HP 3478A Troubleshooting - Page 1
Forum section: electronics / testgear

author: Aphelion
author_rank: Contributor
timestamp: September 17, 2021, 03:47:28 pm
content: I've been trying to fix up a used HP 3478A that I picked up locally here in the philippines. Self test check out. So far, I have replaced the RIFA caps and proceeded to replace all of the electrolytics (main 3300uf filter cap is still in shipping)The problem is that all of the ranges are jumpy with a dead short ie. At the 30mV range it jumps in between a range of values at +-00.0050 from cold start but sometimes after warmup and use it settles at +-00.0010 and returns to -+00.0010mV. Amount of noise is consistent on all dc ranges(for 30v and up the least significant digit is the only one that does not settle)I've checked the 10V line at JM102 and it checks out. Unfortunately for the checking of the U101 opamp I can't check for noise for sure as I only have a UT61e to check for noise beyond 0.001v. Also, the manual didn't state how much noise I should look out for.So instead of getting a signal probe and sig analyser, I am going to try my luck and source U101, U765, U766, U467 and U550. Hopefully after replacing these I can remove the noisy readings of my meter.U101, from searching the net is thought to be an AD544 BiFET, I got a AD711 which seems to be the closest I can get to the original spec of the AD544.Problem is, I can't find any info on the rest of the ICs that the A/D Controller troubleshooting section suggest replacing before replacing the main IC. I can probably guess it is also like the input hybrid which is unobtanium in on itself (been searching for a month now with no luck)I wanted to get another 3478A for parts, but the meter itself is almost unobtanium here in the Philippines by itself. Most US ebay sellers won't even ship to here.And lastly, should I just bite the bullet and get a new multimeter instead of trying to revive this meter?

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2021, 04:38:26 pm
content: Quote from: Aphelion on September 17, 2021, 03:47:28 pmI definitely think you should not try to 'fix' the meter by tossing in parts. Given your description I'm not at all sure that anything is wrong with it.I have an HP 3478A here with me. I consider this meter to be in good working order. I just plugged it in and put a 2-wire short (a Pomona dual-banana short, no wires at all) and I get 00.0047, which is slowly decreasing. Going uprange gives me similar numbers--000.05 mV, 0.00001 V, 00.0000V. That is 4.7uV of offset cold, which doesn't seem excessive to me. I'll see how noisy it is after while. During warmup you can have competing temp coefficients that will cause the offset number to wander around before finally settling somewhere.So what you should do is make sure you have a very short short circuit on the inputs (no wires if possible) and that your room temperature is within 18-28C. Then let the meter warm up for 90 minutes, check each range for reading and noise/instability and then compare that to the specifications. If you still think the input is noisy--and you are absolutely sure that your short is perfect--then turn Auto Zero off (blue shift key, INT/TRIG) and see how much it bounces around with AZ off.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2021, 05:10:33 pm
content: For comparing the noise / drift one should have it properly wamred up and protected from large variations in the room temperature. With a meter not used much for quite some time (which is normal for used meters) there can be some initial drift from humidity getting a new lower equilibrium. So it can initially take more than the normal warm up.For the noise it depends which speed is used. AFAIK it uses averaging over multiple conversions to get the full resolution. So don't expect it to be low noise at the higher speeds. It seems to be reasonable working and no real need to mess around with the ADC. Some parts are special HP specific hybrids.The ADC may need some of the noise so that oversampling works - it may get worse with to little noise.

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2021, 05:53:06 pm
content: So after an hour or so mine is reading between +00.0010 and +00.0020, bouncing around a bit. Auto Zero on or off makes little difference, so this is simply a microvolt or so of input offset and noise. The specification is +/- 4.0uV or 40 counts at zero, so well within spec. My room temp is ~23C with reasonably low humidity and the meter has been in the room for weeks now with the temperature stable to +/- 5C. Temperature variations, humidity, the quality of your short, etc may make some difference.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 05:50:15 am
content: Here's a short clip of the meter with shorted inputs in the mV range. It has been sitting here turned on for more than 2 hours now. My shorting plug is no pomona plugs but I think this shouldn't be normal?.https://youtu.be/uLai8bUrttI

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2021, 06:19:55 am
content: This looks a bit noisy indeed.Is the current reading also noise ? internally it should use a similar setting of the input amplifier.I doubt it would be the U101 to go bad with increasing noise. That would be a rather rare failure mode.Candidates would be more like a wrong setting (e.g. 50/60Hz mains), supply ripple from a bad cap or a bad solder joint, poor relay / switch contacts and maybe resistor in the input path.It sometimes help to excercise the front / rear switch a few times (e.g. 10 x) and see if this chances things. Also test if a short at the rear may work different.If possible get some PC interface ( GPIB or RS232 if the 3478 supports) and record data to show as a graph. That is much easier to judge than a video.Having a PC link is anyway a main incentive to get a bench meter.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 06:35:08 am
content: I'll check if there are any more bad caps that I can replace. Measuring voltage between the main 3300uf cap has unstable voltage. That is so far my primary suspects along with U101.Solder joints are fine from the last time I checked, and the mains switch is set to 60hz, which is correct for my country.No idea how I can check the reed relays though, first time I saw these kinds of relays.All of the carbon resistors along the input path that is 54k is 58k when I measured it, and R107 which is a 1 ohm 1% resistor is 0.68. might be a candidate for replacement. Can these affect the DCV ranges?Also already tried using the outputs at the back and cleaning the input terminals, still no luck. All of the modes are noisy, even ohms and DCA. ACA seems to be fine though(000.001+- mAAC noise)Gonna try to build the arduino GPIB interface if I can so that I can datalog it. I really hope it is not the input hybrid or ADC ☹️

author: Greybeard
author_rank: Frequent Contributor
timestamp: September 18, 2021, 08:02:11 am
content: Quote from: Aphelion on September 18, 2021, 06:35:08 amYou can try to knock on the relays by plastic or wood sticks, handle of screwdriver etc. while watching for jumping display.Measure voltage drop over closed relay contacts.

author: AVGresponding
author_rank: Super Contributor
timestamp: September 18, 2021, 08:34:56 am
content: Quote from: Greybeard on September 18, 2021, 08:02:11 amI think I'd rather use a magnet to test them than hit them with anything; that could easily damage them or the solder joints.@OP, is there a SMPS or LED light withina couple of feethalf a metre of your meter?

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 09:48:38 am
content: Here is a clip of the meter after 5 hours of settling time and turning off everything else in the room that I can see.https://youtu.be/8NUr_PDUXHsBringing a hard drive magnet near the rightmost relay facing the front of the meter adds about 0.1 mv to the readingEdit: forgot to mention that the reported noise is about the same even if I turn off all sources of EMF in the room as shown in the video.

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 09:50:32 am
content: Quote from: Aphelion on September 18, 2021, 05:50:15 amOK, that's an order of magnitude, or decimal place, higher than what I first understood.Turn off CAL ENABLE (turn the small slotted screw 90 degrees clockwise) and reset or power cycle the meter.I can't judge your shorting plug from here, but if it makes good contact internally it should be fine.Press the AC volts button, give it a minute and see what it says and if there is a residual reading, how steady it is.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 10:02:01 am
content: At AC mode the reading is pretty stable, with only a deviation of -+0.005mVAC. It may be a bit larger now as the case is open to check the relays with a magnet which AVGresponding suggested to do

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 10:17:47 am
content: Quote from: Aphelion on September 18, 2021, 10:02:01 amOK, so you can look at the schematic and get some idea what that means as far as what input circuitry is bypassed in AC.The DC input system uses a high impedance input amp U101, with 3 gain settings for the 30mVDC, 300mVDC and 3VDC ranges and then a 100X divider for the 30 and 300VDC ranges. So the U101 gain settings are the same (medium) for the 300mVDC and 30VDC ranges, and then the 3VDC and 300VDC ranges share the lowest gain setting. Only the 30mVDC range gets the highest gain.You might be able to determine whether the noise is before, from or after U101 by comparing the ranges with the same gain setting. So look at the 300mVDC range and see what the noise looks like, then go to the 30VDC range and compare it.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 11:20:43 am
content: Ok, after poking a while in the meter, only off things I can say is that all of the 51k resistors reads 56-58k, the capacitors C102 and C101 reads at around 480-ish pF. Also, R110 reads 1.2k. Might get replacements for those carbon resistors.300mV range has noise at the range of -+000.010mV, and 30v range has about the same amount at -+00.0010V.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2021, 12:23:31 pm
content: The 50 K resistors are there only for protection. So the actual valeue does not matter. An important point is that they can withstand high pulse voltage - here modern thin film resistors are often worse than carbon.The divider is 1:100. So 0.01 mV and with the divider 1 mV noise corresond to the same level of noise. The difference is in the input path before the amplifier. So either both paths are bad, or more likely both are OK, or at least kind of OK. This kind of excludes the 50 K resistors at the input.It could be still the switching part for the auto zero mode. So one may be able to compare the non AZ and AZ mode (I suppose the meter has this option not sure though).It is still hard to tell if the noise is more from before or after the amplifier. For this one would have to compare 2 different gain settings, like the 3 V range and the 300 mV or 30 mV range. Noise from before the amplier would stay the same. Noise from after the amplifier would be scaled with the gain.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 01:02:10 pm
content: Quote from: Kleinstein on September 18, 2021, 12:23:31 pmThank you for explaining each part to me clearly. Kind of learning as I go while I troubleshoot the meter so it already paid for itself even if it turns out to be a dud 😂.Already probably probed everything I can test before U101 that can affect it except the input hybrid itself. Haven't tried measuring the voltage drop in the relays though, gonna have a crack at that later.Also can't check whether or not the ADC is the one at fault, reason why I was going for scorched earth tactics and replace that might be replaced easily. Don't have a logic probe/signature analyzer/pulser etc.bdunham7 already suggested me to check in between 30v and 300mv ranges, and it scales( 00.0005V-0.0010V noise on 30v range, and 000.005V-000.01V for the 300mV range). So if I am correct this points to the ADC section that has the problem?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2021, 01:34:48 pm
content: The comparison between the 30 V and 300 mV range does not tell if the problem is more before the ADC or more at the ADC. bdunham7 was wrong on that, though the general idea is good: The main difference between the 300 mV and 30 range is only the input that. So it kind of checks parth of the input hybrid and they seem to be similar.To see if the problem is more with the amplifier (or before ) or the ADC one would need to compare different gain settings, like the 3 V and 300 mV range.A logic analyzer would likely not be needed. The instrument of choice may be a scope.Even with limited test instruments one can still get a good hint on where to search for the noise and not swap part withut a good reason.The hum measured at the supply could be a hint for example. If lucky, just the still missing caps will fix things.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 03:43:56 pm
content: Replaced the main 3300uf electrolytic, still no dice ☹️

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 04:25:22 pm
content: Quote from: Aphelion on September 18, 2021, 01:02:10 pmNo, I think that test points away from the ADC being the problem, at least when combined with the results from your other ranges. Just to be clear, you've corrected the CAL ENABLE issue and you are observing those number as noise, not a steady offset? Can you compare the 3VDC and 300VDC ranges? I know those noise levels were low, but it still might be handy to see them.Quote from: Kleinstein on September 18, 2021, 01:34:48 pmMe wrong?Actually I didn't say that, I was suggesting using the comparison of two scaled ranges to see where the difference was relative to U101. Comparing them all might help too, but I wanted to start somewhere andAphelionindicated that the noise was very small on the highest range, while the lowest range/greatest gain at 30mVDC doesn't have an equivalent with the 100X divider switched in.So now we know that it has ~10 counts of noise in the 33.3X (medium) gain setting regardless of the input divider setting, and the noise apparently scales up in the higher gain setting and down in the lower gain setting--the former is on video and the latter I've just askedAphelionto check again. Since all of the ranges use the gain of the input to keep the FS of the ADC to be in a -10VDC to 10VDC range, there is no additional scaling or digital magnification which would cause the ADC to read differently from one range to the next--the displayed counts are the (ADC output * calibration correction * a constant) with the decimal point set separately. So to me, the ADC doesn't seem to be the issue. That and the apparent absence of noise on the AC input range.So it looks like the noise is present on the inputs of, or internally to, U101. Since the input paths are mostly separate up to that point, and since the inputs to U101 are effectively shunted with ~100K in either case, it appears to me that the issue will be U101 or the closely associated parts of U102.

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 04:29:20 pm
content: Quote from: Aphelion on September 18, 2021, 03:43:56 pmI just had a thought--is your power supply frequency switch set correctly? On the back, underneath the EXT TRIG connector, there are a series of switches for the HPIB address setting, but the first one is up for 50Hz, down for 60Hz.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 05:54:01 pm
content: Quote from: bdunham7 on September 18, 2021, 04:29:20 pmYep, made sure of that as that was the first thing that was on the manual. Even tried toggling the DIP switches to account for dirty contacts.Strange thing is that I hear some buzzing inside the case that I can't seem to pinpoint. Checked all of the caps, even near the input hybrid and there seemed to be nothing out of spec (all reading 460-480 pf). Maybe it is just the transformer humming.And yep, all testing done was with the CAL ENABLE switch in horizontal position.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 06:20:27 pm
content: Quote from: bdunham7 on September 18, 2021, 04:25:22 pmHere is a clip of the 3v and 300v range noisehttps://youtu.be/PMRGZhfuDbkAnd yes, the cal enable has been in the horizontal position with all of the tests.Gonna probe U101 with a scope to know once and for all to know whether it is faulty or the input hybrid is. Is a hantek dso5102 fast enough? Currently thinking of probing pin 2 and 6.

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 06:41:26 pm
content: Quote from: Aphelion on September 18, 2021, 06:20:27 pmIt's going to be very problematic to probe this with a scope. The input noise floor of the scope will be much higher than the noise you are looking for unless you use a very specialized low frequency preamplifier. You could try shorting the input pins together and then seeing if the noise is still there and changes with range settings.Am I seeing that right--the noise (in counts) is higher on the 30V range than the 3V range? If true, that might be a big clue right there.

author: Aphelion
author_rank: Contributor
timestamp: September 18, 2021, 07:06:11 pm
content: Quote from: bdunham7 on September 18, 2021, 06:41:26 pmWell, that is one option out.As for the 30v range having more noise than the 3v range, yes it seems that way with autozero off. It is more apparent when you turn autozero on.Well, rechecking the input paths it may really be the input hybrid that has a problem. I have already checked all of the resistors and capacitors that is attached to the input going to the hybrid.So basically if 3v is noisier than 30v range, there is a chance that the precharge amp section of the hybrid is dead?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2021, 07:13:06 pm
content: The noise in the AZ mode looks good. It is normal to get quite some extra noise in the auto zero off mode. For the 5 digit mode I would not consider the AZ off mode to be of any use, maybe except in the 3 V and 300 V ranges. The AD544 was a good OP for it's time. There are better OPs now. If you really want to change it, my choice would be an OPA145 - though it would need an adapter board ( SO8 to DIP). Not sure if the lower noise is really needed - it may help a little in the 30 mV range.The speed of the scope is not a problem. The interesting point would be the test-point at the output of U101. The point would be mainly if the settling after AZ switching and the precharge phase is OK and to see if there is significant hum ot other signal that is not supposed to be there.P.S. The precharge part could just have too much offset. AFAIK the same input hydrid is also used in the 3457 and there they have additional offset trim. One could still had an manual (one time) offset trim if needed.

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2021, 10:37:20 pm
content: Quote from: Aphelion on September 18, 2021, 07:06:11 pmThat actually seems less likely, but I suppose it is possible that a switch is leaking there or something.Looking and your videos and your reported results seems a bit confusing. If you have a display bouncing from 11 to 13 counts, I would call that 2 counts of noise and 12 counts of offset. It's important to maintain the distinction. Perhaps you could make a chart of the offset and noise in each range with AZ on and off. So, after a set warmup--say an hour--go through the ranges from 30mVDC to 300VDC, a set time--say 15 seconds--in each range and write down your estimate of the min/max variation (noise) and average (offset). After you do all the ranges, go back with AZ off and do it again. I think this would really help me fully understand what you are seeing.As far as what is normal and what is not, on my presumably working model AZ makes very little actual difference in noise, but it does appear to move faster because you see twice as many readings. Letting it sit for few hours in the 3V range with AZ off resulted in 1 count of drift in that time. If you have noise, say in U101, then AZ won't really help and may make it worse because each AZ reading will be affected by the noise differently--so your zero will be changing constantly. At least with it off, the CPU is using the same value for zero all the time. Short term, that could actually be more stable if the noise is high frequency--less than an A/D cycle-- and random.Normal offset on mine is a few microvolts--about 4.0 when first turned on, then +1.0 to +2.0 (10 to 20 counts) on the 30mVDC range when warmed up. All the higher ranges are 0 or 1 counts. Causes of a steady offset could be actual voltage on the inputs at any point prior to the junction of the input amp U101 and the AZ section (which I believe is S8DC inside U102) or an fixed offset error in the AZ circuit. All of that would be inside U102 and probably would be an issue with the analog switches. AZ should really take care of any other offsets anywhere else along the chain. Of course there will be a residual offset and that is taken care of as a calibration constant, provided that it is stable.You've probably read the manual pages 7-A-1 through 7-A-4, but they're worth studying closely. Some of the tests, such as the part where you supply a precision 3 volts, short pins 2 & 6 of U101 and then make measurements with a precision voltmenter, probably require better equipment than you have. You may have also seen the short part where they essentially tell you that if you have noisy readings in some ranges, replace U101 and if that doesn't fix it, U102. Obviously nowadays that is easier said than done. Try to make up that chart and I'll think a bit more about how you might be absolutely sure what the issue is. U101 may be findable, U102 not so much....

author: AVGresponding
author_rank: Super Contributor
timestamp: September 19, 2021, 09:25:28 am
content: Just to provide you with an additional data point; when I first turn mine on it has around a 2.5uV offset with around 5 counts of noise on the 30mV range, 0 on every other range.This rapidly drops to below 1uV after only 10 minutes or so, with the count noise dropping to 2 or 3.Using AZ makes as you say little difference, at least at turn on. If I switch it off once it's warmed up for a bit it can make the offset and noise jump all over the place, to the tune of as much as 3uV +10 counts.Ambient here is 20°C, and my short is a home-made one, using WAGO 4mm banana plugs and soldered together solid core 2.5mm2copper wire.

author: Aphelion
author_rank: Contributor
timestamp: September 19, 2021, 10:34:12 am
content: Here is what I have gathered for the DCV range Room has closed windows with 30 degC temp with a RH of 60%. I've let it run still for 5 hours before taking measurements. Cover is off, CAL ENABLE is in horizontal position.Nickel plated banana plugs with soldered in 18 AWG wire for the shorting setup

author: Aphelion
author_rank: Contributor
timestamp: September 19, 2021, 10:42:15 am
content: Quote from: AVGresponding on September 19, 2021, 09:25:28 amHmm, mine even after an extended warmup time has an oscilating -+50 counts of noise in the milivolts rangeOffset often varies for some reason. Sometimes it zeroes out, sometimes it does not

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2021, 11:05:19 am
content: The noise seems to be at the amplifier input side and not from the ADC.The offset is quite high, but this could be a calibration / software thing and in parts caused by the high temperature.In the AZ mode the noise does not look to be so bad - depending on how it is measured (peak to peak or RMS).The OPAs used for U101 may differ a bit, some may be more sensitive to temperature than others and the LF noise is also not always the same.If the input hybird is part of the problem, one may also see some effect on the input side, like more input current or larger input current spikes. With the 100 K and cap from the filtering one may not see very much, but it could be worth to just use the scope to look the the input in votlage mode. For a slight votlage for the AZ switching to work against maybe also with some 2 or 3 V (e.g. 2 x AA battery) in series. There should be only minor switching spikes from the AZ swtiching visible.Another simple test is the input bias: e.h. some 10 M accross the input and than read the DC voltage. The input bias should be less than 100 pA and thus less than 1 mV, ideally less than 0.2 mA (20 pA).Another way to test the input is to measure the drift with a 10 nF cap (low loss like polypropylene or polystyrene type) accross the inputs. 100 pA would give you 10 mV per second as a drift rate.

author: bdunham7
author_rank: Super Contributor
timestamp: September 19, 2021, 06:19:22 pm
content: So your offset scales with U101 gain and with the 100X divider, but the noise more or less scales only with the gain of U101.The offset may just be a calibration issue and may not even be a problem, since it seems fairly stable. You might take your other DMM and check the voltage at the junction of R110 and R103 on the various ranges just to see what is there. However, as far as the noise goes, I'm not sure I can offer you better advice at this point than what the manual says--replace U101 first, if that doesn't fix it it needs U102.You can try shorting pins 2 & 3 (inputs) of U101 with a very short wire (perhaps on the back side of the PCB) and then see if there is any noise and if it scales with gain. You can also try the procedure in the manual that involves connecting pins 2 & 6 so as to set the gain to x1, but I think the noise level is just low enough that you won't be able to see it this way.Unfortunately I'm fearful that this is a U102 issue, but I can't see quite how to prove it at this point.

author: Aphelion
author_rank: Contributor
timestamp: September 19, 2021, 08:13:51 pm
content: Initially I kind of came to the conclusion that U101 is defective as ACV takes a different route through the input hybrid instead of the DCV mode that goes to switching FETs and directly to U101. And ACV has almost no noise (+-000.001mVAC)As far as replacements go, I've only read up a single thread on the internet that indicated that it is a AD542KHhttps://www.electronicspoint.com/forums/threads/crossreference-for-a-ad542kh.172618/Is an AD711 a suitable replacement to even just check whether or not the problem lies with U101? Once I confirm that it is the case I might get an OPA145 as the AD711 I ordered is still in transit as Kleinstein suggested it is a better replacement for it. The AD711 is what the analog devices page suggested as a replacement for the AD547

author: Aphelion
author_rank: Contributor
timestamp: September 19, 2021, 08:24:42 pm
content: I've also tried the "short the pins 2 and 6 of U101" from the manual. While the reading in the 3478a is no longer near the 3.5v of the battery that I hooked up, the noise in the reading basically disappeared. I don't know whether or not that matters though

author: bdunham7
author_rank: Super Contributor
timestamp: September 19, 2021, 08:52:30 pm
content: Quote from: Aphelion on September 19, 2021, 08:24:42 pmWell, it reduces the gain of U101 to x1, which means it attenuates any input noise. If it ranged down into the 3V range, that's 3.33X, so you'd have to judge whether the noise reduced by more than that.QuoteI can't see any reason it won't work. I was unable to find any specs for U101 or to confirm that 1826-1056 crosses to the AD542, but supply voltage, input bias current and so forth all look acceptable.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2021, 08:53:48 pm
content: Shortening pin 2 to 6 reduces the gain from some 3 to 1. So the noise would also go down by a factor of 3 and thus more or less get invisible.As far as I understood it, the 3478 uses the average of 10 shorter conversions in the 5.5 .digit mode. So in AZ mode this would be something like 23 conversions short per second. So the relevant frequency should be around 20 Hz and the effective bandwidth for the ADC at about 3 Hz.So the expected noise from U101 should be the noise density at some 20 Hz times sqrt(3 Hz) times sqrt(2) 2 from the AZ mode using a 2 conversions.for the AD711 this would be about 30 nV/sqrt(Hz) * 2.5 Sqrt(Hz) = 75 nV (RMS) or some 0.45 µV peak to peak.So this should be considerably (e.g. 10 x) better than the current state for the AZ mode. The bias may be higher, but that is OK for a test.So the AD711 should be OK (low enough noise) for a test.The non AZ case may however be relatively poor, as the 0.1 to 10 Hz noise is suppose to be quite high (6 µV). Modern parts may be better than the specs, but one never knows for sure with the LF noise.

author: jeffjmr
author_rank: Regular Contributor
timestamp: March 23, 2022, 01:51:03 pm
content: I know this post is ancient history, but folks with 3478As with similar problems (like me) might benefit from something I found in a newly acquired HP3478A.U101 on this unit is an AD18261921 8 pin DIP. There are signs of resoldering on some of the caps and the battery has been replaced, but U101 looks like it came from the factory. My other, slightly older 3478A has the specified 1826-1056. Somewhere along the line HP changed from the CAN U101 (AD542KH?) to the DIP, but the parts list still shows 1826-1056.Supposedly some chip houses have the 1826-1921in stock.I have not yet compared the 1921 data sheet to the AD542KH that other 3478A threads mention as a possible sub.Has anyone else found an AD18261921 in their 3478A?JeffEdit: No dice on the data sheet for the 18261921. Still awaiting a quote though.