Author Topic: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown  (Read 32823 times)

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Offline EEVblog

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EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« on: March 13, 2015, 02:38:59 am »
What's inside Keysight's new 34470A 7 1/2 digit Truevolt bench multimeter?
What are the PCB changes from the 34461A?
What voltage reference does it use to get 16ppm nominal accuracy?

34470A Datasheet:
http://literature.cdn.keysight.com/litweb/pdf/5991-1983EN.pdf

34470A Operating & Service Manual: http://literature.cdn.keysight.com/litweb/pdf/34460-90901.pdf

LTZ1000 Voltage Reference: http://cds.linear.com/docs/en/datasheet/1000afd.pdf

AD8638 Chopper Amplifier: http://www.analog.com/media/en/technical-documentation/data-sheets/AD8638_8639.pdf

 

Offline BravoV

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #1 on: March 13, 2015, 03:28:43 am »


For vref board, curious why using thru hole type of the jelly bean transistor 2N3904 ?  :o

I'm guessing its not just for saving few cents, right ?

Any particular reason why using thru hole instead of using smd like SOT type ? Does it has anything to do with temp compensation thingie ?



« Last Edit: March 13, 2015, 03:30:58 am by BravoV »
 

Offline EEVblog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #2 on: March 13, 2015, 03:33:50 am »
For vref board, curious why using thru hole type of the jelly bean transistor 2N3904 ?  :o
I'm guessing its not just for saving few cents, right ?
Any particular reason why using thru hole instead of using smd like SOT type ? Does it has anything to do with temp compensation thingie ?

I explained a possibility in the video.
 

Online Vgkid

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #3 on: March 13, 2015, 03:42:00 am »
Thanks for the teardown, now to wait for a review.
I believe you are correct about the 2n3904, it is even in the datasheet, though I'm curious about the lack of remaining precision resistors in the vref circuit. Though I'm not familliar with smd's.
If you own any North Hills Electronics gear, message me. L&N Fan
 

Offline gslick

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #4 on: March 13, 2015, 03:59:17 am »
In case anyone is curious, the listed replacement part price for the 34470-66303 voltage reference PCB is $344.

http://www.keysight.com/my/faces/partDetail.jspx?partNumber=34470-66303
 

Offline EEVblog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #5 on: March 13, 2015, 04:04:22 am »
I believe you are correct about the 2n3904, it is even in the datasheet

They mention that detail? Where?
 

Offline EEVblog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #6 on: March 13, 2015, 04:11:56 am »
In case anyone is curious, the listed replacement part price for the 34470-66303 voltage reference PCB is $344.
http://www.keysight.com/my/faces/partDetail.jspx?partNumber=34470-66303

Hang on...
The only physical difference between the 34470A and the 34465A seems to be this $344 retail board. But the price difference is $1500.
So they are charging $1156 retail for turning on the extra digit in the software and (technically) calibrating it to a higher standard? (but likely the same production cal jig)
I suspect Keysight have a lot of margin here to price crush any competition that might come along. I don't see why they couldn't potentially sell the 34470A for $1600 retail...
« Last Edit: March 13, 2015, 04:14:15 am by EEVblog »
 

Online Vgkid

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #7 on: March 13, 2015, 05:38:07 am »
I believe you are correct about the 2n3904, it is even in the datasheet

They mention that detail? Where?
not mentioned, but in the schematic
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Offline EEVblog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #8 on: March 13, 2015, 06:17:28 am »
not mentioned, but in the schematic

I thought you meant my guess about it being a TO-92 package to enable dissipation though system airflow?
That's not mentioned on the schematic.
 

Offline Dr. Frank

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #9 on: March 13, 2015, 06:49:42 am »
The 34470A is not the first 7 1/2 digit instrument (from HP / agilent).

Before that, the 34420A, nV/ µOhm meter already had 7 1/2 digit.
It is based on the elder Multislope III A/D, similar to the 34401A, and had slightly better linearity than this new DMM.

The 34420A  features DCV and Ohm only, and has an LM399H built inside.


The LTZ1000A reference is even stable to 8ppm/yr, as the 10V DCV range has 8ppm 24h uncertainty, and after one year 16ppm, therefore 8ppm drift.
It's a pity, that they did not improve that stability above the 3458A, by also running the LTZ1000A on 95°C. (15k/1k divider for oven)
They could have reduced that temperature to 75°, so that the elaborate selection process would have been easier, as that lower temperature would give stabilities of 2..4ppm/year very easily.
About the monitoring process, see HP journal 4 / 1989, for the 3458A.
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1989-04.pdf
 
Although the circuit is very similar to the one in the 3458A, it only features 1ppm/°C temperature stability, whereas the 3458A has 0.15ppm/°C. Maybe that's due to the usage of all SMD resistors for the oven and reference circuitry, (instead of leaded metal foil ones), which mainly influence the T.C.

The reason, why the 10V DCV range is the most stable on mostly all bench DMMs, is the fact, that this range relies on the stability of the reference plus one or two resistive amplifiers for the A/D conversion reference voltages of about +/-10 or +/- 12V. These two amplifier circuit will also be Autocal'ed for T.C.

All other DCV ranges have additional resistor dividers implied, which create these 5ppm/°C.
That is the 100:1 high voltage divider for 100, 1000V, and a x10, x100 amplifier for 1V, 100mV.
Latter one is also used on DCI, so creating an even higher T.C. as the current shunt resistors add up here.


The 10k VH102Z resistor is used as a real reference resistor, max. 0.8ppm/°C T.C. and (per datasheet) 2ppm/6 year stability.
The Autocal function transfers its uncertainty and stability to all other Ohm ranges, to some degree.

The datasheet of the 34470A therefore is very conservative regarding Ohm 1 year stability, although this reference resistor may  probably be much better than the one in the 3458A!

Frank
« Last Edit: March 13, 2015, 08:06:13 am by Dr. Frank »
 

Offline EEVblog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #10 on: March 13, 2015, 07:34:36 am »
The 34470A is not the first 7 1/2 digit instrument.
Before that, the 34420A, nV/ µOhm meter already had 7 1/2 digit.

Yes, but it's not a general purpose multimeter.

Quote
The LTZ1000A reference is even stable to 8ppm/yr, as the 10V DCV range has 8ppm 24h uncertainty, and after one year 16ppm, therefore 8ppm drift.
It's a pity, that they did not improve that stability above the 3458A

Why would they?
The 34470A is not aimed at the same metrology market as the 3458A. It is not a replacement for the  3458A.
 

Offline hikariuk

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #11 on: March 13, 2015, 08:08:19 am »
It looks like they were even thinking of using that pin header on the old model, if those holes you can see just to the right of the old voltage reference are anything to go by (they look a bit too big for vias and they seem to line up with the header).
I write software.  I'd far rather be doing something else.
 

Offline bundy

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #12 on: March 13, 2015, 08:45:07 am »
Funny how they build their reference. All refdesses are equal to the typ. application schematic. They put a '9' in the 'extra' parts, so e.g. R95 is in parallel to R5.

Once I've designed such LTZ1000 reference. Since the datasheet is very conservative I've asked LT for more info and they said just use a 1:1 copy of the original Jim Williams's design and layout and it will work. I got even some hand written notes about this reference... They could not tell me some specific tolerances of the LTZ (for worst-case designing) or if the 1013 dual opamp could be replaced with a more common type nowadays, etc.

So it is like Keystone got the same advice from LT as I did:)
 

Offline HighVoltage

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #13 on: March 13, 2015, 10:12:50 am »
Thanks for the great teardown. We have ordered one of these meters today.

As to removing the case, if you bring the handle upwards, you can remove the handle by pulling it out and the the case will slide off easily.



There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline Tothwolf

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #14 on: March 13, 2015, 12:05:40 pm »
'I_BOOT' is still present, they just moved the sinkscreen down next to the relay. Part of the second "O" is missing due to the guard groove.

The 2N3904 on that particular voltage reference board is a Fairchild part (the -C43 batch code gives it away, I've been using a lot of these recently).

One glaring question I have is why did they use one of Nichicon's cheapest series for the two large filter capacitors? VR series are general purpose 85C parts which aren't long life, low ESR, or anything. Couldn't they have at least gone with some 105C rated VZ series parts since those would last longer? (Capacitor life doubles for every 10C below its rated temperature.) Better still, a PM or PW series 105C long life part? For $3000 I would certainly not expect to see VR series capacitors in the thing. Heck, if Nichicon was still making VX series radials (which they only make in axial now) I guess Keysight probably would have probably would have even used those...
« Last Edit: March 13, 2015, 12:09:58 pm by Tothwolf »
 

Offline funkyant

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #15 on: March 13, 2015, 12:38:32 pm »
This might be a really stupid comment  :P

On the 24 hour specs, which you mention is pretty much useless unless you are using it in the factory, doesn't it say ACAL, which you can perform yourself?
 

Offline 6thimage

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #16 on: March 13, 2015, 02:26:54 pm »
This might be a really stupid comment  :P

On the 24 hour specs, which you mention is pretty much useless unless you are using it in the factory, doesn't it say ACAL, which you can perform yourself?

ACAL compensates for temperature not time - it is so you can take a multimeter calibrated at 23 deg C and run it in a 50 deg C environment (i.e. in a rack) without having to deal with large temperature-based errors or getting the cal lab to calibrate it at a higher temperature.
 

Offline funkyant

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #17 on: March 13, 2015, 02:30:52 pm »
Ok, so that 24 hour spec is drift in the first day from factory cal?
 

Offline Tek_TDS220

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #18 on: March 13, 2015, 02:52:14 pm »
Regarding the cutouts surrounding the LTZ1000 reference, you mentioned that the board might be a FR4 or a special low coefficient of thermal expansion material.  It is likely that the board is made from conventional epoxy.  In glass fiber boards (as these are), the CTE in the XY axes (in the plane) are dominated by the glass fibers that are oriented in the XY plane.  The use of low CTE thermosets in such boards has a significant reduction only in the Z-axis CTE.

Another important specification for PCB's is water absorption, which causes changes in AC and DC dielectric properties.  That might be a factor in a high accuracy meter like this one.
 

Offline 6thimage

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #19 on: March 13, 2015, 03:15:54 pm »
Ok, so that 24 hour spec is drift in the first day from factory cal?

Yes, that's correct.
 

Offline drago

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #20 on: March 13, 2015, 04:08:31 pm »
Would be interesting to see what Agilent competitor have put into Keithley DMM7510.
DMM7510 at Newark sells for $3,990.00 which is almost 1k more than 34470A.

Looking at the 10V specs it could be LTZ1000.

Datasheet
« Last Edit: March 13, 2015, 04:10:29 pm by drago »
 

Offline schopi68

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #21 on: March 13, 2015, 04:44:27 pm »
I am sure the cutouts on the boards are thermal "moats" for a thermal decoupling of the voltage reference. Otherwise every temperature change produced by airflow and the other parts on the board would change the temperature/temperature gradient on the the LTZ1000. So putting the 2N3904 behind a third thermal moat and outside of the air shielding case (and using a non smd device) makes sense too.
« Last Edit: March 13, 2015, 04:48:41 pm by schopi68 »
 

Offline saturation

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #22 on: March 13, 2015, 04:45:46 pm »
Yes, I would too.  There was a supposed to be a tear down in eevblog mid-Feb of one other Keithley model that has not yet been followed through.  There are not many DMMs [ or manufacturers, for that matter] that do >= 7.5 digit.   Given the long term history of reliability of Agilent, and this tear down, good as always by Dave, it  made me favor the Keysight model, not to mention its more attractive list price.

Would be interesting to see what Agilent competitor have put into Keithley DMM7510.
DMM7510 at Newark sells for $3,990.00 which is almost 1k more than 34470A.

Looking at the 10V specs it could be LTZ1000.

Datasheet
« Last Edit: March 13, 2015, 04:48:04 pm by saturation »
Best Wishes,

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Offline NANDBlog

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #23 on: March 13, 2015, 07:30:37 pm »
The 24H is useful if you have your own calibration equipment inhouse, just to know how often do you need to cal.
I was kinda disappointed looking at the refernce board. The old 3458A had proper vihay foil resistors, these look like (probably still very good) thin film. Also the capacitors...
Anyone noticed how the diode from the reference was also missing the diode in series with the heater? Or how Dave was showing the wrong page schematic for the negative reference?
 

Offline 6thimage

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Re: EEVblog #723 - Keysight 34470A 7.5 Digit Multimeter Teardown
« Reply #24 on: March 13, 2015, 09:59:22 pm »
Well I finally got around to watching the end of the video (it's great as always) - so this is aimed towards Dave.

The chip with the thermal pad and exposed copper (U606) is a TMP75 - an I2C temperature sensor from TI (Burr Brown) http://www.ti.com/lit/ds/symlink/tmp75.pdf. This is used by the multimeter to report its temperature (in the calibration settings).

With the Vishay resistor (R324 - in the metal can) having a very low temp co, it makes sense for them to be using it for the ACAL functionality - hence why it isn't present in the 34461A.

From what I've heard, they have also added a temperature sensor near the front panel connectors for cold point compensation (for thermocouples). As there isn't one on the measurement board, my guess is that it is on the front panel next to the connectors. I don't suppose there is any chance you fancy taking the front panel off, is there?
 


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