EMI-Measurements of a Volt-Nut

[Andreas]

Would not have expected that the offset voltage of a modern device suffers significant from this typical input impedance 
Wondering if an input capacitance at the inverting input to gnd would improve this - I know this is going a bit offtopic...
Andreas, you could populate R23 with ~100pF-1nF (leakage <100pA@10V) with R22=10k, C22=10nF to verify.

Hello,

I did a measurement on LTZ#8 with OPA189 from the worst case scenario: (28.11.2019 with 100nF to PGnd R22=10k, C22=10nF)
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2810142/#msg2810142

And added a 470pF (500pF datasheet recommendation) 1000V FKC3 capacitor (to PGnd = directly to the OPA189 Gnd instead of the long way)
Result: another 5-5.5 uV increase of the "offset" when the 470pF capacitor is populated.
I had expected "no change" since C22 already gives a path to the output of the Op-Amp which is low impedant driven.

with best regards

Andreas

[Andreas]

Another test with LTZ#8:

starting from setup of 28.11.2019 with 100nF to PGnd R22=10k, C22=10nF
I replaced the OPA189 with the EMI-hardened low power version OPA187.
Of cause the 1/f (0.1-10Hz) output noise increases somewhat (I measured ~0.1uVpp).

The good news:
- The output voltage between unbuffered and buffered output measured within 1uV. (so no abnormal offset due to bias current).

The bad news:
- against the measurement of 28.11.2019 with OPA189 (-48 ppm @ 1.78Vpp @ 60 MHz)
  there is a large increase of EMI-sensitivity for the OPA187 (-1392 ppm @ 1.78Vpp @ 60 MHz)
- And additional some distortions on the OPA187 below 100kHz (obviously interference with the internal chopper frequency).

I will have to look if a additional capacitor across output ground gives some improvement.

with best regards

Andreas

[rhb]

You might want to make an RF sweep with a spectrum analyzer.  I strongly suspect that the radio and television broadcast bands are the biggest source of EMI.  That's certainly true for me living in the woods.  Otherwise, most of the noise will be below 100 kHz from nearby sources which can be tracked down and squelched.

Recent Chinese T&M kit is especially prone to producing a lot of EMI from the SMPS.  I shielded my entire mains feed to the instruments because of my concern about radiated EMI being picked up by adjacent instruments.

As I need to set up a new bench for the consequences of a severe TEA binge I am going to build a  12' x 16' x 8' room in my shop lined on all faces with galvanized steel sheet with soldered seams.  That will get covered with plaster board.  I plan to experiment with spraying wall board mud mixed with coal or charcoal  and iron or steel filings as an RF absorber if I can find a cheap source of iron or steel filings.

If you don't have the luxury of doing that a largish box would do for holding an instrument and some references.  I have to build a room to keep out wood dust, so lining it with steel sheet and RF absorber is a small frill. 

I have a 3457A in transit and am looking for a 44492A 10 input two wire relay board for it.  Once I have that all in hand I'll put the 3457A/44492A in a non-rectangular metal box lined with RF absorber and its own Peltier based temperature control system.  The lab area will have its own HVAC zone, but tight temperature tolerances work best with small spaces and masses.

For someone living in an apartment, a completely closed metal rack with a dedicated temperature control system seems to me the only way to avoid EMI.  Put a shield between each section of the rack, use proper feed thrus and EMI will not be an issue.  For hobby metrology I had assumed this was the norm, more for temperature control than EMI.  It's the cheapest solution to tight temperature control.

For cooling a rack, strip the mechanicals from a small refrigerator to build a small cooling system for the box with a holding plate with phase change media to minimize compressor cycling.  For a single instrument, use a Peltier travel cooler.

If you build an enclosure, make all the surfaces non-orthogonal.

Designing for high CMR is vitally important in precise work.  But avoiding high common mode fields is more important.

Have Fun!
Reg

[Andreas]

Hello,

after adding a capacitor from output to output Ground I get a dramatically reduction in EMI-Sensitivity.
(so I actually fear that the Capacitor to power ground is broken somehow?)

So If I am able to handle the 80 kHz interference that will be the best result up to now.

with best regards

Andreas

[Andreas]

@rhb:

my intention is not to avoid EMI but to harden my devices so that I get the same results independant of environment.

Here it is not easy to "live in the woods". The population is simply too dense.
And I have no intention to dig myself into a abandoned salt dome on battery supply which would be the only possibility to avoid EMI here.

with best regards

Andreas

[Andreas]

Hello,

I also did some tests on LTZ#9 with a different approach at the frequency of maximum sensitivity:

I fed in 60MHz 3.56Vss (unloaded generator output) signal into the buffered output of the reference.
Then I touched different parts like battery the LTZ itself and the plastic OP-Amps with a finger.
Of course the output voltage changed a lot by changeing the parasitic capacitances.
The "deltas" are recorded to find a position of maximum sensitivity.

Further I used a "probe" to touch the pins directly with a "defined" parasitic capacitance:
https://www.reichelt.de/miniatur-klemmpruefspitze-0-75-mm-schwarz-pruef-mps-2-sw-p106196.html?

The "deltas" between untouched and touched pins are also recorded together with the "absolute" value (without EMI signal).
It soon got clear that even the housing of the LTZ has some EMI relevance.
Since the housing carries around 0.5V against the ground pin it cannot be connected directly to ground.
I used a capacitor to do a RF short to ground. Which influences somewhat the other sensitivities (especially pin 3).
The soldering caused a permanent shift of the output voltage by -4 .. -5 ppm.
So I will not use this measure (see picture) for my calibrated LTZ´s.

Further from the measurements it gets clear that pin 6 is the most sensitive pin to EMI.
(the base of the temperature sensing transistor).
So by adding C11 between base and emitter of the sensing transistor the sensitivity can be largely removed.

with best regards

Andreas

[Andreas]

Hello,

during my weekly ageing measurements for the LTZ references I mentioned that LTZ#8 (with OPA187 buffer) had a large noise together with my ADCs (ADC13, ADC15, ADC16 and ADC17). This happened only on the buffered output of the LTZ#8 reference. The unbuffered output was unaffected.
Further examination showed up to 150 uVpp on the 2:1 divided output of the LTC1043 dividers. (so 300uVpp on the 10V input side).
Normally the LTC2400 ADCs show only around 10uVpp on the 5V input or 20uVpp on the 10V input.

So obviously the OPA187 has interference with the switched load of the LTC1043 dividers.

I made a measurement to show the difference of the buffered and unbuffered output of LTZ#8.
After exchange to an ADA4522 the noise was in the normal 10uVpp range
(except for some time immediately after connecting to the unbuffered output).

so for me another reason not to use the OPA187 as output buffer.
On my K2000 (10 NPLC) and 34401A (100NPLC) there is no interference visible.

with best regards

Andreas

[Andreas]

Hello,

I updated some EPCOS coil measurements on the coil comparison post:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684085/#msg2684085

with best regards

Andreas

[Andreas]

Hello,

I repeated the heater noise + Lamp switching experiment on LTZ#9 see:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2776212/#msg2776212

This time with C11 populated between base + emitter of the heater sensing transistor.
Under same conditions I measured  much lower excursions of the heater voltage.
Instead of up to 1Vp I measured maximum 0.2Vp excursion.
If I look only on the low frequency part (e.g. with a 1 MHz filter) the maximum is  around 80mVp
So C11 gives a factor 5-12 improvement.

From statistics side
20 measurements in average with 108mVpp (against 287 mVp peak only in formerly measurement)
Standard deviation 44 mVpp (against 274 mVp before)

when filtering with 1 MHz we get
20.5 mVp average
and 21.7 mV standard deviation.

with best regards

Andreas

[SilverSolder]

[...] design from a EMI proof power supply first [...]

One or more large 12V lead-acid battery/ies?

[Echo88]

Regarding suitable power supply here are some suggestions:
LT1533-based DCDC: https://www.analog.com/media/en/technical-documentation/application-notes/an70.pdf
Sine-Wave-Driven-DCDC: https://www.analog.com/media/en/technical-documentation/application-notes/an29f.pdf Page 4
https://x.artofelectronics.net/wp-content/uploads/2019/11/9xp14_low-noise_isolated-pwr.pdf

[iMo]

FYI - a bit harder EMI measurement, hopefully not applicable to your labs

[Andreas]

One or more large 12V lead-acid battery/ies?

I generally use NiMH-cells for my references.
Mainly to avoid one stage (inner housing) of guarding and shielding.

with best regards

Andreas

[iMo]

Below what I see when I switch my wifi router on/off (confirmed, I can send morse keying the router on/off ).
The router is ~4m off the plastic box with the 10V Vref and 34401A.
Notebook 1m off the box, wifi on all the time.

The measurement taken on a standard LM399AH wiring (dead bug 78L15+OP07+LT5400), the heater blocked 4u7 tantalum, the zener with 100nF ceramic close to its 4 pin socket plus 4k7/3u3_foil low pass towards opamp. 78L15 blocked and output of the OP07 blocked 25ohm/100nF_foil.
34401A with 1m long twisted cable rolled to a small 15cm dia bundle.
An stm32 MCU on the RS232 with the HC-05 sending data to my notebook.

The router on/off difference is always aprox -1.5ppm as depicted.
Interestingly - when the wifi router is ON the Vref output is "more" stable and quiet.

PS: y axis is ppm (filtered), blue is the temperature inside the Vref box (not related to the issue).

PPS: a clip-on ferrite bead put on:

a. RS232 cable - no change
b. 34401A close to its inputs - no change
c. close to the Vref output - the on/off ppm difference halves (-0.7ppm)

[dietert1]

Another nice example that a precision reference isn't "ready" if it needs to be treated like a raw egg.
Maybe your Wifi isn't off completely when you think it should be off. Nowadays those things have their own life.
Can you do something similar with a cellphone?

Regards, Dieter

[iMo]

The wifi router is switched off by pulling its power cable off - thus no power off the antennas. I have closed my phone in a metal box and put in a different room during the experiment (actually this is NOT an EMI experiment, but my measurements with 399s I run - I had been thinking it is a popcorn or a defunct 399 or something like that, until I discovered the relationship between the time I switched off the router and the jumps in my data).
The HF energy enters the Vref output via cables, the 100nF foil does not work at those frequencies and it messes up with the OP07 opapm. I will add ceramics and common mode choke at the Vref output tomorrow..

[MegaVolt]

I had been thinking it is a popcorn or a defunct 399 or something like that, until I discovered the relationship between the time I switched off the router and the jumps in my data).

Refrigerator?

[Andreas]

@IMO
thanks for the experiment (I have also ordered some LM399 for that purpose but you were faster).

the 78L15 is blocked with 100nF both at input and output?
(I have seen a 3.3V regulator with about 400mV (>10%) off when using electrolytics according to the data sheet on EMI-Tests (30MHz-1Ghz 3V/m). A additional 100nF keramics at input and output helped).
The LM399 is also sensitive to heater supply changes.
And I would also spend a 100nF in parallel to the heater pins. (additionally to the 4.7uF).

Where does the power supply come from (battery / wall wart)?
Eventually you will also need a common mode choke here.

Does exchangeing the op07 against a ADA4522 give a improvement?

with best regards

Andreas

[iMo]

@Andreas: my power source is a "low noise" 723 well blocked with ceramics. Then follows 78L15 (as a feasibility experiment only) with 220u input and EDIT:100nF ceramic output. I will add 100nF ceramics to the heater. I do not plan chopper there, OP07 perhaps an OPA277 is enough. Chokes and UHF ceramics at input/output are a must, it seems.

[Andreas]

there are also some EMI-hardened other precision OP-Amps (non chopper).

with best regards

Andreas

[iMo]

I had been thinking it is a popcorn or a defunct 399 or something like that, until I discovered the relationship between the time I switched off the router and the jumps in my data).

Refrigerator?

It is the wifi router, powering it off/on changes my Vref output by 1.5ppm, I repeated that many times after the discovery and it works nice

[iMo]

there are also some EMI-hardened other precision OP-Amps (non chopper).

OMG, an LM399 reference does not need this kind of gold plating, imho - it requires a well made decoupling, that is all.. 

[Andreas]

Hmm,

it is not only the reference which gets affected by the EMI.
Also the OP-Amp is affected.
So I think a ADA4177 is shurely not too much.

with best regards

Andreas

[iMo]

I meant 399 reference as the "board" .
I would opt for an opamp with low offset drift, something like <0.5uV/K.
OPA277 looks best with +/-0.1uV/K.

[iMo]

Added in/out filters (2T on a ferrite bead plus 4n7 ceramics) and clip-on ferrites on the RS232 and Temperature cables.
Still 0.4ppm difference between router on and off.