100VDC reference circuit - Page 1
Forum section: electronics / projects

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 19, 2012, 09:27:12 pm
content: Hello there,I am adjusting my 6 1/2 digit DMM, and I need to input several reference voltages. 100mVDC , 1VDC, 10VDC and 100VDC. My idea is to scale the voltages down from a 101V reference voltage to 10.1V, and from 10.1V to 1.01V and from 1.01V to 0.101V using voltage dividers. Then I would use potentiometers to fine tune the voltage to say 10.00000 or 1.00000.In one of Dave's episodes (#210 Krohn-Hite DC Voltage Standard), he says it is possible to make a stable reference voltage from a zener and constant current source.This is my attempt. I was hoping you could supply feedback to improve the design, or suggest a better idea if possible. I love to learn.Attached is my LTSpice simulation and circuit schematic. As you can see, the voltage isn't completely stable. R2 is the multimeter. The two npn are the constant current source.Thank you for your time.Kind regards,MariusEDIT:I see that I have put my capacitor the wrong way around.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 19, 2012, 09:44:17 pm
content: Here is updated schematic with C1 with corrected polarity.M

author: Unknown
author_rank: Unknown
timestamp: February 19, 2012, 10:07:31 pm
content: Weren't the DC voltage ranges OK according to your quick performance test?I'm not so sure if a zener with constant current supply would be the most stable solutions, zeners tend to be fairly noisy. Their ripple rejection is worse than your typical three-terminal regulator, which is why you'll rarely see power supplies with just a zener and a pass transistor anymore. Dividing the output voltage and using a precision op-amp to compare the divided voltage with an IC reference (which is basically what a three terminal regulator does) should be more stable. I'm not sure if you need anything fancy, however. Just using a tree terminal regulator and make sure its adjust terminal stays close (< 30 V) to the input voltage might be enough (just don't short the output). With three lab supplies in series I got 100 V with a standard deviation of about 1 mV, and these were just basic linear bench supplies.Make sure the current is very limited, since you don't need much current and 100 VDC can be dangerous.Try searching theVolt nutsmailing list archive for information, although most only bother with a 10 VDC reference, since they can divide any higher voltage down to 10 VDC and compare them. They also aim for a very low long-term drift (something you don't need) and even lower noise.

author: amspire
author_rank: Super Contributor
timestamp: February 19, 2012, 10:44:25 pm
content: Sorry to say, but the sort of circuit you are considering will not be even vaguely stable enough. If you connect your dvm to it, you will see the voltage just keeps drifting before your eyes.First, high voltage zeners never used in precision references - way too much temperature coefficient. Zeners only can have a zero temperature coefficient somewhere in a range from about 5.1V to 6.2 volts, and so the precision reference zeners are specially manufactured and selected for low temp coefficient at a specified current. Even with a great zener, designing the matching constant current source takes experience too, so forget it.Go instead for a good 10V IC reference like a a REF01, or something else from companies like Analog Devices or Maxim. Take a good look at the specs though. 5ppm / degC means that if you calibrate the DMM at 5 deg C different temperature from the temperature you calibrated the reference, you could be out .0025%.Now the circuit will be a 9:1 voltage divider to divide the 100V down to 10V, a good opamp that regulates the divider voltage and the reference voltage to be the same, and a few transistors on the opamp output to allow it to control the 100V voltage. I can give you some tips there.Next thing is the divider. With your meter, you will find nearly all resistors have a very noticeable temperature coefficient. The coefficient of metal film resistors is just huge, and the 90V across one resistor will heat it enough to start drifting very happily. One solution is to get a thick film 10:1 divider - there is a standard multimeter divider that is excellent.Otherwise the best you can do is to get batch of 100K 1W metal film resistors all from the same manufacturers batch, and make the divider from 10 of them. This way, every resistor will be dissipating the same power, and typically the temperature coefficients will match to within a few PPM with luck.Last to calibrate the reference is just a matter of calibrating the divider, and for that you add an extra switch to turn the divider in to a Hamon divider (see Conrad's pagehttp://conradhoffman.com/HamonResistor.html).What you will do is close the switch and the divider will be 2:1 - ie the output will be 20V. Adjust the divider so that the reference to output voltage (10V) is exactly the same as the reference volts. Your meter can do this even uncalibrated. Then open the switch and the output is a precision 100V DC.Last with reference circuits, even with a stable room temperature, you always have to turn them on and let the temperatures stabilize for at least 30 minutes before using them for precision work.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 20, 2012, 12:06:57 am
content: Quote from: alm on February 19, 2012, 10:07:31 pmYes, they were okay, but I didn't think they were fantastic.Quote from: alm on February 19, 2012, 10:07:31 pmAlm, Richard, just a minute. Are you suggesting I amplify the 10V reference to 100V?Perhaps using something like this:http://search.digikey.com/us/en/products/PA341CE/598-1918-ND/2700702?M

author: vk6zgo
author_rank: Super Contributor
timestamp: February 20, 2012, 01:15:44 am
content: No,Richard is saying to divide the 100 V down to 10V,then compare it with the reference.The resultant control voltage,which is dependent on the error, is amplified & used to control the !00v supply (normally via a pass transistor).The old way is to use a gas filled regulator tube,but these tubes are rare.difficult to use,& need to be chosen on test toobtain any reasonable degree of accuracy.

author: IanB
author_rank: Super Contributor
timestamp: February 20, 2012, 01:22:54 am
content: It's basically true, though, you are taking a low voltage reference and multiplying it up. This will also multiply any errors.The ideal way would be to take a 1000 V reference and divide it down, but 1000 V references are hard to come by.

author: amspire
author_rank: Super Contributor
timestamp: February 20, 2012, 03:20:50 am
content: Quote from: king.oslo on February 20, 2012, 12:06:57 amI am suggesting using a precision divider with an opamp to make in effect a 10x amplifier. I will do a quick sketch.QuoteWay to expensive. You just want to able to use a standard op-amp with some extra circuitry to go to 100V. That will be in the sketch.

author: ivan747
author_rank: Super Contributor
timestamp: February 20, 2012, 03:28:56 am
content: Quote from: amspire on February 20, 2012, 03:20:50 amThey won't send him a free sample, right?

author: amspire
author_rank: Super Contributor
timestamp: February 20, 2012, 05:41:05 am
content: Attached is a sketch of a 100V ref concept.This arrangement will get the best stability and accuracy that you can get from cheap parts (especially the resistors) and the 100V can quickly and easily be calibrated from the10V output.It is only designed to give a few milliamps output, which is all a reference supply needs to do.The op-amp should be a low offset FET-type opamp - the slower the better. The faster it is, the more likely it is you will have stability issues.I added Q2 to eliminate the effect of the miller capacitance of Q1 (to help with stability), so Q1 can be any low voltage small signal NPN transistor. Q2 can also be a high voltage NPN transistor instead, and the lower the power rating, the better (about 0.5W maximum dissipation with 200V input) . Q2 just has to have a voltage rating greater then the maximum possible voltage from the transformer. The reference is inherently short circuit proof, thanks to the 22K 1W resistor.The gate of Q2 ( or base if you use a NPN transistor instead) is connected to the 10V ref output, just because is was a handy 10V source, but any voltage between 10 and 15V is fine, so if you have a +15V supply for the reference and opamp, you can use that for the gate of Q2 as well.For the 100K resistors, get the best quality 0.5W to 1W 100K metal film resistors that you can find at a good price, and you need to select them. The 9 top resistors are organised into 300K triplets, and you want each triplet to match within about 0.01%. The single lower resistor needs to be selected to be lower then any of the other 100K resistors, so we can add the series adjustment pot.Stability issues will come from the fact that the Q1 is increasing the loop gain by about 22 times. This extra gain means the inbuilt opamp compensation probably will not be sufficient to get stability, and some extra compensation will probably be required.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 23, 2012, 10:47:16 pm
content: Thanks Richard,I understand how it works (finally)!I will build this up within the next few days.I am shopping parts, and an enclosure for it (perhaps with a heater? What do you reckon I could use? I can get 17W wire-wound resistors reasonably). I need to work out which parts to buy.With two electrolytics of 22uF the Vppwill circa 5V. Is this fluctuation going to be a problem in order to get very stable 100VOUT? Do you recommend different type of cap? Would it not be better with 2 x 4mF?Second, I cannot get a 10.00000V reference. But I can get one which is 10V +-5mv:https://www.elfa.se/elfa3~eu_en/elfa/init.do?item=73-069-43&toc=0. What is your opinion?Op-amp wise. The slowest I can get is 88Hz. Surely, this slow it will perform poor regulation. What do you reckon is optimal frequency? And how to power it?Would you advice additional output capacitance?Thanks.M

author: amspire
author_rank: Super Contributor
timestamp: February 24, 2012, 12:09:58 am
content: First the reference. All the precision reference IC's allow you to attach a potentiometer for calibration. Add this with a link so you can go from the factory calibration to your calibration. There is every chance your meter calibration is very good. I would test the reference and if it looks like it reads within the manufacturers specs, then adjust it so it reads 10.00000V on your meter. The thing is most of the time, an error of 0.01% is not a big deal. What you want is all the ranges of the meter to match, so if you had 10 of these reference chips all adjusted to be exactly 10V according to your meter, then if you put them in series, you want your meter to read 100V exactly.Now in picking the chip, look more at temperature stability and long term drift rather then accuracy. I would prefer a 1ppm/C 0.1% reference chip to a 5ppm/C 0.02% reference. I cannot go looking at data sheets right now - doing some hard disk recovery work, but Mouser and Digikey searches are a good starting place.Secondly, you need to get a feel for how much resistors vary when you have a meter like yours. Grab a metal film resistor - perhaps 10K, but the value does not matter. Measure it on a handheld DMM. The reading will probably look absolutely rock steady. Now put it on your dmm. Watch the values drift. Just the sub-milliwatt heating of the resistance test is enough to make the resistor change with a 6 1/2 digit meter. Touch the resistor with the tip of a finger, and you will really see the value change. So This is why it is so important that every resistor in the 100V divider is dissipating the same power exactly. Now when you close the switches in the Harmon divider, this is no longer the case, so you have to change your technique. You let it stabilize first with the switch off. Close the switch and adjust the output to be exactly double the reference (as described before) and open the switch. Let it settle again. Quickly switch the switch on and adjust again (you have to do it in a few seconds). Keep repeating until the initial output voltage just after you turn the switch on is exactly double the reference. Then you will know that with the switch off, you have an accurate 100V.Third, the opamp. By slow, I meant that a 1Mhz (or even 100K) opamp will probably be easier to get stable then a 10MHz opamp. Here is the story about stability. In a feedback circuit, there is exactly a 180 degrees phase shift between the output and the opamp input connecting to the feedback at DC (ie you are connecting to an inverting input). In my circuit, we connect to the + input as the transistor is inverting the output. As the frequency increases, capacitances causes gain fall offs and at the same time, phase shifts. If the sum of all the phase shifts reaches 180 degrees and the gain is equal or great then 1 from the opamp input to the circuit output, the circuit will oscillate.Basically if you have the output connected back to an inverting input, and the circuit has a 180 degrees phase shifts due to capacitances, then the signal the input sees is now in-phase (non-inverting) compared to the output and instead of negative feedback, you have positive feedback. Positive feedback is always unstable if the gain of the feedback loop is 1 or greater. Positive feed back means that if the output drops a little (you added a load) the signal the opamp input will see will say "drop the output even more", and this is the start of an oscillation.So the trick they use in internally stabilized opamp is they add a single RC (resistor-capacitor pair) fall off that swamps out all other capacitive effects. So above a few Hz, the opamp has a 90 degree phase shift from this internal RC for AC all the way down to the opamp's unity gain point and so it is stable with just about any load other then a straight capacitive load (which could add another 90 degrees shift and make it unstable). The opamp designers do not care of the phase shifts below unity gain get really ugly as that does not affect stability. Now to boost the output to 100V, I have added an extra gain of 22 times, and the transistor circuit will have some phase shift of its own. This means that we can get oscillations even down to the point the opamp has a gain of 1/22, and as I mentioned before, the opamp can look really ugly for phase shifts down there. So you will probably have to add some extra frequency compensation to get the circuit stable. If you are into LTSpice simulations, it is a good exercise, but otherwise, playing with the 100P capacitor in my circuit is a good starting point.Hope that makes some sense.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 24, 2012, 12:38:42 am
content: It makes sense Richard! Will be back with some pictures or video when I get something done.M

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 24, 2012, 12:01:01 pm
content: Opamp has to go all the way to VIN, right? In my case 325V?M

author: amspire
author_rank: Super Contributor
timestamp: February 24, 2012, 12:25:26 pm
content: No, the opamp will run from a supply like a 15-20V single supply. The input should operate at less then that.

author: Richard W.
author_rank: Regular Contributor
timestamp: February 24, 2012, 12:52:39 pm
content: At the moment i'd NOT readjust the meter. The specs aren't too bad.I'd redo all measurements with improved enviornmental conditions,doing exact measurements isn't easy at this level.~ Let the meter warm up. Many many hours, the longer the better.~ Let the meter in place while do the measurements. The displayed values will drift, if you move the meter. Why?1. Temperature distribution will change2. The boards inside the meter will bend a little bit. Also bad.~ Clean the plugs and sockets; contact resistance and thermoelectric effects are evilHere is a really nice brochure how to do it right:http://www.keithley.de/data?asset=55835But if you want to build a reference, here are several appnotes to pick the right chip:http://cds.linear.com/docs/Application%20Note/an42.pdfhttp://pdfserv.maxim-ic.com/en/an/AN2879.pdfHere you can see what happens if you mechanically stress components (especially voltage references):http://www.amplifier.cd/Technische_Berichte/Spannungsreferenzen/Spannungsreferenz.html(the google-translator link would have been too long...)regards

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 24, 2012, 12:53:24 pm
content: Thanks Richard W. I will compare the meter with another reference before I decide to adjust it. I will post information in the other thread when I have it. Still, this is a great exercise, and nice to have a reference for both DC and AC!Richard (without W), I am probably mistaken: The 20V opamp will only open the npn transistor to conduct 20V-0.65V = 19.35V away from the 100V rail? So the 100V rail can only be about 120VAC?

author: amspire
author_rank: Super Contributor
timestamp: February 24, 2012, 01:39:04 pm
content: Quote from: king.oslo on February 24, 2012, 12:53:24 pmThe NPN connected to the opamp output works at low voltages. It functions up to aboutn the voltage on the gate/base of Q2. Q2 handles all the high voltages and is the only device needing a high voltage rating. I would probably aim at a supply of 130V to 200V,

author: Unknown
author_rank: Unknown
timestamp: February 24, 2012, 10:08:31 pm
content: Quote from: Richard W. on February 24, 2012, 12:52:39 pmSome voltage references, including the LM 199/299/399 used in the later HP 3456A (and most other 6.5 digit) multimeters, are also sensitive to position. Its output voltage will vary slightly depending on orientation. Not sure what the exact mechanism is, I believe there was some speculation on the volt-nuts mailing list.

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 24, 2012, 10:16:12 pm
content: Quote from: Richard W. on February 24, 2012, 12:52:39 pmRichard W, these were very interesting. Thank you.Quote from: amspire on February 24, 2012, 01:39:04 pmOk. There is certainly something which I have misunderstood about the circuit.Say the supply is 150VDC at the drain of Q2. But Q2 will always be conducting current, because the 10V ref connected to the gate is always on. Therefore, almost all of the 150V will be conducted to the collector of Q1. When the opamp output conducts higher than 0.65V, Q1 starts to conduct current away from the 150V supply. If the opamp has 20V supply, Q1 will only be able to conduct maximum ~19.35V away from the supply.The rest of the current is lead away by the hamon divider. Perhaps ca 11V?Is this incorrect? As far as I can tell, the circuit cannot reduce the voltage of the 100V rail to lower than 150V-19.35V-11V = ~120VI think I am about to learn something newThank you for your time.Kind regards,Marius

author: amspire
author_rank: Super Contributor
timestamp: February 24, 2012, 11:19:03 pm
content: The voltage on Q1 cannot get higher, then the Q2 gate voltage as if that happened, Q2 would be turned completely off. Basically all the current in Q1 has to also go through the drain of Q2 (nothing flows out the gate) and the collector voltage of Q1 will settle at exactly the voltage required by Q2 to turn on enough to conduct the current from the Q1 collector. The result is the Q1 collector voltage is always less then the Q2 gate voltage.As I said this circuit works just as well if Q2 is a NPN transistor instead of a MOSFET, and it might be easier getting a small high voltage transistor then a low current high voltage MOSFET. If you have any old TV's or CRT monitors to scrap, they almost certainly have some suitable low power high voltage transistors.What this circuit does for you is it means that the collector voltage of Q1 is only changing by a small amount in in its full range of regulation - perhaps a fraction of a volt, rather then by 200V (or whatever the supply voltage). This greatly reduces the effect of Q1's collector to base capacitance.Now you could just use a high voltage NPN for Q1 and eliminate Q2 totally. Probably worth a try too just to see what happens.The reason I called it a concept circuit is I haven't done any breadboard, simulation or detailed calculations, but the idea is sound, and I am leaving it to you to breadboard it and try it out. There are circuits that have been built and tested - I seem to remember seeing one somewhere in one of the Jim Williams Linear Technology application notes for a high voltage regulator, and all you have to do is replace the divider in the circuit with the Harmon divider. But if you an make this circuit work, it is a good one for supplying a low reference current as the current is limited by one resistor which makes it totally short circuit proof. Even if the circuit doesn't work, nothing disastrous should happen (as long as you keep your fingers off the high voltage).It is the kind of circuit you can experiment with.To breadboard the circuit. you can start off without the voltage reference IC - just have a 15V supply for the opamp, and use a 470 ohm and a 1K resistor as a voltage divider for the 15V to supply about 10V to act as a reference. Add an electrolytic cap to ground. That is all you need to debug the regulator for the output.Richard.

author: Richard W.
author_rank: Regular Contributor
timestamp: February 25, 2012, 01:30:00 am
content: Quote from: alm on February 24, 2012, 10:08:31 pmMy "why?" was a rhetoric question.As i said before, i think it's caused by mechanical stress to the semiconductor itself and temperature variations.Thats also the reason why there are often slots in the pcb around the reference chips.if i finally have a new oscilloscope and some time, i'll check the theory. A LM399 is about 15€ at digikey, i'll order two or three and try it

author: king.oslo
author_rank: Frequent Contributor
timestamp: February 25, 2012, 03:10:57 am
content: Quote from: amspire on February 24, 2012, 11:19:03 pmLatest update: I built the circuit in LTSpice. I used a 50nF + 70ohm as compensation. That seems to make it stable (no idea why).it seems to regulate some current away from the 100V rail, but not much. Perhaps 20V. No idea why. At the minute I regulate the output voltage by regulating the supply. I will look at it in the morning. I am too tired now to make sensible decisions.Thank you for your time.Kind regards,Marius

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 08, 2012, 11:20:00 am
content: In an other thread Richard helped me overcome stability issues. Thank you.I made three tweaks. My goal is to be able to use 230VAC without a step-down transformer to ~115V. I moved the 1k resistor and swapped the 200pF Coutto 470u. The simulation indicate very little fluctuation.I would like to proceed to build this up (and perhaps post photographs or video), unless you have concerns with the big output cap.Thank you for your time.

author: amspire
author_rank: Super Contributor
timestamp: March 08, 2012, 11:52:42 am
content: You definitely do not want to move the 1K resistor. Even the leakage current of the capacitor will cause a significant error.Don't get too worried about getting the AC filtered out completely, as the DC range on the meter is very good at rejecting mains AC noise.About running straight off the 240V. Yeah it can be done, but I don't like it.The mains neutral is still live mains, and the neutral will have a lot of noise to ground. If a socket has the neutral and active leads swapped, then you are connecting your earthed meter straight to an active mains.It is easy to use a voltage multiplier rectifier, so basically you can use a transformer with any secondary voltage. Do you have any transformer at all?Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 08, 2012, 11:59:52 am
content: Yes. I have a transformer I was going to use for a PSU, which was ditched. It has secondary: 2x18V = 36V. Perhaps this can be multiplied by three to get 108VAC? I will search google to figure out how that works.M

author: amspire
author_rank: Super Contributor
timestamp: March 08, 2012, 12:17:32 pm
content: Use a voltage multiplier like this:http://en.wikipedia.org/wiki/Voltage_multiplierThat transformer can supply 100V p-p. So the circuit in the first diagram will give 200V. 4 diodes and 4 capacitors. You will not need that first 22uF cap I put in the circuit (C1) since this multiplier circuit has its own capacitors.The current is low - diodes can be the cheap 1N4004 are more then you need. Even a small signal diode rated at well over 100V is fine. The capacitors need to be something like 47uf to 100uF electros. Rated at over 100V obviously.Richard.

author: Unknown
author_rank: Unknown
timestamp: March 08, 2012, 06:30:23 pm
content: Quote from: amspire on March 08, 2012, 11:52:42 amAgreed.Quote from: amspire on March 08, 2012, 11:52:42 amEarthed meter? I've never seen a DMM which connected one of the inputs to PE, although that doesn't mean they don't exist.

author: amspire
author_rank: Super Contributor
timestamp: March 08, 2012, 10:49:34 pm
content: The meter is earthed. The inputs are not earthed, but there is a limit to the voltage above earth that you can apply. The floating parts of the circuit internally are capacitively linked to earth through the transformer windings.Even if the meter can easily float the input with a 240V AC signal, that is not really the point. The point is you are trying to check the calibration of a 6 1/2 digit meter (or whatever it is), and you are applying potentially hundreds of volts of common mode noise. When you are making precision circuits, it is all about doing everything you can to eliminate sources of errors. Adding a few volts noise to ground via the mains neutral, or hundreds of voltage via the mains active is a very bad start.Richard.

author: Unknown
author_rank: Unknown
timestamp: March 09, 2012, 12:17:29 am
content: Quote from: amspire on March 08, 2012, 10:49:34 pmIs there any meter where the max. voltage from low to ground is less than 240 VAC? It's 500 V peak for the HP 3456 A that is the DUT in this case as far as I know.Quote from: amspire on March 08, 2012, 10:49:34 pmFair enough, your comment "connecting your earthed meterstraightto an active mains" (emphasis mine) suggested to me that you considered it a safety issue.

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 20, 2012, 11:13:53 pm
content: I built it up now. I am pretty excited about the output stability. I am looking forward to showing you.My question today, it seems that it is possible to trim the LT1031DCH voltage reference by driving the ground pin. The example in the datasheet has a potentiometers connected between VOUTand a -15V supply. The wiper is connected to ground.I suppose it is best to use a really stable -15V supply. Can you give me advice on how to achieve this?Thank you for your time.Kind regards,Marius

author: amspire
author_rank: Super Contributor
timestamp: March 20, 2012, 11:52:22 pm
content: That looks like a pretty old design of reference. Doesn't mean it is bad, but all the modern 8 pin references have a much easier way to add trimming that does not need a separate supply or as much stability.It is not just that you need to negative supply - it is also that after adding the negative supply, you may not actually need it. It is only there for the case that the reference voltage needs to be adjusted down. If your regulator is 9.998V, you could adjust it with just a positive supply only.To answer the question, you do not need -15V. -10V is better. I bet they used -15V for the simple reason that the engineer writing the app notes up had a -15V supply already available. To get a very stable -10V, you use a standard inverting opamp circuit connected to the 10V reference out. Since the power in the two opamp resistors will be identical, they will track thermally pretty well.A better option might be something like the Maxim MAX6350CSA+ (about $11). 1ppm temp stability.But you are probably going to say it is only 5V.Ok no big problem. Get two of them and two plastic cases. Power each one up from a 9V battery (They need 8V or more for the supply) and put the two in series to get 10V. Plus you can compare them against each other to check for drift.Richard.

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 29, 2012, 09:56:38 pm
content: Ok, guys.For the AC reference I need a negative and positive supply. I thought I would use two 17V zener diodes plus positive and negative LDO regulators.Attached is my circuit schematic. When I simulate the two zener regulators in LTSpice, it simulates correctly, but when I built it up, it is not.The problem is that I cannot get the correct measurements across one of the zeners. The drop is only circa 0.9V across D11 (the lower one). The drop across tje other zener is showing 18V, which is ok.Any ideas?Thanks.M

author: amspire
author_rank: Super Contributor
timestamp: March 29, 2012, 10:50:32 pm
content: Sounds like you have a dual 56V RMS output transformer.Use each winding for each different reference - then the two are isolated.You can use half wave rectifiers for the +/- 17V. and a voltage doubler for the supply for the 100V reference.Make sure the power rating for the resistors are high enough. If you want say 20mA peak output on the 17 rails, the divider resistors will have to be about 2W power rating.Richard

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 29, 2012, 10:55:10 pm
content: Thanks Richard,Are you able to work out why my circuit doesnt work like it does whilst simulating?Kind regards,Marius

author: amspire
author_rank: Super Contributor
timestamp: March 29, 2012, 11:13:36 pm
content: The wire from the negative of the lower filter capacitor should be connected to the positive of the top filter cap, not the bottom. R16 should be in the wire near the -69V and replaced with a short in its current location.Edit: The big problem with your circuit is the "ground" for the sinewave circuit is different from the one for the 100DC circuit.You could not directly connect both to mains ground at the same time which is nasty. With my suggestion, the two references are seperately isolated so you can connect them how you like without any problem.

author: king.oslo
author_rank: Frequent Contributor
timestamp: January 12, 2013, 06:35:37 pm
content: Hello again,I have been working on this on and off. My motivation took a blow when I one evening wrecked yet another one one of the LTC1052s. Coupled with the frustration of building on Veroboard, I decided I would take a break. But I am feeling fresh again. But I wish I knew why these opamps keep on short circuiting inside.Amspire, I hope you are well. I remember I had at least one concern. When SW1 was closed, I measured 10VDC at the output relative to VREF. That was great. The problem was that the measurements on the DMM would jump around about 300ppm max for each new reading. Output of the voltage reference was stable with 1ppm fluctuations. What do you reckon it could have been? Diode? Potentiometer?And everyone else, I hope you are well too. Have tested the accuracy of the Geller Labs SVR boards against a traceable source? On their website, they say that stating that it has 10ppm absolute accuracy is conservative. Sounds interesting.http://gellerlabs.com/SVR%20M.htmlThank you for your time.M

author: quarks
author_rank: Frequent Contributor
timestamp: January 12, 2013, 07:00:13 pm
content: Quote from: king.oslo on January 12, 2013, 06:35:37 pmI do have two with calibration data from Joe Geller and two more boards to experiment. I can highly recommend it and communication with Joe Geller is always very helpful.Attached are two graphs (TEK DMM4050 was running the recommended >1h to warm up, set to 100 NPLC, the x-axis represents 4:41h time)edit: the one spike in the middle was me moving things while measuring was still running