HP 3456A terminal block and wires - Page 1
Forum section: electronics / general

author: dietert1
author_rank: Super Contributor
timestamp: August 26, 2019, 07:56:33 am
content: Yesterday i unmounted the input terminal block of a 3456A and found the metal parts to be copper with gray plating, maybe zinc. That's good news. It means you can connect a bare copper wire to those terminals with low thermal EMF.I unsoldered the terminal wires inside in order to unmount the terminal block. The solder appeared to have an unusual high melting point. Hope i did not destroy something special. Now i have some doubts on those wires that lead to the front/back terminal switch inside. They appear very hard, not like copper. Anybody knows more details?On another HP 3456A, with a bare copper wire as a short, the residual voltage is -0,7 uV and rather stable against touch. The same connection, but without the front plate hiding the adjustment pots, gives -0,1 uV. Maybe it was adjusted to zero with that front plate open and the 0,6 uV shift could be from the wires i was asking about. Shouldn't i replace them by copper wires?Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: August 26, 2019, 11:52:52 am
content: As long as the offset is stable and known - can it not just be nulled out?

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: August 26, 2019, 03:28:50 pm
content: I think you are going to kill the golden goose.Copper was not expensive all during the time that the HP3456 was manufactured and certainly not at its price point. Why would HP have skimped?At least do some more research and analysis before forging ahead? Seems prudent.At any rate before any major surgery, massive de-oxidation is called for. If copper has a weakness it is corrosion. Why not start there? Isn't that much more likely than that its built incorrectly!?

author: dietert1
author_rank: Super Contributor
timestamp: August 26, 2019, 05:53:44 pm
content: Yes, i cut a piece of the wire and looking at it under the microscope the cuts appeared like copper with a grey/silver plating. Probably it was wishful thinking that i had found a reason for that 0,6 uV shift. A miracle that the insulation of those wires is stiff but not brittle after all those years.Meanwhile i put everything back together as it was in order to keep that 3456A shut and clean. I ordered some copper lugs and copper binding posts (audo parts) to do some more experiments. The nice gold plated Hirschmann banana plugs i have been using yield about 4 to 5 uV thermal EMF in the terminal block just by heating a plug between the fingers.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: October 27, 2019, 09:19:03 pm
content: The copper lugs i got are a good solution for low thermal EMF. Logs indicate that thermal voltages are now well below the 100 nV resolution of the instrument. But i still have that -0,6 uV offset.Now i've got another question regarding the HP 3456A. After reading a measurement over GPIB, i found that the next GET (group execute trigger) command will cause a timeout error, unless i insert a delay of about 50 to 100 msec. A delay of 10 or 20 msec does not solve the problem. The DVM is configured for single trigger and 100 PLC including Autozero, so i don't think this is about triggering into a running measurement.Couldn't find anything about arming or deadtime of the 3456A on the web. Is there a way to know when the 3456A is ready for the next trigger?Also i was wondering how i can use two 3456A in parallel (for redundancy). What may be the reason why the sense lines of 4-wire Ohms are not the usual voltage inputs? Very strange.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: October 28, 2019, 02:27:32 pm
content: I have been logging low voltages like 200 uV and PT1000 temperature sensors recently using a multiplexer and i wanted to hook our second 3456A in parallel. For measuring voltages it is simple, once the GET command works to trigger both DVMs at the same time. For the 4-wire resistance measurements i wanted to have the current source of one 3456A and get resistance readings on both of them.Except: The 4-wire sense lines are not the same as voltage input lines. So one needs another relay to toggle the second DVM connection.Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: October 28, 2019, 03:08:37 pm
content: Quote from: dietert1 on October 28, 2019, 02:27:32 pmCan you be sure the current is precisely the same on both meters - that the reading hasn't been calibrated to whatever current it provides itself?

author: dietert1
author_rank: Super Contributor
timestamp: October 28, 2019, 05:53:40 pm
content: Yes, agreed, there will be a constant correction factor that i can determine. The resistance measurements on the temperature sensors are not that critical.What i'm after is redundancy. I mean when i am running tests on the brink of what is possible with these DVMs, there could be a second independent set of data to check whether it yields similar conclusions.Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: October 28, 2019, 06:54:24 pm
content: I use the same principle. The more meters that agree simultaneuosly, the more confident I feel that the fundamental error I am overlooking is somewhere else! :-)

author: dietert1
author_rank: Super Contributor
timestamp: October 28, 2019, 08:54:42 pm
content: No, i don't think my measurement has a fundamental flaw and i won't use multiple multimeters. My measurements are near the noise limit and maybe a second measurement can help distinguish between noise of the DVM and noise in other parts of the setup.And yes, i know i will have to check other parts by redundancy, too. That's exactly what i am trying now. I tracked a resistance change of 1.1 ppm over more than a week and now i am trying to check linearity of the device, so this time i need to track a resistance change of only 0.22 ppm over a week. Maybe i need to repeat this as soon as i get the second set of data. Then there will be a threefold redundancy and the result will be rather safe.By the way, those Fluke 8502A and 8505A meters seem to have the voltage input on the same terminals as the 4-wire ohms sense lines, somewhat more intuitive than the 3456A.Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: October 28, 2019, 10:55:27 pm
content: Quote from: dietert1 on October 28, 2019, 08:54:42 pmAre you averaging over a very high number of PLCs for this?I do find making accurate measurements at the ppm level extremely challenging. I recently got hold of a ratio transformer... as an exercise, I decided to check the accuracy of a KVD with it. As you probably already know, I soon found that even registering a change of 1ppm accurately takes a lot of thinking things through...everythingmatters at this level, it seems. So your attempt to perform measurements in the order of 0.2ppm is extremely impressive to me!Quote from: dietert1Ah, I did not know that about the 3456A, I have never used one. Seems it is just a matter of whether the 4 wire mode means adding extra leads for the current, or extra leads for the sensing!

author: dietert1
author_rank: Super Contributor
timestamp: October 29, 2019, 11:05:54 am
content: My setup looks like this. It's a resistance bridge fully instrumented with sense wires and temperature sensors. The reference resistor is a SR1010-1K with desiccant in it and boxed for thermal isolation. The MUX is made from bistable relays and rests inside the SR1010 box, again well protected from rapid temperature changes. All wiring is with copper and copper lugs. As Mr. Pettis recommended i have the DUT roughly at the intended operation voltage (3.3 V). Resolution of the HP3456A is 0,1 uV, so noise limit will be about 0,03 ppm of resistance. With additional noise filtering at a time scale of 20 minutes i have seen 0,01 ppm of noise.There is still room for improvement, for example the oven shows crosstalk from ambient temperature. One suspect is the Arroyo 5305 sensor connection, that is close to the fan exhaust and may suffer from thermal EMF.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: November 04, 2019, 03:47:16 pm
content: e61_phil proposed to reverse polarity in order to check for thermal voltages in the setup. Meanwhile i did that and got a 1.4 ppm shift on the resistance measurement when reversing the input voltage (10 V Geller reference). When i also reverse the output side of my setup (connection to HP 3456A, the difference is 0.30 ppm. It's difficult to tell whether the DUT has some diode type asymmetry or thermal voltages are involved.To research further, i disconnected the reference. Then the temperatures in the DUT oven and in the SR1010 box reduced a little due to turning off the self heating by the bridge currents. Without bridge voltage, the sense lines should indicate the thermals generated in the multiplexer and the SR1010. That 3456A is known to have an offset of -0,6 uV with +/- 100 nV variations and the multiplexer "Null" measurement confirms this.The measurement shows rather stable 250 nV on the lower "power wire" from the SR1010 to the oven. The upper "power wire" is no different from the null measurement. The bridge voltage has about 220 nV offset. So there are thermals.Meanwhile i opened another SR1010 and found gold plated steel binding posts with gold plated steel nuts and copper parts inside. This construction generates up to 10 uV thermal voltage on a copper lug when heating the binding post between the fingers! Will get some copper parts to replace the binding posts.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: March 06, 2020, 07:07:38 pm
content: Today i learned how to adjust the null reading of a HP 3456A. Specs on this offset are less than +/- 2.4 uV. The HP 3456A i am currently fixing indicated -2.4 uV.Three steps:1. First check the ADC autozero works by connecting its input to ground using jumper J29. My instrument indicates a stable 0.2. Install a low thermal short on the voltage input. Turn off instrument Autozero on the front panel. Then use R334 to adjust balance of the input amplifier.3. Turn on instrument autozero on the front panel again, close the instrument covers and note the offset after one hour of warm-up. -2.4 uV in my case.To adjust the offset, use the voltage divider made of R149 and R150 near the Autozero FET switch Q104 to create a small offset voltage. Manual says: "Factory installed" for R150. But my instrument did not have R150. After thinking a little about it i put 2x 3M3 = 6M6 to -13V. This should generate about 2 uV.Then check after another hour with covers on. Currently my instrument displays +0.2 uV. Will leave it like that.This procedure is applicable only with small offsets. A larger offset indicates that FET switches in the input section may be faulty.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: April 12, 2020, 09:08:30 pm
content: Today i found a minor lapse in the second generation HP3456A. They discovered they can run the DVM without fan if they mount rectifier and voltage regulator on the back of the instrument (large aluminum sheet). In that instrument the AC wires for the out-guard 5 V supply are routed directly from transformer to rectifier. Those AC signals never arrive on board A3 and noise filter capacitors C7 and C8 remain unused.Cure: Solder two 100 nF caps directly onto the rectifier.Regards, Dieter

author: SilverSolder
author_rank: Super Contributor
timestamp: April 12, 2020, 11:39:11 pm
content: Quote from: dietert1 on April 12, 2020, 09:08:30 pmOops!

author: dietert1
author_rank: Super Contributor
timestamp: April 22, 2020, 08:24:19 pm
content: Recently i extended one of our HP 3456A, following the descriptions of cellularmitosis and TimInCanada. The instrument is one without fan and it got a 3D printed cover over its reference module. No other mods. My extension has two functions:a) Operate a SHT35 sensor inside the HP3456A guarded sectionb) Capture the extended resolution raw data from the isolation logicIt uses one each LP2950-3.3, MSP430F2616, TC4050, 74HC74 and ICL3232 and sends results via an additional RS232 port. The small MSP430 board was something we had. CMOS is used as level converter and to implement a CS* signal to make the HP 3456A isolation interface appear as a SPI master. The SHT35 has an I2C interface. Power consumption of the whole extension is about 10 or 20 mW depending on the RS232 TX load. For RS232 output the outguard power supply was extended to deliver -11 V. Also used to fine-tune +5 V voltage regulator down from 5.1 V to 5.00 V to reduce heat inside instrument.This is a working prototype, nothing more. Until now the extension is unobtrusive - the instrument remains a standard 3456A. Later i would like to add relay MUX control (e.g. automate front/back switch).The interface receives a request packet that triggers the SHT35 sensor measurement and prepares collection of the extended resolution DVM data from the isolation interface. A second packet from the host triggers readout of all data to the host in one packet of up to 58 Bytes of data: Temperature (float), humidity (float) and up to 10x extended resolution readings of 5 bytes each (0xF1 plus 8 BCD digits).2020-04-19 TestTo get a measure of noise i logged a 6,463 V voltage from a battery pack with four alkaline batteries. The battery pack was inside a plastic bag to keep air movement away. The SHT35 sensor was inside the same bag. A measurement was taken every 10 seconds. It consisted of two 100 PLC measurements of the DVM with autozero. Two measurements result in 20 extended resolution readings. Calculated the averages of all 20 readings and medians from from the inner two quartils (10 readings per measurement). Those agree very well. The log exhibits the temperature gradient of the battery pack. Then i focussed on a part of the log where temperature was almost constant and fitted a sixth order polynomial to the averages. That results in a standard deviation of 0,422 uV for the residuals over about 3000 seconds. Almost a factor 25 better than native resolution of the HP 3456A (10 uV in its +/- 12 V DC range). A nice improvement. Thanks again to the contributors mentioned above. Will draw a board to upgrade our other HP3456s as well.After mounting the SHT35 sensor inside the guarded section, closing the HP 3456A and running it for a day, temperature inside is about 9 °C above ambient temperature with relative humidity less than ambient. Heat-up took about one hour.Regards, Dieter