Author Topic: Probes for checking voltage references  (Read 3815 times)

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Online rhb

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Probes for checking voltage references
« on: March 26, 2018, 02:51:11 pm »
What are recommended probe techniques for checking a voltage reference?  I tried probing the LM399 reference for noise  this morning with a DSO and was promptly handed my head on a platter.  With a regular probe the best I could do was ~11 mV of noise picked up  mostly from the LED overhead light.  The LM399 is holding nicely at 6.917 V on my 4 1/2 digit Tenma meter + 2 mV if I disconnect the meter or power down the LM399.  But it settles back after a few minutes.  Probably about a minute if I disconnect the DMM and a bit longer if I power off the LM399.

I finally did a search and reread Bob Pease's comments on the subject.  I followed Bob's advice and made up a socket to fit on the power header of the board Jason designed that slipped over the probe tip with the grabber removed.  Even getting that to work was not a slam dunk.  I think I'm going to put hot melt glue on it to hold things in place.

But I'm still a factor 0f 100 from the 3478A headed my way. I realize that the DMM has a much longer time constant, but clearly EMI becomes a very serious issue playing with voltage references.
 

Offline David Hess

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Re: Probes for checking voltage references
« Reply #1 on: March 26, 2018, 03:09:02 pm »
The low input capacitance and low bias current which are necessary for the 1 megohm input buffer of an oscilloscope also yield high noise.  References are not particularly quiet but a low noise amplifier will still be necessary to make a meaningful measurement with an oscilloscope.
 

Offline cellularmitosis

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Re: Probes for checking voltage references
« Reply #2 on: March 26, 2018, 03:55:28 pm »
Hey Reg,

You want a "spring tip adapter".  Looks like Mouser has a kit with four sizes for $3.60.

https://www.mouser.com/ProductDetail/Cal-Test/CT3668?qs=sGAEpiMZZMtbPwtnQMMkA0r1RXhinUnLxXKNNibaEQMIqT%252bnMgkUXQ%3d%3d

Insert the probe tip and ground spring into the holes labelled "test".

Also, to reduce EMI, you'll want to bodge in a 0.1uF cap on the output of that board.

Edit: alternatively, if you are trying to measure "1/f" noise (0.1Hz to 10Hz), you'll need a low-noise amplifier (LNA).  I just did a write up about measuring some LTZ1000 references for 1/f noise.  http://www.eevblog.com/forum/metrology/diy-low-frenquency-noise-meter/msg1461828/#msg1461828
« Last Edit: March 26, 2018, 03:57:02 pm by cellularmitosis »
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Online BravoV

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Re: Probes for checking voltage references
« Reply #3 on: March 26, 2018, 04:04:33 pm »
You want a "spring tip adapter".  Looks like Mouser has a kit with four sizes for $3.60.

https://www.mouser.com/ProductDetail/Cal-Test/CT3668?qs=sGAEpiMZZMtbPwtnQMMkA0r1RXhinUnLxXKNNibaEQMIqT%252bnMgkUXQ%3d%3d

Or much-much cheaper alternative, use steel wire for guitar string, wind it on nail with similar or smaller diameter, and/or reheat if needed to fix the coil from unwinding by it self.
 
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Offline cellularmitosis

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Re: Probes for checking voltage references
« Reply #4 on: March 26, 2018, 04:09:28 pm »
this video from TI does a good job of covering spring tip probing (starts about 1 minute in)



oooooooh fancy!

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Offline montemcguire

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Re: Probes for checking voltage references
« Reply #5 on: March 26, 2018, 06:10:30 pm »
When measuring LF 'signals' like a DC reference, the type of cabling from the DUT to the meter is more important than the actual contact and grounding to the DUT. At frequencies above a MHz, e.g. an oscilloscope probe, getting a proper wideband ground connection and HF signal 'launch' is probably more important, but below a couple of kHz, none of this matters.

Shielded twisted pair cables can essentially place an enclosure around the probe leads, all the way to the meter input jacks, and hopefully also remove a bunch of interference. By using a dedicated conductor (one conductor of the twisted pair) for 'ground' from the DUT, as well as an overall shield, more interference can be rejected. The standard procedure is to ground the shield at only one end of the cable, usually the measuring instrument side, so that it behaves as a pure electrostatic shield. At the DUT end, the twisted pair is used for signal + and 'ground' connections, just as in a normal probe.

In the audio world, star quad cable is used as an improvement over standard shielded twisted pair cable. Star quad uses two conductors for each of the conductors in a twisted pair, and because of the mechanical symmetry of such an arrangement, star quad is able to reject about 20dB or more of random EMI compared to a similarly shielded twisted pair. An overall shield will reduce electrostatic interference, as in the case of a coaxial cable, but unless a twisted pair (or quad) is used, there will be no rejection of current induced interference. The advantage of a star quad cable is that the twisted pair geometry is more precise, has more effective 'turns per inch', and thus rejects EMI better.

A good plan would thus be to use a twisted pair (or quad) connected with one conductor to the V+, one conductor to the V-, and the shield cut at the DUT end and connected only to the guard, chassis, or V- connection as appropriate to your meter or test instrument. Mogami 2799 is a nice miniature star quad cable that is very small but still not a huge pain to terminate. If your connectors can handle larger diameter cable, Mogami 2534 is a better cable - it has less capacitance, and because of its larger overall size, you get more copper and better dimensional stability. I have some 2534 cables that are 30 years old and they still work beautifully despite being flexed and used without great care. Conventional shielded twisted pair cables can work well, probably better than coax, but if their construction uses a drain wire, the mechanical displacement of the drain wire against the real twisted pair will mess up the symmetry of the twisted pair, and thus it will not reject as much EMI as possible.
 
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Offline hwj-d

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Re: Probes for checking voltage references
« Reply #6 on: March 26, 2018, 07:18:36 pm »
@rhb

first of all avoid ground-loop: pwr-earth-psu to grnd-dut to pwr-earth-dso. Don't connect shielding, dut-casing, at both sides to ground. Maybe take batteries to prevent that. Scope ground is pwr-earth in principle, dmm is floating measure.

Keep in mind, that dmm measuring in 1ppm/┬ÁV area is done normaly under 10 or 100 plc condition, what you don't have with a scope. Take 20mhz bandwith limitation.

Good luck  ;)




« Last Edit: March 26, 2018, 07:58:09 pm by hwj-d »
 

Online rhb

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Re: Probes for checking voltage references
« Reply #7 on: March 27, 2018, 12:11:43 am »
Thanks to all.

I'd already made a spring tip from copper wire and put a pin header socket on the end.  I play guitar and *really* should change the strings as they have been on there for a year or two.  It's an archtop, so muted treble is part of the sound.  So I'll wind a bunch of the ground springs for all my probes.

I have a lot of noise pickup on the power from the PS to the DUT.  I twisted the zip cord, but it didn't seem to help much.  The PS and DMM are floating.   I'll look for some shielded twisted pair to use for power with the shield grounded  at the PS only.

I'm not familiar with "10 or 100 plc condition".  Could you explain the acronym?

I hope I can get some long historical records of voltage refences.  When I read the comment about "seasonal" effects in the Fluke paper that @cellularmitosis linked I got very interested in getting records for multiple devices at one location.  That told me the author at least did not know what was causing the changes.

Even without temperature, pressure and humidty, I should be able to make some useful inferences just based on the location. While humidity shouldn't effect a reference in a hermtically sealed enclosure,  the op amps and other parts usually  are susceptible.  At ppm, everything matters.  A good analogy is the water elevation in a well.  At first glance that would seem to be quite independent of the phase of the moon.  But you'd be wrong.  It is easily measured and observed.  As is the effect of a train pulling into the station and then pulling out.

Geophysics is full of stuff like that.  If you have a large mass underwater, there is a *very* small bump on the surface of the water.  That's how they make maps of the seafloor using satellite altimetry data.  In my primary playground, reflection seismic data,  one is measuring motions in the subnanometer range and the noise is much larger than the signal.  It's only possible by having multiple channels and a very precise model of the physical relationships among the channels.  Many millions of dollars are spent every year creating models of the elastic wave propagation velocity in the earth so that each data sample can be summed with the correct samples.  The direction dependence of velocity is a major cause of problems and topic of research.

To create a 3D image of the subsurface,  10e7 to 10e9 samples get summed together to produce each of 10e12 to 10e14 image pixels.   The summation gives a 1000 to 10,000x improvement in SNR  There are warehouses scattered around Houston filled with computers doing this.  A major limitation is how much power you can get to the building from the grid.
 
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Offline hwj-d

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Re: Probes for checking voltage references
« Reply #8 on: March 27, 2018, 01:27:31 am »
Quote
I'm not familiar with "10 or 100 plc condition".  Could you explain the acronym?

"Power Line Cycles (PLC) or Number of Power Line Cycles (NPLC) indicates how long an input signal is integrated to obtain a single measurement.  Noise introduced from the power line tends to be periodic.  If the A/D converter integrates for an amount of time equal to one cycle of the power line noise, then the signal components from the periodic noise can be canceled.  Generally speaking, the longer a signal is integrated by the A/D converter, the more accurate the reading result. The (N)PLC setting of an instrument allows adjustment of the tradeoff between speed and accuracy."

https://de.tek.com/support/faqs/speed-what-nplc

Same measurement of one of my ltz's with 1 plc, 10 plc, 100 plc



For 60Hz power, an instrument operating at 1 NPLC can report a new value no faster than at 16.67 msec intervals.  For 50Hz power and 1 NPLC, a new value can be reported no faster than at 20 msec intervals.
« Last Edit: March 27, 2018, 01:32:47 am by hwj-d »
 
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Online Conrad Hoffman

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Re: Probes for checking voltage references
« Reply #9 on: March 27, 2018, 02:15:35 am »
Maybe too basic, but remember that noise levels are entirely meaningless unless the bandwidth is specified.
 

Offline hwj-d

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Re: Probes for checking voltage references
« Reply #10 on: March 27, 2018, 03:23:02 am »
Quote
Maybe too basic

Yes that's right. Sometimes it's difficult to clarify these basic things without immediately compromising someone. The last sentence in the op prompted me to do so.
 

Online rhb

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Re: Probes for checking voltage references
« Reply #11 on: March 27, 2018, 04:08:19 am »
I haven't got far enough to worry about quantifying the noise.  I'm still trying to suppress it.  At the moment I'm debating wire sizes to get  for shielded twisted pair power supply cables.  My bench supply is 6 A max if i parallel the outputs, so 20 AWG ought to do.

I never worked on the acquisition side of things, so I'm not used to specifying or measuring SNR.  Just dealing with whatever we actually got.  But at several million dollars for a survey, the specs are quite specific and comprehensive ranging from allowable sea state to tow noise of the cables and noise from other vessels.

I have enough education and experience to realize that there are always gaps in what one knows and remembers.  So I'm not inclined to take umbrage at having something very basic pointed out unless I get beat over the head with something which is not relevant to the instance in question.
 
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Offline cellularmitosis

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Re: Probes for checking voltage references
« Reply #12 on: March 27, 2018, 04:14:21 am »
A trick I picked up from Conrad is to use the twisted pairs from either cat5 ethernet cable or telephone "bell" wire.  Usually 26AWG, solid core.  Great for hooking up voltage references, and more than adequate for powering them too.

When you get into situations where leakage in the cable causes measurable error (I think this comes into play when measuring something with a high output impedance), the PVC insulation on those twisted pairs is no longer good enough, and you'll want something with Teflon (PTFE) insulation.  EEVBlog forum member "ap" sells some cable like that.  https://www.ab-precision.de/products/accessories/
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Online rhb

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Re: Probes for checking voltage references
« Reply #13 on: March 27, 2018, 04:18:56 am »
Thanks.  I was looking at 20 AWG silver plated shielded PTFE on eBay at $16 for 10 ft with shipping.
 

Offline montemcguire

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Re: Probes for checking voltage references
« Reply #14 on: March 27, 2018, 04:31:00 am »
I have a lot of noise pickup on the power from the PS to the DUT.  I twisted the zip cord, but it didn't seem to help much.  The PS and DMM are floating.   I'll look for some shielded twisted pair to use for power with the shield grounded  at the PS only.

Maybe try grounding one of the devices? If everything is floating then there's a lot of trash on the shields / enclosures that isn't going anywhere but into the common mode of the devices. I'd ground the PSU if there's a choice, just to see if anything changes.
 

Offline hwj-d

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Re: Probes for checking voltage references
« Reply #15 on: March 27, 2018, 05:03:05 am »
Thanks.  I was looking at 20 AWG silver plated shielded PTFE on eBay at $16 for 10 ft with shipping.

I'm looking for somewhat of this type too. But see the prices in germany ...

https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&LH_Complete=1&LH_Sold=1&_nkw=10m+HF+TEFLON+LITZE+%C3%98+5mm%2F2x1%2C8mm+GESCHIRMT%2FVERSILBERT&LH_PrefLoc=2
 

Online rhb

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Re: Probes for checking voltage references
« Reply #16 on: March 27, 2018, 07:48:27 am »
Ouch!
 

Online Conrad Hoffman

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Re: Probes for checking voltage references
« Reply #17 on: March 27, 2018, 10:25:52 am »
I don't remember which ones, other than AN-47, but read anything you can find on probes and noise from the late Jim Williams of Linear Technology (now part of Analog Devices).
 

Offline thermistor-guy

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Re: Probes for checking voltage references
« Reply #18 on: March 27, 2018, 10:34:41 am »
...
In the audio world, star quad cable is used as an improvement over standard shielded twisted pair cable. Star quad uses two conductors for each of the conductors in a twisted pair, and because of the mechanical symmetry of such an arrangement, star quad is able to reject about 20dB or more of random EMI compared to a similarly shielded twisted pair....

Cat5 cable outperforms star quad here:
https://www.prosoundtraining.com/2010/03/15/the-emi-project-part-2-point-source-field/
 
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Offline cellularmitosis

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Re: Probes for checking voltage references
« Reply #19 on: March 27, 2018, 10:44:12 am »
pasting the graph in case that article suffers from bit-rot in the future

edit: interesting that they could measure the difference between the various twist-rates of the CAT5 pairs!  Looks like the blue pair is the clear winner.

edit 2: part 1 of that series: https://www.prosoundtraining.com/2010/03/15/the-emi-project-part-1-introduction/
« Last Edit: March 27, 2018, 10:47:38 am by cellularmitosis »
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Online rhb

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Re: Probes for checking voltage references
« Reply #20 on: March 27, 2018, 10:53:42 am »

Thanks.  I'm a big Jim Williams fan.  I grabbed AN-47 and took a quick look.  Very nicely done.  I wish EDN & AD  would put out all of Jim's work in book form.  I've pointed a lot of times to "Max Wien, Mr. Hewlett and a Rainy Sunday Afternoon".
 

Offline Svgeesus

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Re: Probes for checking voltage references
« Reply #21 on: March 27, 2018, 01:10:56 pm »
Thanks.  I was looking at 20 AWG silver plated shielded PTFE on eBay at $16 for 10 ft with shipping.

I'm looking for somewhat of this type too. But see the prices in germany ...

For good cable at good prices in Europe, try Thomann Music who sell high quality Sommer cables for microphones etc.
https://www.thomann.de/de/kabel_meterware.html
 

Offline CalMachine

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Re: Probes for checking voltage references
« Reply #22 on: March 27, 2018, 01:24:13 pm »
Here is what I use :popcorn:   Not sure why the datasheet claims it is tinned copper conductors, though.  It is most definitely bare copper conductors.

https://www.ebay.com/itm/162965561759

https://catalog.belden.com/techdata/EN/5500FE_techdata.pdf?ip=false


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Online rhb

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Re: Probes for checking voltage references
« Reply #23 on: March 27, 2018, 01:36:10 pm »
I have read online that oscilloscope cables are typically 200 ohms to reduce reflections at the 1 Meg scope input.  But I have been unable to find a source of 200 ohm coax.  The highest impedance I could find was 95 ohms.   Moreover, the magnitude of the reflection coefficient is almost unity with 200 ohm cable.  So it seems rather silly even if scope makers have secret sources of cable.

The 2 & 4 conductor PTFE shielded twisted pair takes care of power and DMM probes, but I'd like to make up a set of test cables with various connectors at the DUT end.  Should i just use 50 ohm cable with a thru terminator?  That's a lot simpler.  And seems to me to make more sense so long as the DUT side is close to 50 ohms.

What do people who do this stuff for a living do?
 

Online splin

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Re: Probes for checking voltage references
« Reply #24 on: March 27, 2018, 01:44:51 pm »

Thanks.  I'm a big Jim Williams fan.  I grabbed AN-47 and took a quick look.  Very nicely done.  I wish EDN & AD  would put out all of Jim's work in book form.  I've pointed a lot of times to "Max Wien, Mr. Hewlett and a Rainy Sunday Afternoon".

Well they have put a number of them in book form:

https://www.amazon.com/Analog-Circuit-Design-Applications-Solutions/dp/0123851858

I don't have volumes 2 and 3 but there are 41 Llnear Technology application notes in the first including AN47 and  AN86. Vol 1 includes 21 power management, 5 data conversion, 15 signal conditioning and 2 HF/RF  design AN's
 


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