Fluke 732B - blue 10V ref module - current cancellation circuits - Page 1
Forum section: electronics / general

author: iMo
author_rank: Super Contributor
timestamp: September 15, 2019, 05:19:35 pm
content: Is there any knowledge here how the current cancellation circuits on the blue 10V ref board are wired? I've been referring that 3 current cancellation boxes below..It seems to me the 2x AD706 plus some transistors are involved (an active cancellation with sensing the voltage drops and driving the currents?).

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 15, 2019, 06:47:16 pm
content: The full schematic of the 732B is unknown to this, and to other forums. Nobody did a reverse engineering, yet.You might have a look in the 5440 schematics, there they also use several current cancellation circuits, accordingly.Frank

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 18, 2019, 11:09:06 pm
content: I sure would love to see that circuit!Bill

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 19, 2019, 07:07:08 am
content: Sorry, I confused the Fluke instruments...You'll find such current cancellation circuits inside the 5700A/5720A.I append a cut-out from page 514 (7-50) of the service manual, that's A11 DAC PCA.That circuit might be used inside the 732B in a similar manner, probably with different dimensioning.FrankPS: The 2ndcut-out from the same page obviously shows thenegativezener current cancellation circuit

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2019, 09:13:06 am
content: For the reference - I've been referring this moduleOn the left hand side that is the OP97 and the opamps in the mid and right hand side are the AD706 - probably the active current cancellation. There are thick film resistors (plus?) under the blue epoxy paint too.PS: "lymex" is elaborating too -HERE..

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 19, 2019, 11:29:40 am
content: I think, Lymex again has very well re-engineered the 732B module.First time ever that I see this ceramic hybrid.You can very well see the resemblance between the 5700A and the 732B cancellation circuits.No wonder, as both instruments are from the same design era.Frank

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2019, 11:56:08 am
content: He is just guessing, he writes the actual wiring of that module is unknown yet (you have to remove the blue epoxy layer)..PS: afaik nobody here is using that with their LTZ1000/LTFLU/399 boards, would be great to reverse engineer - no need for thick golden wires anymore

author: Theboel
author_rank: Frequent Contributor
timestamp: September 19, 2019, 12:33:38 pm
content: could he use X-ray machine to see the trace ?

author: dietert1
author_rank: Super Contributor
timestamp: September 19, 2019, 01:22:13 pm
content: What the Fluke engineers call current cancellation others call 4-wire method. And i think nobody needs to reverse engineer the Fluke device like the Chinese do. The problem with reverse engineering: Who doesn't understand the design makes copy mistakes without noticing. I have seen Chinese products with unbelievably stupid design errors that probably happened when reengineering existing designs.4-wire method is fairly basic analog technology and every electronics engineer should be able to do that. Lymex linked TI AN-1515 which explains a little how to make a current source if you want to do it the way Fluke engineers did it.Interesting that the photo shows "SZA263" and the circuit diagram has "LTFLU" label. So this analysis isn't really up to date, since we know that those two chips are pretty different. Also i am wondering what may be the TC curve of that reference.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: September 19, 2019, 02:21:43 pm
content: Whether SZA or LTFLU - it does not matter, imho (when talking the CCC).https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/msg214537/#msg214537They laser trim the R-network based on the actual Vreference "Zero TC" current.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 19, 2019, 04:12:32 pm
content: Dieter,these current cancellation circuits have only faint similarities with 4 Kelvin connections, but are really different in their principle.They passively or actively sink/source the driving currents, and 4 wires are not really needed.I really think that Lymex has well explained this subject, and the 3 methods being used.Also, as discussed in depth in this thread, SZA263 and LTFLU are compatible, though their interior might be very different.Also, the basic precision RefAmp circuit is always the same principle, with either chip.The 732B series started on the SZA263 chip, which was later replaced by the LTFLU, very probably using the exact same hybrid.Both chips can be trimmed to zero T.C. @3mA zener current, with 20..100µA collector current, which is obviously done with this glass hybrid resistor array.In the end, I really don't dig your antipathy against this Chinese group, calling them copyists; they are just doing the very same what we are doing here, and the reverse engineering sometimes with much more depth.Frank

author: dietert1
author_rank: Super Contributor
timestamp: September 19, 2019, 05:23:26 pm
content: Every PC power supply nowadays employs separate drive and sense lines for variable current outputs.The current compensation circuitry employed in that Fluke reference is a little overdesigned in my taste. I mean for a voltage reference there is basically one variable current that may create unwanted voltage variations: the load current. All other currents are local and known (delta U = very small resistance * very small delta I). For example my current LTFLU setup has a 500 Ohm resistor for the zener current that sees 2 mV/K change, times 50 milliOhm trace resistance = 0,02 ppm/K in relation to a 10 V line. This is ridiculous in comparison to other effects like the 300 ppm/K of the zener itself.Concerning Chinese reverse engineering habits: This is my personal opinion based on my personal experience. Whenever i looked closer into Chinese electronics products - for example to try a repair - i found horrible errors. One of them was a Behringer measurement microphone that was supposed to deliver a differential audio signal on two pins. It had an amplifier stage to produce the two signals, but one of the signals wasn't used. Instead that output pin was connected to Gnd. Nobody should confuse reengineering results with design documents.Regards, Dieter

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: September 19, 2019, 06:57:29 pm
content: For the current compensation op-amps, I would use the opa187 instead of the AD706. The AD706 (even if in an oven with the Zener) is going to exhibit some offset drift over time, and it also drifts with humidity changes ("seasonal variation"). The opa187 mitigates these drifts, uses only 1/7 of the supply current, and can even be used outside the oven.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2019, 08:02:40 pm
content: Using AZ OPs can be tricky, due to high frequency signal injection that can cause trouble in other parts of the circuit. In addition the OPA187 can have quite some bias current.At most places the current compensation is not that critical. Even if there is some offset error at the OP this would usually give a relatively small net current and if the trance / connector resistance is not too high the final effect to the reference would be rather small. It depends on the details, but for most case I would expect an error attenuation by a factor that is more like 1000 than 100.As the compensation is usually not perfect, the trace resistance should still be low. So a small residual current change due to the OPs offset would not be a large factor.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: September 19, 2019, 08:35:44 pm
content: Quote from: Kleinstein on September 19, 2019, 08:02:40 pmI think everyone is well aware that the inputs of a zero-drift op-amp have spikes on them due to charge injection of the switches (as do the IC power connections). On this forum, and many places on the Internet (and in many application notes) there are circuits that mitigate this charge injection to the point where it is a non-issue. I think that people sometimes make a bigger problem out of things than they really are. For this particular DC application, there is no other solution that I can think of that is superior to the opa187-- and as I already stated, in THIS application, it is FAR superior to the AD706. The opa187 is relatively new, and if it had been available at the time of the initial design of the 732B (and 732C), Fluke would probably have used it.The 732(A/B/C) was designed to operate loads with some current-- for example, the Fluke 720A-- and this current compensation then becomes important. For hobby purposes, unless your reference is going to be driving more current than a high-impedance DMM input would need, these current compensation circuits are probably not needed. That said, I cannot say IN ADVANCE what someone is going to do with a reference I design.

author: AG7CK
author_rank: Regular Contributor
timestamp: September 20, 2019, 12:26:37 am
content: Quote from: dietert1 on September 19, 2019, 01:22:13 pmWrong.The 732B uses 4-wire / remote sensing up to the output posts. So the 4-wire or Kelvin-connection is around 10-20-? centimeters between the reference module and the front binding posts.Inadditionit uses current cancellation of certain internal currents between the Refamp and the output posts ( and reference common). This (localwithin the 732B) current cancellation is active all the time and is independent of whether you plug in any DMM or load to the binding posts.The most import current cancellation is the cancellation of the current in thesensewires themselves, as well as cancellation of internal ground return currents.As it seems that more or less only Dr. Frank has got this point, I would suggest that posters look at the block diagram in the first posts over, and read Dr. Frank's post.And as always: RTFM.

author: AG7CK
author_rank: Regular Contributor
timestamp: September 20, 2019, 12:42:37 am
content: Quote from: Magnificent Bastard on September 19, 2019, 08:35:44 pmThe 732B can deliver 10v at 12 mA. The current cancellation hasnothingto do with loading of the 10v at the binding posts. It is an internal current shifting on micro- and milliampere level that seeks to keep the "10v" and "zero volt common" constant within the instrument itself.

author: AG7CK
author_rank: Regular Contributor
timestamp: September 20, 2019, 01:57:42 am
content: Quote from: Kleinstein on September 19, 2019, 08:02:40 pmWell I don't know. I leave the 732B to the design engineers at Fluke. Since the 732B probably is the best 10v ref around, I guess that they know what they are doing.However, in the sub ppm context of the 732B, the sense leads to the feedback sample divider R401+R402 carries more than half a mA (R402 is ca. 12kOhm). Given that the leads partly are composed by a flex-PCB-strip it is probable that there can be ohmic losses at (sub-) microvolt level. For 10v this could result in reference level differences of several hundred nanovolts (i.e. several 0.1ppm).Since the sense leads electrically are part of the sense divider R401+R402 (10V BETA STRING; whatever that means ..), the effect from the sense leads will deteriorate the performance of this ovenized divider. Hence, the current cancellation drives and sinks the divider current so as to reduce the current (and hence the ohmic loss) in the sense leads to almost nil.As for the avalanche diode in the Refamp U4, the return current is some 3 mA. This current must obviously not return via 10v reference common, and is hence shifted so that it is sinked by an op amp and returned to the psu.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: September 20, 2019, 01:59:57 am
content: Quote from: AG7CK on September 20, 2019, 12:42:37 amAll they are doing is making the binding posts the reference point for 10V and GND for the bottom of the Zener, the bottom of the resistor divider, and the top of the resistor divider. They need the op-amps to do this because the place where these are generated (in the oven) is FAR away from the binding posts-- and the connections are done through polyimide flexible PC ribbon cable-- with very small conductors. If you have even 1mA load on the binding posts there will be a significant voltage drop at the binding posts, and this drop will be terribly temperature dependent. Using the op-amps, they can reduce the current in the sense lines so that there is no drop in the sense lines-- and then the circuit compensates for the voltage drop in the 10V and GND main connections coming out of the oven. Yes-- this circuit works all the time, but if it were not there, you would see a significant drop in voltage at the binding posts with increasing load current. There is also a significant amount of current flowing in the Zener and in the resistor divider that need to be virtually connected to the binding posts (even though they are physically far away).You should not presume that I (or anyone else on this forum) do not know what we are talking about. I have studied the Fluke references for decades, and am VERY familiar with how they work, and why they were designed the way they are. We are not idiots here, and it is insulting to insinuate that we are.Perhaps it's semantics, or miscommunication, but I think we are saying the same thing, but you seem to want to have a fight about it, and I'm not interested in a fight.

author: dietert1
author_rank: Super Contributor
timestamp: September 20, 2019, 07:39:55 am
content: AG7CK: "several hundred nanovolts (i.e. several 0.1ppm" Sorry, 0.1 ppm of 10 V is 1 uV!Maybe the Fluke engineers didn't have stable amplifiers at that time. So the amplfiers had to be inside the oven, which results in this mangled current compensation design.For my own references i will take a more structured approach that separates the task of making a good voltage reference from the task of bringing out/distributing the reference voltage. I guess this leads to a design with two compensation amplifiers, one for plus and one for minus. With modern chopper OpAmps those buffers can sit outside the oven. I guess a third amplifier isn't really needed then.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: September 20, 2019, 08:23:56 am
content: The current compensation is a way to implement 4 wire sensing with sense circuits that are not low current. The current compensation is there to reduce the usually rather constant input current for the sense terminals. This way they can use direct feedback to things like the divider R401,R402 without and extra amplifier for buffering.The current compensation is needed, as a high current could cause quite some errors: For the voltage sense input to R4041/R402 about 0.5 mA and something like 0.1 Ohms of uncertainty in the wire resistance would be 50 µV or 5 ppm of error. The current compensation delivers approximately the right current, so that only a small fraction (e.g. 0.01 - 0.1 %, mainly set by the accuracy of the resistors in the compensation circuit) of the current still flows at the sense terminals. This would reduce the error by that same factor.The voltages in the compensation circuit are on the order of several volts at the critical resistors. So an offset error of the amplifiers of even 1 mV would be less than 0.1 % error in the current, which in the example above would result in 0.1% * 5 ppm = 5 ppb of uncertainty.So there is no absolute need to use OP with super low offset.Also no real need to have the current compensation inside the oven - it is just more convenient to have it there as the current is needed inside the oven. Otherwise if would need more wires in and out.Besides the high frequency spikes, that need some care, the OPA187 also has a higher input offset current (100 pA typical, 500 pA max). In high impedance circuits this gives an offset comparable to the AD706 ( < 50 µV). So it is not even sure the error would be smaller in an unchanged circuit.For the compensation circuit the resistors are usually more critical than the OPs.