HP 3478A 5.5 digit meter as null detector - Page 1
Forum section: electronics / general

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 27, 2020, 04:45:08 am
content: I've found the 3478A to be a handy bench meter with good low range sensitivity and low ohms capability. It does have to warm up a few hours for minimum drift; mine drops about 30 uV over an hour or three. After that it's pretty good. I logged data for half an hour on the 30 mV range to see what the noise was like. Excel file attached. Curious what people think of it as a null meter.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2020, 06:58:44 am
content: The 3478 is nice for low voltages. The 30 mV range gives sufficient resolution for a low level measurement. It is a nice meter to buy used: only 5.5 digits, but still with sub µV resolution.The possible weak point is the input current. One should at least first check the input current, e.g. with a large resistor at the input, or watching a 1-10 nF cap charge up.Chances are the simple null-meter circuit with AZ OP (LTC1050/LTC1052 or simialar) and a modern handheld DMM could give better performance as a null meter. Battery operation reduces common mode leakage to mains.The choice of DMM can make a difference as old ones only sample for a small part of the time and this way can increase the noise. This is also a slight weakness of the 3478: in AZ mode it the input is read only 50% the time.It depends on the signal sources how important input current is.

author: retroware
author_rank: Contributor
timestamp: February 27, 2020, 08:22:07 pm
content: I tend to use my null meters (keithley 155, fluke 845a and hp 419) to balance resistance bridges and also to compare the outputs of KVDs. Thus, input bias current is very important to me. But almost as important are the user interface advantages of an analog meter. Nulling typically involves very small movements of a pot, be it on the bridge or the meter itself. Even on the slower 1 and 3 uV scales of the analog meters, one can get a feel as to whether they are getting close. Having a DMM set to an NPLC of 10~100 makes this much trickier. The other trick one can play with an analog meter is null at say the 30uV scale where things are a bit less noisy. On a DMM, once you are on its lowest scale, you are sort of stuck. But, to be honest, I haven't actually tried using my 3458a as a null meter, maybe there's a NPLC setting that is suitable for this purpose.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 27, 2020, 08:34:23 pm
content: I have an 845AB and two HP 419s. I still use the 845 but it's a bit noisy and needs service (again). I really liked the 419s but mine have been idle for a long time and need serious service before I could use them. I've also got a couple top-hat L&N galvos, but that's probably going too far back! OTOH, I can imagine that an old galvo like that, combined with a laser pointer and a couple passes across the room, could be seriously sensitive. My 3478A is always right there ready to go. I'll check the input current next.

author: The Soulman
author_rank: Super Contributor
timestamp: February 27, 2020, 11:05:36 pm
content: I've been looking for a meter like this, didn't know the 3478a had a 30mV range, that combined with 5,5 digits means 100 timesthe resolution from what I'm currently using..Is it me or is something wrong with the excel sheet? can't open it..Noise is something to consider, curious about that.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 28, 2020, 01:10:06 am
content: I couldn't get any useful numbers with resistors but the meter changes by about 1 mV per 10 seconds with a 0.1 uF cap on the input. Check my math, which is almost always wrong on the first three tries, but is that 10 pA? BTW, I was wrong about the initial offset. It's not 30 uV, but 3 uV.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2020, 10:41:11 am
content: 0,1 mV/s and 100 nF would be 10 pA. However such a large capacitor may show some extra dielectric absorption effects if it was charged before. So this looks reasonable good.The offset of the meter should be lower. At least there should be a kind of user zero adjustment so one could set the offset back to zero.It's quite some time I last used a 3478, but I do no remember much noise when measuring thermocouples.The amplifier is a precision JFET OP of that age. There may be a little more noise as with modern meters, as the ADC is likely relatively slow and thus using a low frequency AZ cycle.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 28, 2020, 02:02:55 pm
content: I think I used a polyester, so will try again with styrene or propylene, but I think the number is close.While riding the exercise bike this morning, and waiting for the torture to end, it occurred to me that one might build up a little plug-in circuit module with a battery and high value trimpot and higher value resistors, to cancel out the input current. Is that something people have done?As for the noise, it's only moving a few counts, with a periodic spike a bit higher and lower. Not terrible. The offset, as I mentioned above and is wrong in the spreadsheet text, is about 3 uV at turn-on, coming down to maybe 0.5 uV after a couple hours warm-up, as the graph shows. I'm sure there's an adjustment, but I've never calibrated the unit as it's not far off enough to worry about.There's an interesting Fluke document on using a DVM as a null meter when working with the 752- can't seem to find it again, but it was in this forum.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2020, 02:48:33 pm
content: One could build a compensation current source. However I don't think this is really needed at 10 pA.For only 10 pA or a similar small current one could also consider a combination of photodiode and LED to generate the small compensating current. The get 10 pA with less noise from a resistor is needs more than some 50 mV at the resistor and thus at least some 5 Gohms.

author: retroware
author_rank: Contributor
timestamp: February 28, 2020, 05:17:06 pm
content: 10 pA is quite a bit larger than what one typically finds on an opto chopper null meter such as the Fluke 845a. I measured around 100 fA on my 845A which is important when measuring high impedance sources.What's interesting is that my Keithley 155 (which uses a mosfet chopper) seems to have a much higher bias current - around 3 pA. Looking at its schematic and adjustment procedure, they are using a voltage source through a 1 Gohm resister to adjust the bias. I'm going to play around with that to see if I can get it lower. But, if I'm reading the adjustment procedure correctly, it seems that anything within 5 pA is acceptable (5uv across 1M) so for balancing bridges, the 845a seems to be a better choice.I have two hp419a's, one with a dead chopper. I'm considering a couple of different retrofits for the dead one ranging from replacing the chopper with an H11F3 to replacing the amp with a modern AZ amp. The problem with the latter is getting a low noise, low bias current amplifier is challenging. I had contemplated the voltage through large resistor solution but like Kleinstein I was worried about the resistor noise. I do however like his idea of using a photodiode.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 28, 2020, 06:08:22 pm
content: I like the photodiode solution. I think this comes down to the impedance you want to measure. The 752A is on the high side, so it really matters. Comparing my voltage references with low impedance it shouldn't matter much, if at all.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2020, 07:55:29 pm
content: With a good quality high value resistor, there is no real problem from resistors noise. The photodiode has shot noise from charge quantization. The Johnson noise of a resistor gets lower, once the voltage across the resistor is larger than some 50 mV (I may be off by maybe a factor 2 with this number). So if a voltage source is available or doable the resistor tends to be better. The photo-diode may also be part of the clamping diode - photo-diodes also tend to be low leakage, though also expensive. The noise from the resistor is not that high: a 1.5 GOhms resistor would be at the 1 fA/sqrt(Hz) level, less than most amplifiers. Something like 10 GOhms resistors seem to be still affordable.An AZ OP and current compensation sounds very plausible. The AD8628 would be may favorite as it is reasonable low noise (~ 3 times lower than the resistors at the 845 input) and still not that much bias and current noise.It still needs some filtering at the input, but not that much capacitance. Filtering for noise purpose is anyway better not done directly at the input.For the 752 the divider output impedance during adjustment and use should be at some 40 K. So 10 pA of bias current would be only 400 nV.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 03, 2020, 10:38:40 am
content: Quote from: Conrad Hoffman on February 28, 2020, 02:02:55 pmThis is all that I managed to find from Fluke, mentioning DVM as a null meter.

author: calija
author_rank: Contributor
timestamp: March 03, 2020, 12:19:56 pm
content: For me the best is Nanovoltmeter Keithley 2182A, range 10 mV.Best regards!!!

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 03, 2020, 02:00:32 pm
content: Quote from: calija on March 03, 2020, 12:19:56 pmKeysight 34420a range 1mV best!

author: retroware
author_rank: Contributor
timestamp: March 03, 2020, 05:38:44 pm
content: Quote from: MegaVolt on March 03, 2020, 10:38:40 amThanks for posting those links. I had seen the first and third but not the second.With regards to using a DMM to calibrate a 752, it seems like a lot of extra work (calibrate for the bias current, possibly resort to a cap to increase the filter time) over the 'true' null meter approach. I'd be curious to see an uncertainty comparison. Each of the bias calibration steps seems to add to the overall uncertainty.Plus, the first line in the summary of the 'migrating from dc voltage dividers to modern reference multmeters' document was a bit amusing:"In summary, a comparison of measurement uncertainty between dividers and 8508A will often favor traditional dividers."It always amazes me what one can do with a bunch of well connected resistors and a relatively simple, but well designed, null meter.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: March 03, 2020, 05:52:54 pm
content: Quote from: retroware on February 28, 2020, 05:17:06 pmMr. Carlson just repaired the photo chopper in an HP 419A:https://youtu.be/vrmwql2msbU

author: David Hess
author_rank: Super Contributor
timestamp: March 03, 2020, 11:43:42 pm
content: Quote from: Kleinstein on February 27, 2020, 06:58:44 amA bench meter also has considerable common mode capacitance to ground.

author: retroware
author_rank: Contributor
timestamp: March 04, 2020, 06:20:44 pm
content: Quote from: Tomorokoshi on March 03, 2020, 05:52:54 pmThanks for the link to the very nice video.I've pretty much convinced myself that the problem with my unit are the actual photocells themselves. My neons appear to be firing correctly plus I've tried several different types. The chopper waveform compared to my working one suggest that they have become slow. I don't think its the circuit board, if I swap boards with the working unit, the working unit continues to function.Still, there might be something to be had by trying to drive the photocells with LEDs. If I remember correctly someone else with supposed photocell issues tried that with little success.

author: retroware
author_rank: Contributor
timestamp: March 04, 2020, 06:23:28 pm
content: Quote from: MegaVolt on March 03, 2020, 02:00:32 pmHave you measured the input bias current? I'd be curious how it compares to say a 3458A. I have the same question with regards to the Keithley 2182A.

author: MiDi
author_rank: Frequent Contributor
timestamp: March 04, 2020, 09:07:00 pm
content: Attached the specs for input bias currents.

author: retroware
author_rank: Contributor
timestamp: March 05, 2020, 02:24:01 am
content: Quote from: MiDi on March 04, 2020, 09:07:00 pmHmmm, not so great for balancing high impedance sources.

author: MiDi
author_rank: Frequent Contributor
timestamp: March 05, 2020, 10:29:59 am
content: Quote from: retroware on March 05, 2020, 02:24:01 amI do not have expertise on that, but what about balancing by current?Perhaps a high resistor in series with electrometer in amps mode between sources could do the job.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2020, 03:49:44 pm
content: Quote from: MiDi on March 05, 2020, 10:29:59 amThe electrometers low current range use a TIA and thus can have an offset (several mV and not very stable). So they are not an alternative.The input current of the DMMs makes thinks complicated, but in theory one could compensate for it: do a zero reading with the same impedance. The usual bridges are not that high impedance. Some 40 K and 400 K for Fluke 752 is about the highest one would encounter. 20 pA trough 40 K gives 0.8 µV of offset - that is not very much, so that the compensation does not have to reduce it that much.

author: calija
author_rank: Contributor
timestamp: March 06, 2020, 11:57:44 am
content: For DCV calibration of DMM 3458, 8508 and 8081 (8.5 digit) and for calibrators 5720 and 5520 we used 732 + 752 + 2182.A lot of national metrology institutes do that and don t have problem.When you do ACAL and REL on 2182A it is excellent on range 10 mV (temperature must be +-1C)Best regards!!

author: retroware
author_rank: Contributor
timestamp: March 06, 2020, 06:05:21 pm
content: Quote from: calija on March 06, 2020, 11:57:44 amWhen using the 2182, do you measure and account for the bias current using a technique similar to that described in (using a DMM to calibrate a 752):https://www.eevblog.com/forum/metrology/hp-3478a-5-5-digit-meter-as-null-detector/?action=dlattach;attach=942832

author: Kleinstein
author_rank: Super Contributor
timestamp: March 06, 2020, 06:52:07 pm
content: The Fluke instructions about using a DMM with the 752 is a little odd: At least from the drawing inside this PDF the auxiliary bridge is 2x120K and thus 60 K output impedance. So the source impedance for the self adjustment would be 20 K + 60 K = 80 K and not 40 K. So the bias compensation should be done with 80 K and not 40 K ! If in doubt - use a 0 V source (short) for the excitation and measure the resistance.For other tests (especially in the final 1:100 mode) the impedance can be higher / different.

author: David Hess
author_rank: Super Contributor
timestamp: March 07, 2020, 01:14:54 am
content: I wonder what the input current of the HP 419A DC Null Voltmeter which uses a chopper (not chopper stabilized) amplifier is. It also uses a single ended input.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2020, 12:30:51 pm
content: The HP419 is relatively similar to the Fluke 845, but using lower impedance.I would expect a slightly higher input current and less noise.On the positive side the HP419 has a well defined dead time and an extra zero adjustment for the DC amplifier part (1 V range zero adjustment). So one may be able to adjust the input current to some degree and reduce the residual AC needed at the input.The specs for the current ranges suggest < 1 pA for the input current.

author: retroware
author_rank: Contributor
timestamp: March 07, 2020, 04:00:11 pm
content: I have two HP419s, one working (part of an ESI 242 resistance measurement system) and one that's not. I've been struggling a bit to measure the working one's bias current. The method of simply measuring the difference between the zero mode and regular operating mode doesn't work for the HP419 due to its low input impedance of 100K. The meter moves around 100~200nv in the operating position which isn't that much different than when I have it shorted. Thus, the noise floor seems to be limiting this particular procedure. Also the lack of a shorting switch on the ESI doesn't help either as I'm always leery of my low thermal short. Ideas for measuring the current in light of the 100K input impedance would be greatly appreciated.With regards to DMMs, their bias current and suitability for balancing high impedance sources, there are some DMMs that require one to adjust the bias current as part of the cal procedure. The Datron 1061,71,81 series comes to mind. Indeed, my Keithley 155 meter also has this adjustment. Mine was originally quite high (thanks mrdiode for pointing this out) and adjusting it brought it down to the femtoamp range. I'm guessing with careful adjustment, DMMs with adjustable current one could do the same.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2020, 05:07:42 pm
content: A 100 K input impedance makes it tricky to measure the input current in deed.To really measure the input current would need some modification. One could get away with a switch in series to SIR20 to optionally add a large series resistor that. The low voltage ranges could than have some 10 M or similar input impedance. So the bias current test would be much more sensitive.The internal 1 V zero adjustment should also have some effect on the input current.For adjusting the input bias: this only works well if the input current does not change that much with temperature. The old Datron 1061/1071 use a BJT bases input stage and thus need the compensation to get rid of quite a bid of input current (e.g. 1-5 nA).I would consider some compensation possible for an AZ OP. Here the input current can be quite small to start with (e.g. 10s of pA).

author: guenthert
author_rank: Frequent Contributor
timestamp: March 07, 2020, 05:17:42 pm
content: Quote from: retroware on March 07, 2020, 04:00:11 pmThe procedure as far as I understand goes like:apply a (low thermal EMF) short (like a copper strand between inputs), set the meter to SET NULL, adjust bucking voltage until needle dances around 0, set meter to READ NULL and the then indicated voltage is the voltage generated by input bias current across the input resistance(100k in the lower ranges, i.e. for each 10pA expect 1uV deflection).Edit: unfortunately neither is the input bias current of the HP419A specified, nor a performance test described which would measure it. After having another look at the schematics, I realize that above won't yield the desired result (on READ NULL the input is shorted and the polarity of the bucking voltage reversed, hence it'll yield exactly the opposite of the SET NULL with inputs shorted). I'd think now that just ZERO'ing the meter and the setting it to VM mode with open inputs should show the input bias current (of the amplifier) across the input resistance (SIR20 through SIR24, total of 100kOhm) plus SIR27 (15k).

author: retroware
author_rank: Contributor
timestamp: March 07, 2020, 11:40:08 pm
content: The problem with that procedure is that one runs into to the noise floor of the instrument when one gets down to around 2pA. 2pA across 100K resistor is 200nV which is less than the noise spec of the instrument. So, if the bias current is less than that, one won't be able to measure it. On my unit, that seems to be the case.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 08, 2020, 04:04:45 pm
content: The bias current could be measured with an electrometer?That's what I did with my meters, and found them to be to spec.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2020, 04:24:52 pm
content: The relatively low 100 K resistance across the input makes it essentially impossible to measure the input current as one has one the very low voltage of some 100 nV. A typical electrometer would not work on this case as the TIA will also have some offset on its own. The measured current would be more like the TIA offset divided by 100 K, often orders of magnitude more than the real input current.From the circuit I would have expected less voltage noise than from the Fluke 845. The visual comparison could be tricky with much more filter action in the 845 and thus a slower response.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 08, 2020, 11:22:55 pm
content: One could perhaps chop the current and use a lock-in amplifier to detect it?

author: guenthert
author_rank: Frequent Contributor
timestamp: March 09, 2020, 04:45:15 pm
content: Keep in mind that the input-bias current will vary with time. I observed peaks in the nA range for the input-bias current of a HP3456A, while the average was in the order of tens of pA.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 09, 2020, 07:15:35 pm
content: Quote from: Kleinstein on March 08, 2020, 04:24:52 pmI'm not sure I understand this. Won't the electrometer input impedance (in current measuring mode) be much lower than the 100K resistance, so the 100K essentially won't matter - and so, the current seen by the electrometer is substantially all of the bias current?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2020, 07:54:33 pm
content: The electrometers measure small currents as a trans-impedance amplifier. So the input is essentially at the ground potential. However no OP is perfect and there usually will be a small offset (e.g. mV range). One can still measure sub pA current from a higher impedance source, and get a good zero with an open input. However with a low impedance current source there will be current due to the offset voltage. The offset voltage is an unwanted small voltage source in the circuit.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 10, 2020, 12:58:47 am
content: Quote from: Kleinstein on March 09, 2020, 07:54:33 pmAaah, thank you, yes, I see it now.A Keithley 614 here has 0.62mV offset, which would result (does result!) in 6.2nA of current in a 100K resistor... which will of course "drown out" any current one might be looking for at the pA level - the current suppress function doesn't have enough range to compensate.

author: retroware
author_rank: Contributor
timestamp: March 10, 2020, 05:19:55 pm
content: Would bucking this offset with the 419's NULL feature work? Unfortunately, my working 419 is a modified version that ships with the ESI 242 resistance measurement system and it has neither the zero short switch or null feature. I tried using an external voltage source together with a Keithley 616 electrometer but was unable to get any results that I trusted.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2020, 06:03:40 pm
content: The offset of the usual very low current TIA in the electrometers is not very stable - so even if one would adjust or compensate it would another voltage source the TIA is still not suitable to measure the current from a low impedance source.If at all one could use an external nV meter (e.g. Keithley 2182) to measure the voltage at the 419 input.The other option would be to modify the 419 to allow a high impedance measurement:This could be an additional resistor (e.g. 10 M) in the ground connection of the input divider from (SIR20) with a switch in parallel. With the high resistor active one would have high Z for the lower few ranges (til 3 mV), but non working higher ranges. With closed switch there might be a minimally higher error for the higher ranges (maybe + 1% at 1000 V, less with lower ranges) - so probably not a big deal. The input current would than be better measurable with some 100 times finer scale than the original.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 10, 2020, 07:07:59 pm
content: Quote from: retroware on March 10, 2020, 05:19:55 pmThe offset is simply too big for the nulling / compensation features to be able to kill it off.I'm thinking that an external voltage source to cancel out the offset might work, provided the source is stable enough...

author: retroware
author_rank: Contributor
timestamp: March 10, 2020, 07:12:23 pm
content: Quote from: SilverSolder on March 10, 2020, 07:07:59 pmI was using a Fluke 5440B but I wasn't particularly careful with my connections. Maybe I'll try again.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2020, 07:15:30 pm
content: Even of the external source is stable to compensate the electrometer offset, the electrometer offset itself is not stable as it is from a high Z JFET or CMOS amplifier. So expect at least 10s of µV of variations. If in doubt just measure the voltage at the TIA with a reasonable good DMM.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 11, 2020, 12:14:06 am
content: Quote from: Kleinstein on March 10, 2020, 07:15:30 pmI did notice the instability that you are talking about. - but the magnitude of the instability is much less than the magnitude of the offset voltage?

author: calija
author_rank: Contributor
timestamp: March 11, 2020, 08:49:43 am
content: Quote from: retroware on March 06, 2020, 06:05:21 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2020, 03:43:03 pm
content: Quote from: SilverSolder on March 11, 2020, 12:14:06 amHow large the absolute offset is, depends on the individual units. One could even have an adjustment for the offset. Without adjustment I would expect up to a few mV, so considerably more than the usual variations. Just have a look at the specs of an OP suitable for an eletrometer (e.g. LMC6001 : some 1 mV range offset and 10 µV/K drift - 0.1-10 Hz noise is expected to be in the 10 µV range). A TIA is just the wrong way to measure the bias current for a low Z input. It is kind of hard to separate a bias from an internal offset. If there is some thermal EMF at the resistor, these can also cause some kind of bias. With 100 K it only takes 0.1 µV to get 1 pA. So if low bias is needed it helps if the null meter is also high impedance. The 100 parallel resistance can be a limiting factor for the HP419.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 11, 2020, 07:57:29 pm
content: Measuring the open circuit voltage is the way to measure the bias. However the resolution is limited, as 1 pA corresponds to only 0.1 µV. So it needs a good meter - ideally better than the 419, but than there is bias from that other meter.It would be easier if the input resistance would be higher: than the 419 could have sufficient resolution.