HP 34401A DMM settling time comparison / questions - Fixed - Page 1
Forum section: electronics / testgear

author: radioFlash
author_rank: Regular Contributor
timestamp: February 14, 2015, 09:11:03 pm
content: I have two 34401A units I got at separate times, and both pass all self tests without any issues. However, I noticed that one of the units doesn't seem to settle in a reasonable amount of time from power on, while the other is pretty stable from within a few minutes from turn on.I used the DMMCheck 5V as reference. Meter A is physically stacked on top of Meter B. Both meters are connected to the same reference.Below is a chart of the data from the two meters. The data actually starts several minutes after both were powered on after being turned off the night before.Is there a fault of some sort in meter A?EDIT:March 3, 2015:* I checked the power supplies and outputs were in spec and I found no oscillations. It turned out that U400, an Analog Devices AD706 op-amp was defective. Based on Dr. Frank's suggestion, I replaced the op-amp and it's no longer drifting like it was. Thanks all!

author: Andreas
author_rank: Super Contributor
timestamp: February 14, 2015, 09:46:20 pm
content: Quote from: radioFlash on February 14, 2015, 09:11:03 pmYes, as you can see in the 100uVpp noise.with best regardsAndreas

author: HighVoltage
author_rank: Super Contributor
timestamp: February 14, 2015, 09:56:30 pm
content: Yes, something is wrong with that meterI have an older 34401A from 1994 and another one from 2014 and both of them settle really fast within a few minutes.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 14, 2015, 10:37:01 pm
content: I haven't heard mention of a fault like this before--is it likely to be a failure of the heater on the LM399 voltage refrence, or something else?

author: dom0
author_rank: Super Contributor
timestamp: February 14, 2015, 10:48:03 pm
content: Always check the supply first. Excess noise, drift and so on can be caused by a faulty (ripple to high, bad regulation, improper voltages) supply.

author: DJ
author_rank: Unknown
timestamp: February 15, 2015, 05:17:02 am
content: That's a long time constant. Doesn't look thermal.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 15, 2015, 05:28:02 pm
content: I measured the voltages from the voltage regulators. Using the multimeter (a BM869) in DC+AC mode, I didn't see any significant ripple and the voltages seem to be in spec. No visible damage to any components that I can see.

author: Andreas
author_rank: Super Contributor
timestamp: February 15, 2015, 06:12:42 pm
content: Quote from: radioFlash on February 15, 2015, 05:28:02 pmsorry but you have to think twice:6.94 V at reference output and 7.0695V at buffered output of U400 do not fit together.Either U400 has 130mV offset on one of the Op-Amps (unlikely) or there is something oscillating in the cirquit.If U400 is working perfect all + and - inputs should have the same voltage.Edit: and also the -15V seem to have a problem:It is generated by a -18.2V regulated voltage and a 3.3V Zener.So I would expect more -14.9 V than -14.5V except when the current through the zener is very high.But on the other side the voltage drop at the +15V seems to be similar.With best regardsAndreas

author: HighVoltage
author_rank: Super Contributor
timestamp: February 15, 2015, 09:58:01 pm
content: Have you looked at the noise ripple at those DC rails.I had once a failing LM317 in a 34401A and it injected noise to the +18V rail.Replacing the LM317 and the attached tantalum capacitor solved that problem.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 15, 2015, 10:26:06 pm
content: What's the best way to hookup a scope to check the rails for ripple which aren't earth ground referenced? If I connect the scope ground to the rail's ground, I would expect a possible problem.

author: dom0
author_rank: Super Contributor
timestamp: February 15, 2015, 10:40:46 pm
content: The analog part of the DMM is floating.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 17, 2015, 01:14:11 am
content: Since the analog power supply is floating, I'll need a differential probe to check for ripple, right?

author: joeqsmith
author_rank: Super Contributor
timestamp: February 17, 2015, 03:49:54 am
content: Quote from: radioFlash on February 14, 2015, 09:11:03 pmNice plot. How are you collecting the data?What happened at the center of the data? Looks like they both hung for a while.Are you sure you have both meters setup the same way?

author: dom0
author_rank: Super Contributor
timestamp: February 17, 2015, 10:37:09 am
content: Quote from: radioFlash on February 17, 2015, 01:14:11 amNope, it's all floating, just connect your probe. But don't connect anything to the inputs of the DMM, ya?

author: nctnico
author_rank: Super Contributor
timestamp: February 17, 2015, 01:31:39 pm
content: Quote from: DJ on February 15, 2015, 05:17:02 amUnless the reference heater isn't working.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 18, 2015, 01:19:48 am
content: I used Benchvue to capture the data. The glitch in the middle is because Benchvue has a 1 hour capture limit and I had to manually restart the capture. I've also been having flakiness problems with Benchvue and my serial connections. Benchvue often has problems starting captures and times out, but once the capture is going, it doesn't have problems. I've tried 3 different kinds of usb-serial adapters (with FTDI, PL2303, and some other kind of chipset I can't remember) and two different USB hubs, but still the same problem.

author: LaurentR
author_rank: Frequent Contributor
timestamp: February 18, 2015, 01:28:07 am
content: Quote from: radioFlash on February 18, 2015, 01:19:48 amTo go around the 1hr limit of BenchVue, use BenchVue's predecessorDMM Connectivity Utility. It shows one fewer digit on the 34401Abut the graphs still have all the precision. The utility also has histogramming, which is not in BenchVue's basic version.As for the RS232 issues, unfortunately, it's a general problem due to the 34401A's old school flow control. There are some discussions about it here:https://www.eevblog.com/forum/testgear/34401a-through-rs232-with-matlab/I built a cable with the flow control rewired. If I use Matlab first to set the serial port in the right mode, then the Agilent software (either version), it's 100% success. If I launch the Agilent sw first, it's hit or miss.

author: radioFlash
author_rank: Regular Contributor
timestamp: February 18, 2015, 03:39:42 am
content: Thanks for the info on the serial port, I'll have to look into it.I measured the output of the voltage reference for 15 minutes after turn out, see attached. I don't notice the same trend as when I was measuring the 5V input. I didn't have the best setup in the world for measuring it and there may be noise because of vibration (my measurement platform wasn't particularly steady and of course the cover and shield were off the unit).

author: Andreas
author_rank: Super Contributor
timestamp: February 18, 2015, 07:02:44 am
content: Hello,is the zener warm if you touch it (carefully its hot).Is the same noise measured directly at the zener too or only at the buffered output.With best regardsAndreas

author: Zucca
author_rank: Unknown
timestamp: February 18, 2015, 08:41:26 am
content: Quote from: radioFlash on February 14, 2015, 09:11:03 pmBefore to dig into the schematics I would have done different tests:0) VERY IMPORTANT! AsDr. Franksuggested me many times, try to measure R with inputs in short circuit; both in 2W and 4W configuration. Is the measure stable? Check/compare the results also rear vs front inputs. (The input switch is famous to fail for oxidation.. check the other 34401A posts in this forum)1) change the voltage range and test another stable voltage source, your DMM supports those DC V ranges:100.0000 mV1.000000 V10.00000 V100.0000 V1000.000 Vdepending on what V source you have available check if the transient is still there also in different ranges.2) Try to check the measure the VAC voltage mains of your home with the 2 multimeter. Is the transient still there?3) does the 4 wire configuration solve the problem or not?4) What about a known R measure? Is the transient still there?5) and what about a current measure? Is everything OK there?I suggest to understand better what is working and what is not in the DMM before to look for a HW failure inside. The smaller the circle, the less you need to check/measure on the DMM board.PS: Create a new post in the repair section will maybe help you better!Good Luck!

author: radioFlash
author_rank: Regular Contributor
timestamp: March 09, 2015, 02:00:51 am
content: Thanks all for the suggestions and input.* The reference is warm to the touch, so the heater is active* I have checked both front and rear input terminals and the behavior is the same* I have replaced the electrolytic capacitors and input switch and still see the same behaviorI've run some other comparison tests between my two 34401A's, and below is what I've done so far. All measurements were done after turning on the meters for a few minutes (it takes me a while to get Benchvue loaded and setup before capture). All readings were normalized against the average reading for the hour to make it easier to compare between the two meters. The suspect meter is in blue and my reference, presumably good meter, is in orange.In summary, the issue seems to be present on DC Voltage in toe 10 V and 100 V range, and to a lesser extent in the 1 V range. It doesn't seem to be an issue in the AC Volt .1 V, 1V, and 10V range. It also doesn't seem to show up in the 2W resistance mode testing a 1K resistor.DC Voltage.1 VDC Input / .1V Range: Meters agree almost exactly1 VDC Input / 1 V Range: suspect meter's readings change more over the hour, but overall is a very similar shape to to the other meter5 VDC Input / 10 V Range: suspect meter's change significantly over the hour, while other meter is nearly flat10 VDC Input / 10 V Range: suspect meter's change significantly over the hour, while other meter changes very little30 VDC Input / 100 V Range: suspect meter's change significantly over the hour, while other meter changes very littleAC Voltage.1 VAC Input / .1 VAC Range: Meters agree very closely3.2 VAC Input / 10 VAC Range: Meters agree very closely, but there is some odd differences in the first 5 or so minutes. (periodic dips in the readings are likely due to my cruddy signal generator)DC Resistance2W / 1 KOhm Resistor: Suspect meter takes about 20 minutes to settle, then it gets to within the range of the other meter4W Short: Suspect meter is pretty flat, but my other meter takes about 40 minutes to settle down and shows periodic oscillations in readings (but that odd behavior will be for another thread...)

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 09, 2015, 07:51:32 am
content: Quote from: radioFlash on February 18, 2015, 03:39:42 amThe stability figure of the DUTs LM399H is not stable enough.Should be < 1ppm stable.I assume, you use the Brymen 869 for that measurement. So you see the instabilities of the Brymen only!Please use your 2nd, known good 34401A for such measurements only.Cover off and any "vibrations" (??) should not make any deal, as far as you have stable contacts to the test points.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 09, 2015, 08:15:11 am
content: Quote from: radioFlash on February 15, 2015, 05:28:02 pmAs others already said, something is fishy here!Again, only use your 2nd 34401A for measurements!Use 6 digits, high impedance (> 10GOhm) DCV mode. Let the known good instrument warm up for 1h.Please check all internal reference voltages again, see the six red marked locations.That is LM399H direct out, divided 5V reference, +/- 10V A/D reference, and buffered / unbuffered 7V reference .Please measure all six points several times, i.e. directly after turn on, and then maybe 5min and 10 min after turn on.The LM399H should be stable to ppm level, 1minute after turn on.I suspect U400 to be defect, as these 7.0695V should be exactly 6.94..V , like the direct LM399H output..Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 09, 2015, 08:42:03 am
content: Quote from: radioFlash on March 09, 2015, 02:00:51 amSorry, that comparison stuff is much too complicated and (for me) not understandable.The error shows up in the 10V DCV mode /range already. That's the most simple one, and therefore you should only track the error in that mode. That's much more systematic, then.Supplementary to the reference voltages, now check the input path, also.For that, please put the DUT 34401A in DCV, 10V range, 6 digit SLOW mode, .Apply10V externally, from your 731B.5V externally, from your DMMchecker.Check briefly the stability and value of the731BDMMchecker by the other, known good 34401A.Then check if the input amplifier / multiplexer is ok, you should see exactly these10V5V also on pin 4 of U101.It may be, that this buffered output is also switched on /off, due to AZ measurement, then put your other 34401A in fast DCV mode, so you will see 5.00V briefly flashing, maybe.You may also measure the 5.000V input voltage before the buffer (maybe relays K103 is defect), that is marked Buffer input, pin 5 of U101.For that measurement, you have to put your known good 34401A in high Z mode again.FrankPS: The last step, if that up to now would all be ok, would be checking the MC output of the multiplexer, and the DCV amplifier.

author: radioFlash
author_rank: Regular Contributor
timestamp: March 10, 2015, 01:00:10 am
content: Thanks, Dr. Frank, for the suggestions. I'll measure at the points you suggest.

author: radioFlash
author_rank: Regular Contributor
timestamp: March 20, 2015, 10:20:43 pm
content: I measured the test points in the AD Converter section as Dr. Frank suggested.* Measurements were made with Keysight 34465A in High-Z with 10 NPLC. Values shown are average of 5 readings* Approximate time after turn on for the measurements are shown.* There is an odd jump in the readings at 40 minutes--this may have been due to my AC unit turning on or off

author: radioFlash
author_rank: Regular Contributor
timestamp: March 21, 2015, 01:22:09 am
content: Quote from: Dr. Frank on March 09, 2015, 08:42:03 amWith 5V on the 34401A's inputs, this is what I measured right after turn on:U101 / 4 : 5.00578 V (drifting downward)U101 /5 : 5.00024 (stable, same value as directly measuring the DMMCheck with the 34465A)

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 21, 2015, 07:59:38 am
content: Hi,Definitely U400-A is defect.It has 100mV at least of offset, but it should have 100µV maximum offset per datasheet.Its output at "B" drifts about 500pm, totally uncorrelated to the LM399H reference "C", which drifts 100ppmThese 100ppm drift are also much too high and lasts too long..I assume that the input transistors of U400-A are broken , drawing several hundreds µA or mA of current, instead of specified 50pA, so affecting the LM399H output.U400 is an AD706, which should still be available.Before replacing it, please make sure, that nothing oscillates , neither noise on +15V supply of U400, nor its output..U101-4 is drifting, because the processor detects some internal offsets and tries to compensate this by adding an offset, or something similar.. this may vanish also, if the reference circuitry is stable again, after replacing U400.Frank

author: radioFlash
author_rank: Regular Contributor
timestamp: March 28, 2015, 10:06:02 pm
content: Quote from: Dr. Frank on March 21, 2015, 07:59:38 amI replaced U400, the Analog Device AD706, and it's not drifting like it was before. I also checked for any oscillations in the supply rails and found none. The fixed unit's behavior is now the same as my other 34401A. Now, I have to get the repaired unit calibrated. Thanks for all your help and suggestions!

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 12:02:01 pm
content: Rather older thread but the topic is the same - I've got a 34401A here (1999), where from the cold start till settling (ie against a hot LM399) takes 3.5 hours typically (23.2C ambient, 41.5C inside the box at the end).Below is the typical warm up curve.My question is whether to start to mess with the AD706 (for example, when reading this success story), or is it something not worth of the effort..Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 16, 2019, 12:46:51 pm
content: If using the auto zero mode, there should not be much effect of the OPs. Much of the drift in the warm up period is more due to resistor drift. So there is little hope to improve things. If really concerned with the warm up, I would consider an extra heater resistor that can be turned on for the first 15 minutes or so to speed up the warming.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 12:51:20 pm
content: Thanks, AZ is on, there is also a thermal isolation from inside the box (shown in an different thread).PS: added the internal DMM temperature - looks like the temperature needs the same time to settle

author: Kleinstein
author_rank: Super Contributor
timestamp: November 16, 2019, 01:02:44 pm
content: Extra thermal insulation tends to slow down thermal equilibrium and make the warm up effect larger.What is actually shown in the last curve ?The small effect of the OPs is valid for OPs that are OK. Broken OPs can have a large effect.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 01:16:38 pm
content: The curve shows the relative voltage diff against an external "hot" LM399 (Vrefext, >1.5khours, on for several days), meter at 10V range, AZ on.Edit: Display ON.The ppm is (actual_V - Vrefext)/Vrefext.The measurement started 15.8secs after switching the meter on.I've added above the internal DMM temperature graph.Tamb=23+/-0.2C during the measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 16, 2019, 02:04:45 pm
content: The temperature drift does not look excessively large: about some 8 K for the last 5 ppm. This is within the normal range (some +-2 ppm/K) for such an instrument. The very fast initial part can be higher due to local effects, but I would not worry about the first 10 minutes.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 02:32:28 pm
content: I've calculated the regression (from start+10minutes till the end) of the measured absolute Vrefext voltage against Tdmm and I've got 3.198uV/C.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 16, 2019, 03:40:48 pm
content: 3.2 µV/K is a little less than 0.5 ppm/K. This is quite good for the gain stability - so nothing to complain about.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: November 16, 2019, 08:26:38 pm
content: 34401A stipulates an one-hour warm-up time to meet the spec. after 60 min of warm-up, it appears the drift is around 2ppm below the stabilized reading in your case. For a 6.95V reference, you are talking about a deviation of 0.0000139V.If that level of accuracy is required, precision bench equipment is constantly left on in most labs to minimize thermal deviation, and cal annually to compensated for aging drift. If you truly need 6.5 digit precision, you should let it warm up for at least an hour before taking measurement. So if I were you, I would not be messing with it to make it do something it is not designed to do.