Author Topic: EEVBlog #426 - HP 3457A Multimeter Teardown  (Read 24619 times)

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Offline MetraCollector

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #1 on: February 19, 2013, 10:24:48 pm »
Thank you David ! :)
There is 3478A available 3468A, 3458A, I own 34401A, but I have never seen 3457A yet. :) (I mean photos of internals)
And finally today :)

Isn't it that Zener Oven identic to one used in 34401A ?  :-//

Regards Ondrej
« Last Edit: February 20, 2013, 11:06:18 am by MetraCollector »
 

Offline free_electron

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #2 on: February 19, 2013, 10:37:57 pm »
The optocouplers are for the high standoff voltage between the measuring section and the earth referenced logic.
there were optocouplers with long bodies but i guess this was easier to get ...

That's already a new cell. the originals are yellow. There's two versions of the processor board. you seem to have the older one.... could be a 2-into-1 machine... maybe someone fried the analog block and it got swapped.

the 18xx codes are simply internally HP part numbers. they are indeed off the shelf parts there is a PDF file with all cross references. look for the HP journal archives.

http://www.hparchive.com/hp_PARTS.htm
http://home.cogeco.ca/~jbarnes20/hp_ref.htm

That custom chip is the multislope logic. it's not an adc but only the logic portion. the integrator and jfets to switch the current sources around sit external. same principle used in the later model 34401.

they make + and -10 volts to derive the charge and discharge currents for the integrator. during runup they measure that voltage for a fixed amount of time and then rundown using the input voltage. they can assist rundown using the opposite voltage.
they don't care about absolute voltage or absolute resistors used to derive the current. as long as they don't drift they can compensate using software.

the same LT reference zener is still used in the 34401. basically an lm299.  Linear tech was second source for those

That large hybrid still exists in the 34401 but now sits in a ceramic PGA package. it is NOT the same like what fluke does. Fluke has resistor networks. the Agilent network actually has the switching fets inside. Opening that hybrid is a no-no. The little blob on the top is the hole they used to fill that thing... it's potted.

i can't find the detail right now but i seem to recall that the motherboard was used in another machine as well. The analog section was designed by another group. if you look at the pcb layout you can clearly see that these were done by a different group , even using different cad tools. the fonts , routing shapes etc are all completely different.
« Last Edit: February 19, 2013, 10:55:54 pm by free_electron »
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Offline poodyp

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #3 on: February 19, 2013, 10:41:50 pm »
Just an FYI, the 3457A by default comes with rear terminals. The ones with empty slots in the back likely had one of the plug in cards, but somewhere along the line it was removed, either by the seller to be sold separately for extra profit, or by someone who used it in another machine.

It also looks like your's is missing either it's feet or the rack mount lugs on the side, I'm not sure if there was an option for no feet.
 

Online Jay_Diddy_B

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #4 on: February 19, 2013, 11:11:14 pm »
Hi,

Linear Tech also makes the LM399.

Link: http://www.linear.com/product/LM399

This is probably what was used in the HP 3457A.

Jay_Diddy_B
 

Offline Watermelon

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #5 on: February 19, 2013, 11:52:00 pm »
What about the copper pours underneath most passives components? Is it for shielding purposes or maybe better heat distribution?
 

Offline BravoV

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #6 on: February 20, 2013, 03:12:56 am »


The cal backup battery, even measured at 3 volt, it can not be used even as the "ballpark" indicator for it's capacity, cause these low drain, long life Lithium battery voltage will stay flat and will drop like a rock, no smooth slope what so ever, just sudden death.

The one used at 3457A is lithium sulfur dioxide since it's at 3 volt, the typical discharge curve, can be seen the lower discharge current, the steeper the drop. For the cmos ram backup in this 3457A, is approx at 10 uA drain.
Source, Saft Batteries, well known for military grade battery maker , example -> http://www.saftbatteries.com/doc/Documents/primary/Cube657/G04_3_1005.ee086bbd-4c54-4f23-8e7a-d128295ddac5.pdf



Similar to above, another long life battery type, lithium thionyl chloride with typical 3.6 volt instead of 3 volt, the discharge curve is even steeper, they look vertical to me.  :o
Source, Hitachi Maxell Energy, another long time player in LTC battery -> http://biz.maxell.com/files_etc/9/er/en/ER18-50_DataSheet_12e.pdf



While there is other long life battery type called lithium carbon monofluoride or "BR" that has smoother slope, still steep though and just tad better than two types above, but BR battery technology is owned and manufactured only by Panasonic, and its very unlikely the Crompton Vidor company used this type


..
« Last Edit: February 20, 2013, 03:42:12 am by BravoV »
 

Offline uprightsquire

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #7 on: February 20, 2013, 03:23:52 am »
Dave, since you've upgraded from the 3478a maybe you could give replacing the backup battery in that a go... I'd be interested in seeing how replacing these hp backup cal batteries is best done
 

Offline BravoV

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #8 on: February 20, 2013, 03:28:09 am »
Dave, since you've upgraded from the 3478a maybe you could give replacing the backup battery in that a go... I'd be interested in seeing how replacing these hp backup cal batteries is best done

+1, its the preparation, the details during the replacement process will help others old HP dmm owners, imo definitely worth it's own video.

There are thousands of these old HP dmm owners out there.
« Last Edit: February 20, 2013, 03:34:51 am by BravoV »
 

Offline CarlG

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #9 on: February 20, 2013, 04:57:23 am »
The device with the ferrite block seems to be a 74HC132. One gate buffers the clock (from the oscillator to the left of the 'HC132), that's the only reason I see a need for the ferrite block.

I didn't find the 1820-3174 part number in the hp parts list link that free_electron provided but I found an NTE parts number equivalent.
 

Offline BravoV

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #10 on: February 20, 2013, 05:50:34 am »
Curious about that ferrite block too, isn't that thing is electrically conductive and will short those IC's pins ?  ???

Offline c4757p

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #11 on: February 20, 2013, 05:53:29 am »
No, those aren't conductive.
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Offline krish2487

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #12 on: February 20, 2013, 08:02:09 am »
Absolutely interesting video dave!!

HP were far ahead of times when they came up with the multi-run multi-slope converters..

I am attaching the pdf file of the HP journal 1989 where they published the theory behind the 3458A operation, for
anyone who is interested in the details behind.

Regarding free_electron's post about the custom chip and calibration.

I understand how the absolute accuracy and precision are not as important as the temp co drift..

Can anyone here enlighten me as to how the actual software calibration is or can be performed in software??

I have always wondered how do they store the calibration constants and how does the multimeter
take into account the cal values when making a measurement??

I am just trying to understand electrical metrology a little better.. thats all
 :-//


PS : Unable to attach a pdf file larger than 2 MB

Any ideas on where to post the file for file sharing
« Last Edit: February 20, 2013, 08:11:47 am by krish2487 »
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Offline pgross

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #13 on: February 20, 2013, 08:06:39 am »
Hi Dave,

Thanks for your teardown session - very interesting and informative - as always.  :-+

I've recently replaced the back up battery in my vintage hp 3478A. Even though the remaining voltage was exactly 3.0 V, you just never know
how long these batteries will last. I'm aware that the cal storage will be completely erased if the battery is removed from the instrument - there is
no capacitor in parallel to hold the charge during battery swap. Instead I exchanged the battery while the instrument was mains powered.

Just make sure that your solder iron is potential free and look out for Murphy as he will be watching you!

The operation went fine and I'll assume it is good for another 20 years - oh, I forgot to mention that I recapped the meter completely (only 4 caps
in total) with some brand new Panasonic FC caps - this was performed without mains on the instrument of course  :o

BTW. The new installed battery is this: http://www.fdk.com/battery/lithium_e/pdf/data_sheet/hcc_type/CR17335SE_spec-sheet.pdf

/ Peter
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Offline Kryoclasm

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #14 on: February 20, 2013, 08:23:27 am »
Dave,

Great video! I always learn a lot from everything you show us.

BTW was wondering if you planned on selling the 3478A? I'v been looking for one and I know yours works.  :-+

It would be a huge upgrade from my BK 389A.

Anyway, always fun and interesting learning from your vids.

Shawn
“I predict that very shortly the old-fashioned incandescent lamp, having a filament heated to brightness by the passage of electric current through it, will entirely disappear.” -Nikola Tesla
 

Online EEVblog

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #15 on: February 20, 2013, 08:45:10 am »
BTW was wondering if you planned on selling the 3478A? I'v been looking for one and I know yours works.  :-+

It's already on ebay:
http://www.ebay.com.au/itm/271158344204

I did some teardown photos of it:
http://www.flickr.com/photos/eevblog/sets/72157632806646675/
I thought I had done a teardown video on that, but it turns out I haven't.
Very similar to the 3457A in many ways, as you'd expect.

Dave.
 

Offline mikeselectricstuff

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #16 on: February 20, 2013, 09:18:08 am »
What about the copper pours underneath most passives components? Is it for shielding purposes or maybe better heat distribution?
Probably to keep both  ends at the same temperature - you have a junction of different metals at the solder joints, and if they are at different temps you will get a thermocouple effect, producing a small voltage across the part (microvolts or nanovolts, but significant on a 6.5 digit meter)
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Offline codeboy2k

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #17 on: February 20, 2013, 12:20:27 pm »
PS : Unable to attach a pdf file larger than 2 MB

Any ideas on where to post the file for file sharing

you can try fileslap.com, or filedropper.com

http://www.senduit.com/  can set an expiry time

pipebytes.com is good for one time use, person to person. The link is only valid once. Not good for this purpose, but I thought I would mention it.

 

Offline Lukas

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #18 on: February 20, 2013, 01:44:12 pm »
HP labs have all HP Journal issues: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/hpjindex.html
Apr-1989 is the issue with the 3458A.

The  HP journals are good reading on vacation or so, you never get bored  8)
 

Offline krish2487

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #19 on: February 20, 2013, 01:55:01 pm »
@Lukas

Thanks!!!

That was precisely the issue i wanted to share...
You saved me the trouble...

@codeboy2k
Thank you, I will keep that in mind, the next time I need to share something here.
Lukas's post kinda made it redundant to share the issue now...


My question still stands..
Can anyone here explain how does the software in the multimeter store the cal constants and how does it use those calibration constants in making a measurement?
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Offline andyturk

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #20 on: February 20, 2013, 03:35:38 pm »
HP labs have all HP Journal issues: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/hpjindex.html
Apr-1989 is the issue with the 3458A.

The  HP journals are good reading on vacation or so, you never get bored  8)
Wow, that's some awesome material. I remember when free_electron explained that multi-slope ADC stuff a while back. Most of it flew over my head. The journal article seems pretty easy to understand, even for a code monkey.

You can see why HP was a great company. They didn't view technology as dark magic hidden behind NDAs, but used it instead as a sales tool.
 

Offline free_electron

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #21 on: February 20, 2013, 04:35:50 pm »
The multislope system is basically a very precise 'time' meter. Every incoming parameter is translated into a voltage first.
The convertor translates this level into a time.

Base operation: you have an integrator using a capacitor and resistor and an opamp. If you apply a fixed voltage at the input the capacitor will charge linearly over time ( that's the opamp straightening the charge curve ).

The capacitor value is not accurately known. Neither is the resistor value, but we don't care.
The input of the comparator can be switched to four possible values: ground, input, a +10 volt reference and a -10 volt reference. The absolute value of the reference is unimportant. As long as it is temperature stable and does not drift. We will calibrate all that stuff away.

To calibrate this thing. : apply a known voltage at the input, lets make it easy: +10 volts.. Start the stopwatch and wait for a known amount of time.this is called runup.( count up) This is a matter of starting a digital timer with many many bits. The more bits, the more accurate you can measure. So you clock fast ,or you integrate long... (That's why in 61/2 digit mode the measurement takes longer than in 31/2 digit mode. They simply integrate longer to be able to work with larger numbers. Why? We'll see in a moment)

So, we have charged the capacitor for a known time with a known voltage. What the exact charge across it is? We don't give a rat's ass.
Now we switch the integrator input to ground and we start the stopwatch again. The capacitor now discharges. A precision comparator compares the output of the integrator with the ground level. When the capacitor voltage has reached 0 the comparator flips and signals the digital system to stop counting. Rundown is complete.

To make it clear : we have charged from a KNOWN source using a KNOWN time (a fixed number of clockticks) . we have discharged with a KNOWN source (GND) for a MEASURED time.( we have counted how long it took. ).

The number now obtained in conjunction with the runup value( which is a constant) is now used as a fraction. This fraction tells us what the integrator 'time' gain is. Boom. The whole integrator is calibrated. We dont care about resistor or cap. We have a number. ( number of ticks per volt. )

Now we will calibrate our 'own' sources. We again do a runup charging the capacitor from the external source , compensating for the first number. So now we do know the voltage across the capacitor!( it is a 'time' that directly relates to a voltage. Calibrated four our particular r and c in the integrator )
Now we switch the integrator to our internal 'to be assumed 10 volts' and we let the thing rundown again. We get another number. Apply fractional math and we know the exact value of the source. we now how far our source is off from the applied 'standard'
Switch over to our 'assumed -10 volt' repeat. Another number.

So now we know three things :
A first number which is a voltage/time relation for our particular integrator. We don't directly need this one but it was necessary for the other two.
We know how many volts per second we can charge from the 10 volt ref , and we know how many volts per second we can charge (discharge) using the second reference.

Calibration complete.

Apply unknown value at input. Runup from known internal source for a given time( the one we obtained during cal) , switch to unknown. Rundown until we hit zero. Make fraction : we know have the fraction of input vs/ known voltage as a fraction of two numbers.

The rest is a matter of calculating with very large numbers. The larger the numbers are the more digits behind the comma you get. The more digits behind the comma the more precise you can calculate and the more digits you can show on screen.

Since the whole hoopla is time driven this means more digits = longer time needed...

So the actual standard used in the machine is 'time'. Slap in a crystal oscillator... Grin. All problems solved. We don't care about absolute resistor or capacitor values. Its all calculated away.

Theses machines actually compensate their whole internal circuit path this way. They store a set of numbers for every range of every unit possible. So internal switch resistance, relay contacts, pcb traces ..  It's all compensated. Simply by 2 numbers that form a fraction.

Now, why do we need a negative 10 volt ?

Well if you measure a negative input voltage.. Duh ! Before runup they compare input voltage with ground and the precision comparator tells them if it is a positive or negative input ( above or below ground). So you switch to the other reference in that case for runup.

But. Agilent would not be Agilent if they stopped there.
Since we know how many volts per second these calibrated sources add or subtract from the integrator... We can shorten measurement time.

Runup is still a fixed time. No escaping that one. But rundown can be 'assisted' if this is taking too long. Lets pull some charge out by injecting some current from the known references. So the fraction becomes unknown volts/second from input +known volts per second from reference You don't need to be a math wiz to figure that one out.

That is the multislope ii algorithm. Multislope iii goes even further. They continuously switch currents around. The attempt is to keep the capacitor discharged. If no input is applied they switch +10 volt for a known time, then -10 volt for a known time. The comparator should toggle if the sources have not drifted ( so the machine can do a confidence test! Aha ! Didn't see that one coming did you? )

When an unknown voltage is applied it upsets this balance. So now the charge/discharge ration from the known sources needs to alter. So they still shuffle 'time around' but now as a fraction of the internal sources (which are again time) . That is the 'fast mode' in these machines.

There is some other trickery they can do. One of the hp bench briefs explains the whole shebang in detail better than i can.
One of the tricks involves a real ADC as we know it. If after a certain time the comparator has not toggled they digitize the remainder using that one. Its a 10 bit adc. Also used as a confidence test. If what that thing gives does not match , within reason, with the multislope output : there is a problem in the meter. The sources drifted, or it is damaged.
« Last Edit: February 20, 2013, 07:10:50 pm by free_electron »
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Offline Christe4nM

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #22 on: February 20, 2013, 05:20:13 pm »
I've got two questions regarding the PCB design in this unit, hopefully someone with more knowledge than I have can answer these:

1) It seems a lot of traces vary in width all the time. I suppose the smallest width is calculated as minimum for the current that could flow through. But why vary in width so much?

2) I've seen this in other (very recent) precision designs as well: instead of 'solid'copper pours as "ground"-planes they use a mesh. Here it's placed between quite wide traces. What is the reason to use a mesh structure instead of a solid reference plane? Does is have something to do with signal integrity, or the physics of current flow in a reference plane maybe?
(Maybe this should be asked in a separate topic, but for now it's directly related to the HP 3457 teardown)

 

Offline Christe4nM

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #23 on: February 20, 2013, 05:30:27 pm »

<another great post by free_electron>

One of the hp bench briefs explains the whole shebang in detail better than i can.

HP Journal regarding the 3458: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1989-04.pdf

Patents:
Multislope Converter http://www.google.com/patents/US4357600
Continuously integrating ADC: http://www.google.com/patents/US5117227
Calibration of ADC: http://www.google.de/patents/US6433713

Look at the patents cited and referencing to, and you'll find even more
 

Offline M0BSW

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Re: EEVBlog #426 - HP 3457A Multimeter Teardown
« Reply #24 on: February 20, 2013, 06:18:04 pm »
Lovely meter , just saw one on E-Bay USA £356.12 +£198.24 P&P   = £554.36 + custom charges, not such a bargain in the UK.
« Last Edit: February 20, 2013, 06:21:33 pm by M0BSW »
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