Author Topic: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown  (Read 35069 times)

0 Members and 1 Guest are viewing this topic.

Offline Unixon

  • Regular Contributor
  • *
  • Posts: 149
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #25 on: April 09, 2015, 07:51:06 pm »
At 20:55 there's a guard trace around Q90 and other circuitry looks like cut/interrupted by an isolation slot.
I wonder if this was intentional or not...
However, at least above Q90 it goes through via to somewhere else, a ground plane perhaps.

Even more cut guard traces at about 24:20. Looks like cutting guard traces with isolation slots is a common practice here...
« Last Edit: April 09, 2015, 08:00:21 pm by Unixon »
 

Offline thewyliestcoyote

  • Regular Contributor
  • *
  • Posts: 100
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #26 on: April 09, 2015, 08:07:53 pm »
Never thought I would as about Nyquist sampling on a DMM. With a sample rates as high as 1 MSps has any one thought about doing some testing see the frequency performance of the DC measurement. Haha never thought I would as that about a DMM. 18 bits is almost pushing the limits of 32 bit floating point math for a FFT.

Dave maybe you could do a frequency response testing? Maybe just a frequency sweep from  DC to maybe 2 MHz, or a couple static frequency measurements. This would also show if there is some aliasing possible. Crazy to be thinking about this for a DMM.

Thanks,
Wylie

 

Offline nilium

  • Contributor
  • Posts: 19
  • Country: 00
    • Nil's Corner
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #27 on: April 09, 2015, 08:13:18 pm »
Excellent video indeed.

Probably a basic question but you mentioned their grounding being improper. Would you please elaborate on that and mention how it would be proper?

Thanks!

NV
 

Offline bktemp

  • Super Contributor
  • ***
  • Posts: 1623
  • Country: de
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #28 on: April 09, 2015, 08:17:27 pm »
At 20:55 there's a guard trace around Q90 and other circuitry looks like cut/interrupted by an isolation slot.
I wonder if this was intentional or not...
However, at least above Q90 it goes through via to somewhere else, a ground plane perhaps.

Even more cut guard traces at about 24:20. Looks like cutting guard traces with isolation slots is a common practice here...
Looks almost like an afterthought: They did put the guard traces between the pins but then decided the guard traces are not enough and they did cut the slots instead.


I have no idea if it is possible, but could the laser trimming on those resistor networks been used to adjust the temperature coefficient? The traces have different colours, they are probably different materials. Lets say those have slightly different coefficients. By trimming only one material it could be possible to trim the coefficent to zero.
 

Offline Marco

  • Super Contributor
  • ***
  • Posts: 4366
  • Country: nl
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #29 on: April 09, 2015, 08:33:07 pm »
That secondary PSU for the analogue board is frankly bizarre. It must be switching a fair amount of current at a fair few MHz

There's a lot of ferrite in that thing ... would be cool if Dave could measure it's operating frequency.
Quote
is that really going to be a lower-noise system

Maybe they want to minimize capacitive coupling to ground as well?
 

Online dr.diesel

  • Super Contributor
  • ***
  • Posts: 2123
  • Country: us
  • Cramming the magic smoke back in...
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #30 on: April 09, 2015, 08:33:35 pm »
A while back, I pulled apart the firmware upgrade for the SMU 2400. Nested zips, then S-Record. IIRC, they used Greenhills Integrity as RTOS. So not your standard Linux / Windows CE job.

I specifically asked this question when I bought my K2450.  I really confused the sales lady but eventually got to the right guy.  He would only state that is does not run Windows/Linux.

Offline Unixon

  • Regular Contributor
  • *
  • Posts: 149
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #31 on: April 09, 2015, 08:34:12 pm »
The U34 has a funny looking long package. What is this IC, what is it doing?
 

Offline SeanB

  • Super Contributor
  • ***
  • Posts: 15091
  • Country: za
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #32 on: April 09, 2015, 08:36:27 pm »
Guard trace is to reduce leakage current across the board surface. Cutting the board means no more surface, and having the guard trace right to it means no leakage path possible. The slots were probably added to increase both breakdown voltage, reduce stress on the package and ensure that there is no possible leakage path. Cheaper than PTFE standoffs and solid wiring floating in air, and more repeatable and easier to compensate.
 

Offline bktemp

  • Super Contributor
  • ***
  • Posts: 1623
  • Country: de
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #33 on: April 09, 2015, 08:43:54 pm »
The U34 has a funny looking long package. What is this IC, what is it doing?
https://www.idt.com/document/dst/qs34x2245-datasheet
Looks like it is disconnecting the GPIB ic.
 

Offline f4eru

  • Frequent Contributor
  • **
  • Posts: 550
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #34 on: April 09, 2015, 08:54:13 pm »
"Individually Hand selected and tweaked someone with a grey beard or nude virgins, or both"

To attain this kind of precision, it has to be a grey bearded nude virgin.
Very hard to find such a guy, and this makes the cost of these meters...

This free standing vishay resistor is a bit howya doin', could have been fastened somehow.
 

Offline mikeselectricstuff

  • Super Contributor
  • ***
  • Posts: 12001
  • Country: gb
    • Mike's Electric Stuff
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #35 on: April 09, 2015, 09:00:36 pm »
When the plastic box/lid is in place, it holds it all tightly in place, bridging the gaps in the spiral. Effectively the box hangs directly on four tabs to the main PCB, and the internal circuitry hangs by four tags inside the box.

Thermal isolation. Not seismic isolation.
I suspect some of the cut-outs are also to do with decoupling mechanical stress - on a big PCB, hotspots in various places, as well as the mechanical fixings  are bound to set up some mechanical stresses within the PCB, which you don't want transferred into the leads of sensitive devices.
The slots round the reference for example will minimise the transfer of any board warp or other movement onto the section of PCB containing the reference.

Something else that struck me - all those slots between IC pins - but you still have the potential for surface leakage on the IC body -  is the leakage across the PCB enough that it makes a difference to do cutouts when you still have a short path on the IC body?

Youtube channel:Taking wierd stuff apart. Very apart.
Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Online NANDBlog

  • Super Contributor
  • ***
  • Posts: 4443
  • Country: nl
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #36 on: April 09, 2015, 09:13:52 pm »
Nice multimeter, but I still don't understand why they need the laser trimmed resistors. I understand this for older multimeters, with less processing power, but should be no problem to use just 1%, low tempco resistors and then calibrate it in software. The Freescale microcontrollers on the board have even a double precision floating point unit, and they run at 800 MIPS, so even at 1 MHz samplerate should be no problem to do one more multiplication per sample.
I've seen 2% resistors even those were laser trimmed. Things like the 10 Mohm input impedance doesnt tolerate too much difference.

The general feel of the meter it is like it was snapped together. I really dont like the construction,  all those million cables zapping everywhere and boards in every direction... That coax transformer especially. If you need to design a low noise power supply, design it properly. There are options like PCB planar transformers ZCS, and countless methods to generate low noise power. Probably half the guard rings and slots are not necessary. When I say those pins of that dual Jfet in the air  :o This dual gigahertz CPU is also... I mean cmon, I know that processing power is cheap but do you really need that?
Dave the you cannot appreciate the 3458A until you used it. The first few days I used it I was yaw dropping and smiling, saying that "I can measure stuff that I didnt even thought I can measure". Like potential increase on a solid ground plane after turning on a LED. Probably it even has faster display update than this Keithley.
Probably the end result is still good though. I mean the specs are very good, and it does the job.
 

Online dr.diesel

  • Super Contributor
  • ***
  • Posts: 2123
  • Country: us
  • Cramming the magic smoke back in...
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #37 on: April 09, 2015, 09:14:07 pm »
This free standing vishay resistor is a bit howya doin', could have been fastened somehow.

It may look funny, but clamping it down is just not required.  You could toss that thing in the back of a log wagon on an epic coast to coast voyage and it would arrive fine.  (well the vishay would be fine anyhow, LOL)

Offline open loop

  • Regular Contributor
  • *
  • Posts: 52
  • Country: gb
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #38 on: April 09, 2015, 09:15:08 pm »
I personally would be very careful  working on that analogue board during a repair. With all of those cut outs I can imagine it would be easy to damage the PCB if one is a little "ham fisted". Can just imagine the repair tech saying, "just broke the PCB going to voltage ref, is that a problem?"  :palm:

I wouldn't expect this to break down for at least the next 10 years of careful use. With the meter having a 1 meg sample rate I assume you could record audio  >:D

Is that something Dave would like to have a go at?


 

Offline DanielS

  • Frequent Contributor
  • **
  • Posts: 798
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #39 on: April 09, 2015, 09:45:26 pm »
This free standing vishay resistor is a bit howya doin', could have been fastened somehow.
The "Do Not Bend" silkscreen next to the resistor implies that the resistor should not be subjected to any external stresses, likely because it could mess up calibration. Fastening it to something would introduce additional drift from mechanical stresses such as thermal expansion and package compression/deformation from being constrained.

When you are at a resolution level where looking at components sideways can affect readings, many things that do not seem to make sense may become critical.
 

Offline lampbus

  • Contributor
  • Posts: 7
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #40 on: April 09, 2015, 09:54:50 pm »
The boxes are not off the shelf, you can see the diagonals have 2 locating pips, and a corresponding hole in the PCB for them as well, and the bottom sections have the same 2 pips on the other diagonal. Makes it easy to put on the board and align so the screws are going to be in the centre of the holes in the board and will not bind on the GRP material, and positions the box very accurately.

Not cheap styrene or PVC boxes, more like a precision moulded polycarbonate, polypropylene or glass filled nylon, in a rather expensive mould.

Yes, you are right, I noticed the little pins, but failed to notice that there were no mating holes in the opposing  cover. They were never intended to mate without the PCB in between.

I googled the number in the cover - it is a reference to glass re-enforced nylon. (2040-310) but I don't know what the B means that follows the number.

The mold tool may not be all that expensive - they will not make that many of these parts ? Soft tooling ? They may even use a standard mold tool with different inserts to make the little pips, but I cant see any tool joint lines.

On the other hand, the wall thickness is high to limit thermal conductivity - most enclosures I use have quite thin section so on balance probably all special tooling. I think I would have taken the opportunity to make it look a bit nicer  - they went overboard with the logo on the bail...so why not put one on the internal parts that help make this kit different from the lower accuracy manufacturers ?

« Last Edit: April 09, 2015, 09:59:13 pm by lampbus »
 

Offline JackOfVA

  • Supporter
  • ****
  • Posts: 350
  • Country: us
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #41 on: April 10, 2015, 12:44:03 am »
There's something odd about the binocular core isolation transformer.  It looks as if it is constructed from two different material types. An outer tube of ferrite (rougher and dark gray) and an inner tube of a smooth light gray material. The light gray color appears to come from an epoxy or parylene coating. Some ferrite material is parylene coated, but I normally associate that with powdered iron material. If the inner core is from MicroMetals, gray corresponds to powdered iron "mix-3" optimized for 20 KHz - 2 MHz for resonant circuits. 


Or, perhaps the outer sleeve is plastic.

If the outer sleeve is ferrite, perhaps it's function is to work as a magnetic shield, although this strikes me as unusual.

« Last Edit: April 10, 2015, 12:46:18 am by JackOfVA »
 

Offline thm_w

  • Super Contributor
  • ***
  • Posts: 1380
  • Country: ca
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #42 on: April 10, 2015, 12:44:53 am »
I suspect some of the cut-outs are also to do with decoupling mechanical stress - on a big PCB, hotspots in various places, as well as the mechanical fixings  are bound to set up some mechanical stresses within the PCB, which you don't want transferred into the leads of sensitive devices.
The slots round the reference for example will minimise the transfer of any board warp or other movement onto the section of PCB containing the reference.

Something else that struck me - all those slots between IC pins - but you still have the potential for surface leakage on the IC body -  is the leakage across the PCB enough that it makes a difference to do cutouts when you still have a short path on the IC body?

Epoxy appears to be a few orders of magnitude worse than FR4. The leakage of the IC itself is again a few orders of magnitude worse than the epoxy encapsulation (DG411 = nA's for example). These values were pulled from random datasheets for epoxy/FR4:


FR4:
Volume resistivity: 3x10^13
Thermal expansion: 10-70ppm/K

Epoxy:
Volume resistivity: ~1x10^11
Thermal expansion: 15ppm/mC

http://c-component.ru/sites/default/files/KCC%20Organic%20Material%20EMC%20%28Epoxy%20Mold%20Compound%29.pdf
http://www.leiton.de/formulare/Datasheet%20-%20rigid%20FR4%20PCBs%20-%20Rev%202.2.pdf
http://www.andus.de/_pdf/probimer-77.pdf

I can see there is less chance for dust/contamination to build up between leads. But other than that, not sure, maybe someone else can comment further.
 

Offline JoeN

  • Frequent Contributor
  • **
  • Posts: 984
  • Country: us
  • We Buy Trannies By The Truckload
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #43 on: April 10, 2015, 02:49:17 am »
The U34 has a funny looking long package. What is this IC, what is it doing?
https://www.idt.com/document/dst/qs34x2245-datasheet
Looks like it is disconnecting the GPIB ic.

I just dropped by this thread to ask what that guy is.  Never seen a SSOP-80 before.  They are calling it "MilliPaQ".  Good name.  I want one on my project just because it looks cool.   :P

Well, at least they are cheap if anyone cares.  Sucky on resistance, though.  Needs an update.  http://www.digikey.com/product-detail/en/QS34X2245Q3G/800-1742-5-ND/1916043

Spoke too soon, they have a much better 3.3V version with Ron of 4 ohm instead of 23 ohm.  http://www.idt.com/document/dst/qs34xvh245-datasheet  http://www.digikey.com/product-detail/en/QS34XVH245Q3G/800-2196-ND/2018992


« Last Edit: April 10, 2015, 03:09:09 am by JoeN »
Have You Been Triggered Today?
 

Offline Marco

  • Super Contributor
  • ***
  • Posts: 4366
  • Country: nl
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #44 on: April 10, 2015, 03:26:37 am »
National did opamps with fA range leakage currents in epoxy packages ... and even then most of that was probably in the circuit.

Worry about drenching them in flux rather than the epoxy.
 

Offline victor

  • Regular Contributor
  • *
  • Posts: 110
  • Country: 00
  • Boy who writes code and take things apart
    • vitim.us
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #45 on: April 10, 2015, 04:03:25 am »
That fan fixed on a huge pcb...  :palm:

I'm sure that will act pretty much as a amplifier for the noise. I bet you can attach strings to that board and make a guitar out of a pcb.
your body is limited, but not your mind
 

Offline rs20

  • Super Contributor
  • ***
  • Posts: 2150
  • Country: au
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #46 on: April 10, 2015, 09:46:57 am »
I know this question must have an answer, because guard rings are so widely used, but:

What's the point of all these guard rings & isolation slots if the guard rings don't extend, for example, to the plastic on the underside (and top) of the relays & SOIC packages? Are the plastic packages of these components somehow treated to make them far less susceptible to contamination/humidity than the PCB surface? The length of "missing guard trace" on the packages is small, but so is the distance between pins, so it cancels out as far as I can tell from that point of view. For a given level of contamination, there'll be just as much leakage happening on the package surface as on the PCB. Why am I wrong?

Like, look at 26:07. Am I the only one who sees a MASSIVE piece of black plastic completely defeating the clearance and creepage advantages afforded by those intricate slots?  :wtf:
« Last Edit: April 10, 2015, 09:49:34 am by rs20 »
 

Offline rf-design

  • Regular Contributor
  • *
  • Posts: 57
  • Country: de
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #47 on: April 10, 2015, 11:21:12 am »
When the plastic box/lid is in place, it holds it all tightly in place, bridging the gaps in the spiral. Effectively the box hangs directly on four tabs to the main PCB, and the internal circuitry hangs by four tags inside the box.

Thermal isolation. Not seismic isolation.
I suspect some of the cut-outs are also to do with decoupling mechanical stress - on a big PCB, hotspots in various places, as well as the mechanical fixings  are bound to set up some mechanical stresses within the PCB, which you don't want transferred into the leads of sensitive devices.
The slots round the reference for example will minimise the transfer of any board warp or other movement onto the section of PCB containing the reference.

I think your are right. The box and the board spiral are for both purposes. For thermal and mechanical isolation. The buried zener diode themselve is less sensitive to mechanical stress transfered from the board to the package over die glue to the silicon. But the temperature regulator within the LTFLU could be impacted by mechanical stress. The bandgap voltage of the biplar transistors change with the stress and I think the regulator is based on comparison of the scaled temperature voltage to the bandgap voltage.
 

Offline rf-design

  • Regular Contributor
  • *
  • Posts: 57
  • Country: de
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #48 on: April 10, 2015, 11:28:06 am »
No way, guys!

Even though Wayne C. Goeke, designer of the HP3458As ADC, is the person to join Keithley, the 7510s ADC by no means comes near the 3458A!
Lay their specs side by side, and you will promptly see a big quality difference, yet.

Its linearity is mediocre 1..2ppm, again, like most all other 6 1/2 ... 7 1/2 DMMs, and still no other DMM has got this unique , true AUTOCAL function...
Obviously, the true value of this function (i.e., a complete cal lab built inside) is still not clear to everyone...

Frank

Do you know what is the specific feature which take the linearity from 2ppm to 0.2ppm in the ADC?
 

Online NANDBlog

  • Super Contributor
  • ***
  • Posts: 4443
  • Country: nl
Re: EEVblog #731 - Keithley DMM7510 7.5 Digit Multimeter Teardown
« Reply #49 on: April 10, 2015, 11:28:52 am »
I think your are right. The box and the board spiral are for both purposes. For thermal and mechanical isolation. The buried zener diode themselve is less sensitive to mechanical stress transfered from the board to the package over die glue to the silicon. But the temperature regulator within the LTFLU could be impacted by mechanical stress. The bandgap voltage of the biplar transistors change with the stress and I think the regulator is based on comparison of the scaled temperature voltage to the bandgap voltage.
Even thoght the LTFLU is less susceptible, the resistors and capacitors connected to it will be affected by bending the board a lot. I dont know all that well the LTFLU, but I could imagine that changing the current going through it is a bad thing.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf