i read about the high capacitance of zeners and how similar their construction is to varactors.

Another question i was thinking about: You show how external RF can shift the reference voltage. Did you ever have a look at how it affects noise/stability? Maybe we should add some RF to make the zener "glow better".

Regards, Dieter

[Andreas]

This may be a strange proposal, anyway: Do you have a spectrum analyzer? Did you ever check your references for RF emission? A LTZ1000 probably has enough gain inside to be a RF oscillator. Frequency would depend on length of signal tracks, probably in the 100 MHz - 500 MHz range. I am asking this because i read about the high capacitance of zeners and how similar their construction is to varactors.

Hello,

I fear that is not easy. The buried zener is no "power" device so the chip area is relatively small giving only some pF capacitance below the breakdown voltage.

We are operating the zener above the breakdown voltage so the capacitance is near zero.

In the standard cirquit there is a 22nF in parallel to the output transistor which dampens any RF.
Additionally I have on my devices 100nF across the zener voltage output and 100nF across the BE junction of the transistor, giving effectively 50nF across the zener. So how should the some pF in parallel at a very constant voltage give a RF signal in the MHz range?

Besides this: I usually measure 1/f noise 0.1 .. 10 Hz and also wideband noise 10Hz .. 100kHz which are according to the data sheet.

Sorry no spectrum analyser available. I would also never put a 50 Ohms input in parallel to a unbuffered LTZ1000 (it ages very fast under this condition).
When using the Oscilloscope with FFT I see more or less the FM radio stations nearby. So without a very good shielded room you will measure only garbage.

Another question i was thinking about: You show how external RF can shift the reference voltage. Did you ever have a look at how it affects noise/stability? Maybe we should add some RF to make the zener "glow better".

Stability will be worse since you never get the same RF energy into the zener depending on where the capacitance of your body is located versus the reference. So this is no option for me.

with best regards

Andreas

[Andreas]

Hello,

Strange: today I got a updated PCN revision C notice from Mouser for the ADA4522-1.

https://www.analog.com/media/en/PCN/ADI_PCN_18_0171_Rev_C_Form.pdf

Now I am somewhat confused.

Did I do my verification of EMI behaviour too early? (and need a DC later than Aug 20th this year?)

Whats going on there?

with best regards

Andreas