Newbie Precision Reference Calibrated with the 3478A Dinosaur - Page 1
Forum section: electronics / projects

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 10:26:43 am
content: Hello,I have just finished the Precision Voltage reference I was working on. ...Finally!A used HP 3478A that I scored from ebay for $99 arrived just in time for calibrating my reference.The ebay seller was kind enough to calibrate the voltage range with a voltage standard he had.Resistance measurements look pretty accurate. I cannot say the same for its current range thoughand I don't like the lack of uA range.The multimeter arrived in a relatively good condition apart from some dust that has settled on theboard permanently... I really like this multimeter! A joy to use. The display has a nice contrast,but it lacks a back light. A definite future mod. The on board battery reads 3.042V which is verylow. I would have to replace it very very soon.Some photos:Its has a Rev. C board.The reference is based on the MAX6325 chip with an output of 2.5VVery simple circuit with a chopper amp and two precision resistors.You can calibrate multimeter voltage range with the 2.5V, resistance with 1K & 100Kand current with 2.5mA & 25uA.Here it is:(I will have to agree with Dave on that. Just look where they did print their serial number...!I chose these test points because I think that there are much better than the U shaped ones.The top is a cone and the multimeter leads have a larger area of contact and at the same timethey don't slip.The circuit also has a low battery indicator which I think is a must for a portable voltagereference in order to be 100% sure for its accuracy. The green LED starts to fade when thebattery reaches 8.1V and its completely off at 7.9VThe trimmer pot is a Bourns. It is pretty good but if anybody know a better quality one pleaselet me know.I have been running it for 3 days now and it has finally stabilized at 2.50005VThe last digit is a 5 because I prefer the voltage to be on the positive side.I don't like .99999s...I think its accurate enough to calibrate my UT61E or similar multimeters.Now I would have to see its long term stability.The UT61E was 3mV out and the UT136C was 5mV out. Although the resistance measurementof the UT1236C is painfully slow, it was spot on with the 1K and a couple of counts out with the 100K.Its current measurement was spot on! 2.50 mA & 25.0 uA!The leads you see in the picture above are a set of ProbeMaster leads that I saw in a Mailbagepisode. They are simply perfect. Thank you Dave !! (and the kind person who sent them)Thank you for helping me finish another little project.Regards,George.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 02, 2014, 11:59:24 am
content: I've got the same meter. I agree, the lack of a backlight is a definite drawback. I also have a benchtop Fluke 4 1/2 digit that I use more often because the vacuum florescent display is more pleasant to read.Beware, I have read that you have to supply a temporary backup voltage if you change the onboard battery or you will lose the cal settings.

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 12:06:57 pm
content: Indeed, I've heard. I will replace the battery with the power on. My soldering iron is not groundreferenced. I will also include a backup battery just in case... One user left the following commentin a youtube video which I don't understand:"+GBPPR2 I don't think you need to worry about that SRAM battery getting disconnected,according to the manual (Agilent still supports this). I'd suggest putting a big cap in parallelwith that or'ing diode since removing the battery could cause a rail bounce".What exactly is a rail bounce??

author: Andreas
author_rank: Super Contributor
timestamp: May 02, 2014, 12:20:45 pm
content: Quote from: hgg on May 02, 2014, 10:26:43 amHello,Perhaps you like to choose one of the AccuTrim series of vishay?http://www.digikey.de/product-search/de?v=804&pv1=112&FV=fff40004%2Cfff80338&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25With best regardsAndreas

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 02, 2014, 12:26:26 pm
content: Rail bounce is just a term for a transient created on the rail due to a high momentary current that flows due to capitances etc. The track inductance and parasitic capacitances can cause an oscillation.

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 12:33:52 pm
content: @Richard Head: So, removing the backup battery can cause a negative or a positive transient?@Andreas: The Bourns trimmer I used has a 100ppm/C while the one you just suggested has 10ppm/CNot bad. An order of magnitude better! Andreas thanks again for your suggestion and your help with myquestions.

author: Andreas
author_rank: Super Contributor
timestamp: May 02, 2014, 12:53:46 pm
content: Quote from: hgg on May 02, 2014, 12:33:52 pmHello,The price is also at least an order of magnitude better.The question is if you will really need this.If you use the trimming pin of the reference the influence of the trimmer TCis already scaled down by at least a factor of 100 or even higher."The external trimming does not affect tem-perature stability."On the other side. My MAX6350 both have a relative constant ageing drift (at controlled temperature) of about 15 ppm / year.So it would be interesting how your MAX6325A behaves.With best regardsAndreas

author: rdl
author_rank: Super Contributor
timestamp: May 02, 2014, 12:56:50 pm
content: Wow, those AccuTrim resistors from Vishay are very nice."smooth and unidirectional output"15ppm/C for $10-$12Quote from: Andreas on May 02, 2014, 12:20:45 pm

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 01:18:27 pm
content: QuoteDo you mean does not affect at all, or that the amount is very small?Thank you.

author: Andreas
author_rank: Super Contributor
timestamp: May 02, 2014, 01:33:02 pm
content: Hello,the citate is from the datasheet.I think that you / one will not be able to measure it.with best regardsAndreas

author: hgg
author_rank: Frequent Contributor
timestamp: May 02, 2014, 01:36:56 pm
content: Great! Thanks again.

author: hgg
author_rank: Frequent Contributor
timestamp: May 19, 2014, 05:38:15 am
content: Just an update.The 2.5V reference is rock steady. 17 days have passed and its still at 2.50005 Volts.It might be more accurate than 5 digits but I cannot measure that.I have replaced the old battery of the 3478A and luckily I did not loose the calibration data... :-)The battery was the following:http://www.antoniadis.com.gr/product_info.php?products_id=1097which fitted perfectly.When the reference was ready, I calibrated both the UT61E and the UT136C.The 61E has a very fine adjustment pot but the 136C has a single turn adjustment pot.Despite that, after 17 days the 136C reads 2.500V but the 61E now reads 2.499V ...So the 61E indeed has a drift problem !The UT136C although is slow as a donkey, proved to be very accurate and stable.(Quite accurate and repeatable current measurements especially with Frankie's leadsand some very low resistance fuses.)

author: cellularmitosis
author_rank: Unknown
timestamp: May 19, 2014, 06:07:09 am
content: Quote from: Richard Head on May 02, 2014, 11:59:24 amI posted my backlight mod for the 3478a at the end of this thread:https://www.eevblog.com/forum/projects/hp-3457a-lcd-backlight-mod-(also-for-3478a)/I used EL tape, which turned out to be only just bright enough, and unfortunately the controller makes an annoying audible whine.Soon I think I'll add a backlight kill switch so I don't wear out the EL tape while using the meter to log data over extended time periods.

author: hgg
author_rank: Frequent Contributor
timestamp: May 19, 2014, 06:18:12 am
content: Hi,I've seen your mod. Not bad actually. Brightness level for my taste is perfect.Is that a transparent el-tape?

author: Dunk_c
author_rank: Contributor
timestamp: June 26, 2014, 07:55:12 pm
content: HiIs this beautifully crafted reference available for purchase?

author: hgg
author_rank: Frequent Contributor
timestamp: June 28, 2014, 11:28:37 am
content: Hi,Its difficult for me to find the parts again, but I can send you one of theprinted circuit boards and the terminals if you like.Thanks.

author: Dunk_c
author_rank: Contributor
timestamp: June 28, 2014, 08:35:12 pm
content: HiI am a beginner and have just purchased a max6325cpa and some 100ohm and 5kohm resistors (0.05%, TCR 5-10ppm) to make a simple and cheap check for my meters. Until I saw your beautifully designed board I was going to simply join the chip point-to-point, and put the whole lot in a box with some 2 mm banana plug sockets for my probes. I haven't made it yet. What are the hard to find parts?

author: hgg
author_rank: Frequent Contributor
timestamp: July 02, 2014, 02:26:27 pm
content: Hi,Sorry for the delayed reply. I don't have an internet connection here...Anyway, what I meant was that it was not easy for me to find some of these parts here in Greece,like the precision resistors for example, so if you have all the components I can send you a pcb ifyou like.Regards,George.

author: Dunk_c
author_rank: Contributor
timestamp: July 02, 2014, 09:35:02 pm
content: Hi GeorgePlease email me with purchase details and a component list. Much apprecaited.duncan

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 07:51:13 am
content: Just an update,I've checked again the reference with the 3478A and after 5 months its still showing 2.50005VEither its very stable or both the reference and the 3478A are drifting together.26-10-2014I had though a problem with the 3478A. When I received it I had to change the transformer tap to 240V.It also had a 220V option but here we have 230VAC so I chose the 240V.In the service manual it says that for option 345 which is 240V/50Hz I have to use an 125mAT fuse.I used a Schurter F125mA/250V and it was working fine until today. Now it blowed all the 125mA fusesI've tried. Do you think its because I was using the fast type and not the time delay type?Now I am using a 250mA.

author: tautech
author_rank: Super Contributor
timestamp: October 26, 2014, 08:03:27 am
content: QuoteMost likely.Unless a fast blow fuse is specified I would use a STD fuse for mains.Has it a Linear or SMPS power supply?Smoothing caps on PSU could cause the inrush.

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 08:08:55 am
content: It has a linear power supply and the specified fuse for 230V is a 125mAT,so I assume the T stands for Time Delay?

author: SeanB
author_rank: Super Contributor
timestamp: October 26, 2014, 08:34:58 am
content: Fast fuse will blow, that is why they use a slow blow one, otherwise the fast fuse has to be very overrated to handle the surge within it's time envelope. Buy a pack of slow blow ones and use them, if it does blow them then you might have either high mains, a faulty mains switch or power outlet, or a faulty mains cable or finally a faulty meter.

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 08:42:54 am
content: I will try some slow blow ones and see what happens.Thanks!

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 10:24:16 am
content: I am not sure that Utilboard can export to other programs but here are the schematics of the circuit:vRef2.5VI can send you the Gerber files if you are interested.

author: daqq
author_rank: Super Contributor
timestamp: October 26, 2014, 10:53:03 am
content: Hi,Great device! But how exactly do can you calibrate the 25 uA/ 2.5mA ranges with this?Also, the 1uF cap (C3) directly on the op amp output could lead to some nasty stuff.David

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 11:30:13 am
content: Hi,The current reference was not designed to be calibrated... but if you can accurately measure its output,then you can use the measurement for calibration. It is dependent on the absolute values of the resistors.The UT136C with its voltage range calibrated shows 2.50mA & 25.0uA. UT61E with its high burden voltageshows 2.472mA & 24.78uA...QuoteLike what? Can you please explain?

author: daqq
author_rank: Super Contributor
timestamp: October 26, 2014, 11:51:36 am
content: Short version: It adds a direct capacitive load to the op amp output, which it might not like and start oscillating - essentially, going into constant overshoot, desperately trying its best to regulate the output. It might not happen and it might not apply, but it might. You probably wouldn't see it on the output on the HP, but you might see it on a scope.Long version:http://www.analog.com/library/analogdialogue/archives/31-2/appleng.html

author: daqq
author_rank: Super Contributor
timestamp: October 26, 2014, 11:57:02 am
content: If you have LTSpice, run attached simulation. Look at the output voltage.

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 12:03:54 pm
content: Thats exactly what happened when I did not use it...From my little experience, Chopper Amps are a bit picky and when I was powering the circuitfrom my power supply that was connected to the mains, I had some problems with it whichdisappeared when I used the capacitor. There is no problem with the battery and if you lookthe output voltage on the oscilloscope (ac coupled) it is a very steady flat line down to 2mVwhich is the limit of the scope.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: October 26, 2014, 05:40:34 pm
content: Quote from: daqq on October 26, 2014, 11:57:02 amHi George .I remember you had a thread about this opamp 'ltc1250' . It's not very good as a buffer amp because it's got very little phase margin at unity gain,( it can't source much current either about 5mA? (bet the ref can source that). It's got About ~ 10 deg Ph margin, so it's not very stable at a gain of 1, even without any capacitive load. If you want to use an opamp like this at a gain of 1 with a large capacitive load then we have to isolate the output of opamp from the CAP with a resistor say~ 20 - 100. Then use a resistor and smallish cap in the feedback to lower the bandwidth and so set a decent phase/stability margin. If your having probs (it WILL oscillate with a load cap that big on that opamp, your's might only just about be stable due to a high esr cap). To ensure stability try something like a 20R on the output and 1k and 47n in feedback like below .( Vi,Ii are just probes)

author: hgg
author_rank: Frequent Contributor
timestamp: October 26, 2014, 06:15:20 pm
content: Hello Kevin,I had some difficulties finding a chopper amp that could easily source more than 2mAand have a low drift as well. The LTC1250 can output full swing into 1k load and caneasily drive the 2.5mA that I needed. The output voltage is pretty stable as it is rightnow, but I will try your suggestion as well. I remember that it was stable with a muchsmaller capacitor as well.Maybe I was lucky but the current circuit is rock solid with repeatable and accuratemeasurements down to 5 significant digits after 5 months.(always at the same ambient temperature though...)

author: Andreas
author_rank: Super Contributor
timestamp: October 26, 2014, 08:59:33 pm
content: Quote from: hgg on October 26, 2014, 06:15:20 pmYou could try the LTC2057.Quote from: hgg on October 26, 2014, 07:51:13 amThe MAX6325 is a buried zener reference -> is relative stable over time.I have here similar (5V) references (MAX6350) from the same series which have drifted about 25-30 ppm over 800 days.(the blue line is the drift of my "instrument" against a LTZ1000A reference).So with your instrument resolution of 4 ppm for the 2.5V you will need some further time to recognize the drift.BTW: which reference is used in the HP3478A?In the service manual it seems to be a LM399.So if its well pre aged the drift of the instruments reference should be even lower.(buried zener within hermetically sealed package).With best regardsAndreas

author: Kevin.D
author_rank: Frequent Contributor
timestamp: October 26, 2014, 09:26:55 pm
content: Quote from: hgg on October 26, 2014, 06:15:20 pmWithout the extra compensation it looks to be stable loop as long as the esr of the load cap > ~ .5 Ohm . So 1 uF electrolytic should be stable (esr 2-8ish) .But change it for a low esr ceramic and it will start.

author: hgg
author_rank: Frequent Contributor
timestamp: October 27, 2014, 07:46:44 am
content: Hello Andreas,Nice graph. You tracked the drift pretty nice. The red graph is interesting because I read thatthe ageing of the zeners is unidirectional, meaning that if it starts lets say positive, it will staypositive. In your graph although it starts negative it then goes positive for some reason.The HP3478A is well pre aged but I don't remember which reference it has. I would have to check.Maybe I will also check my reference once a month and make a crude graph.Kevin all three capacitors used on the reference are 1uF tantalums which have an ESR of around 1 Ohms,so maybe thats why its stable.

author: Andreas
author_rank: Super Contributor
timestamp: October 27, 2014, 08:04:21 pm
content: Quote from: hgg on October 27, 2014, 07:46:44 amHello,dont forget that this may only be valid for the bare zener. (usually 7V references).A monolithic 2.5 or 5V reference with plastic housing has several ageing effects:- the zener (+ the current source).- the resistor divider which may oxidize due to humidity- the (plastic) housing (or the epoxy pcb) which swells with humidity and creates mechanical stress to the chipon some chips you can see this as seasonal changes (autumn / fall)- and finally the die attach which is used to cement the chip to the lead frame.....with best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: October 27, 2014, 09:00:00 pm
content: Hi,I want to show yo guys a way of not loading your chopper opamp.Look @ the schematic, it uses various techniques to kill noise and to be shure not to load the opamp.The J310 HF Fet is cheap. and most of the time it can source 8 a 10 mA.The opamp sees no load and you wil get a better output commonmode, dropout is about 0,2V.There a to techniques in this schematic to kill noise, he first one is the "bootstrap" capacitors C1 and C2, its a low leakage currentlowpass filter of about 3,5Hz.There is almost no leakage current through C1 because the voltage over C1 is very low(serveral mV).The second filter is C3 and C4 and R8.This is just an schematic i'am testing at the moment to see how much noise is filtered.The LM10 is not good enough to be ussed as a good Reference circuit.But... Use for the 5V section a LT1021, Max6126, LTC6655, and for the second stage a LTC2057, LTC1250 and you wil have a super reference :-)Happy building!Kind Regarts,Blackdog

author: hgg
author_rank: Frequent Contributor
timestamp: October 28, 2014, 10:03:51 am
content: @Andreas,So, its not so simple...Thank you.@Blackdog,Thank you for the schematic. I like the JFET solution but I have some questions.Have you built the circuit? Why all this filtering? What is the source of the noise?Have you measured the noise you are trying to filter?When you say "dropout is about 0.2V", dropout voltage of which component?Do you fine tune the unity gain of the op-amp with the P1?Is the long term stability of the reference now also dependent on resistors R10 & R5?Regards,George.

author: blackdog
author_rank: Frequent Contributor
timestamp: October 28, 2014, 12:33:57 pm
content: Hi hgg,A fast reply, buissy... ;-)Yes i build it, its testing now, for temperature stability, and no, the LM10 is not a good Reference.Why, the reference voltage is multiplied by 50 times!.I made this, to do measurements on the noise behavior, and still have to do this measurements, but no time, work, work, work..To pictures, the first one is the "stability" in 16,5 hour, the upper meter is the 5V output, the lower meter is the 10V output.The temperature shift is about 10C, lower voltage is colder, Temp coefficentie is about 0,5mV/C @ 10V DC output.And this is the box and the test circuit.Later more...Kind regarts,Blackdog