need a precise voltage reference!!! - Page 1
Forum section: electronics / beginners

author: saipavan
author_rank: Contributor
timestamp: July 11, 2014, 02:15:55 pm
content: lately i started building a power supply for my projects.i started with lm317 voltage regulator.lm317 regulator did not satisfy me.its output was varying widely from a few milli volts to about 10 milli volts.experiments from different lm317 chips from different sources gave me the same result.so i thought of driving a darlington pair with voltage references and opamp.i tried to get voltage reference from lm336 and tl431.But the output from these chips is also fluctuating in the range of few milli volts.I powered all my voltage references from a rectified smoothedout voltage from a transformer.i measured all the voltages with dt830d multimeter.can any one help me to get out of this problem?also is there a way of getting a stable reference voltage from discrete components?thanks in advance!

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 11, 2014, 02:38:27 pm
content: In the pursuit of "precise," you have a number of additional terms missing:Over what load?Over what Temperature?What value output?Budget constraints?http://www.linear.com/product/LTZ1000Is a highly popular reference when "precision" matters. But it's just a reference. If the rest of your system is not designed with equal precision, you may find yourself getting the performance of a $3 regulator out of $100 in parts.

author: Dave
author_rank: Super Contributor
timestamp: July 11, 2014, 04:07:52 pm
content: Quote from: XFDDesign on July 11, 2014, 02:38:27 pmAre you serious? He is measuring the voltages with a DT-830 and you suggest an LTZ1000. You can buy twenty of those "multimeters" for the price of one LTZ.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 11, 2014, 04:37:59 pm
content: What does "power supply for my projects" mean? Are you building a PS for some specific project, or is this some kind of general purpose "bench" PS for multiple projects?What kind of circuit requires such precision power supply voltage? That seems like a design flaw. We typically try to design circuits without that kind of dependence.If you really have a circuit that requires a precision voltage at some point, why are you relying on the general supply rail for that? That doesn't seem like a great design, either.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 11, 2014, 04:51:08 pm
content: Quote from: saipavan on July 11, 2014, 02:15:55 pmSure, just go to any reputable reseller of test equipment and tell them you need an accurate multimeter because the one you have is anything but.

author: rdl
author_rank: Super Contributor
timestamp: July 11, 2014, 04:52:55 pm
content: The resistors in the voltage divider used to set the LM317 can cause the output to drift if they are not temperature stable. Also, where did you get your parts? Were they purchased from a reliable supplier? And yes, it could just be the <$20 meter is the problem.

author: leppie
author_rank: Frequent Contributor
timestamp: July 11, 2014, 04:57:23 pm
content: Quote from: rdl on July 11, 2014, 04:52:55 pmYou seem to giving too much credit for a $6 multimeter ;p

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 11, 2014, 08:09:13 pm
content: Quote from: Dave on July 11, 2014, 04:07:52 pmHe asked for "precision." Is the LTZ not an excellent standard of precision? You were aware of the earlier questions I asked, too, right? They matter to better shape the application, but in the same sense as "how long is a piece of string" the request on "precision" - it's subjective. One person's dog of a reference is another person's high-precision standard. Additionally I was not aware that knowing all existing products of various T&M gear, including garbage meters, was a requirement to contribute. Now I've learned!

author: c4757p
author_rank: Super Contributor
timestamp: July 11, 2014, 08:15:33 pm
content: Quote from: XFDDesign on July 11, 2014, 08:09:13 pmIf someone asks how to make his car run faster, one does not typically suggest strapping a large jet engine to the roof, even if one happens to be an airline pilot.There's a lot more to "LTZ1000 precision" than just having an LTZ1000. Should probably keep those cans of worms sealed for now.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 11, 2014, 09:48:16 pm
content: Quote from: c4757p on July 11, 2014, 08:15:33 pmLet's not forget there are some characters who actually have strapped jet engines to their car, so the analogy isn't the strongest. But it is true in a general sense, and perhaps why additional questions were asked about the matter - even specific to mentioning that unless the rest of the system is designed to match the same level of precision as the source, even a $100 system can behave as poorly as one built for $3. I must have somehow made all of my post outside of "LTZ1000" invisible to the readers.

author: c4757p
author_rank: Super Contributor
timestamp: July 11, 2014, 09:54:26 pm
content: Quote from: XFDDesign on July 11, 2014, 09:48:16 pmNope, still fits, as I'm fairly certain obscenely overspecced standards get used in people's hobby projects once in a while too.

author: saipavan
author_rank: Contributor
timestamp: July 12, 2014, 08:38:42 am
content: all the voltages from the references as measured from dt830 multimeter were fluctuating at a few tens of millivolts.by precision i want the voltage to remain constant at millivolt range.iam designing a lab bench power supply.so, is my multimeter responsible for these "fluctuating" references?

author: AlfBaz
author_rank: Super Contributor
timestamp: July 12, 2014, 09:03:41 am
content: Simple multimeter check, hook it up to a 1.5 or 9V battery. If it's still fluctuating then it's almost certainly your meter

author: Electronics-Repairman
author_rank: Contributor
timestamp: July 12, 2014, 09:19:49 am
content: We are not going to get a Fluke salesman jump in are we

author: rolycat
author_rank: Super Contributor
timestamp: July 12, 2014, 09:26:57 am
content: The DT830D is a 2000-count meter, which means that for any voltage above 1.999 V you only get 10 mV resolution. It's common for the last digit to fluctuate - that doesn't mean the actual voltage is fluctuating by 10 mV.I have just connected an old home-built LM317 supply to a 6.5 digit Keithley multimeter. With no load and over a period of a few minutes, the output voltage is stable to within a tenth of a millivolt or less.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 12, 2014, 10:03:34 am
content: Some '830' meters even use their own battery as a voltage reference, they're that bad. For checking whether a fuse has blown, or whether a 5V rail is up or down, even that is sort of OK up to a point, but for precision measurement it's worse than useless.If you want to build something that's accurate and stable to +/-1mV then you need a measuring instrument that's guaranteed accurate and stable to +/-0.1mV or better.

author: electron_misfire
author_rank: Contributor
timestamp: July 12, 2014, 11:57:03 am
content: Quote from: leppie on July 11, 2014, 04:57:23 pmYou'regiving too much credit to a $4 multimeter.

author: rolycat
author_rank: Super Contributor
timestamp: July 12, 2014, 12:15:42 pm
content: Quote from: electron_misfire on July 12, 2014, 11:57:03 amThe DT830Bis a $4 multimeter. The OP's DT830Dis the deluxe model with a continuity buzzer

author: amyk
author_rank: Super Contributor
timestamp: July 12, 2014, 12:30:15 pm
content: The '7106 in the 830s and related DMMs is capable of ~0.1% accuracy, it's the circuitry around it that's usually the letdown.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: July 12, 2014, 01:17:15 pm
content: Quote from: saipavan on July 12, 2014, 08:38:42 amYou will have to test your meter to some standard to determine that. Someone mentioned testing a battery. This is a great standard since they're happy to output a voltage without a load.As to your supply design, you still haven't specified over what kind of load conditions, or the range of output voltages you intend on generating. It makes it quite hard to give you a meaningful answer. None the less, may I ask what precise application demands 1mV of resolution as far as your power supply goes?To be quite blunt, the majority of use cases for a power supply is a semi-precise power source to get you started. You would likely still apply point-of-load regulation or some kind elsewhere on your actual circuit. With any amount of actual loading, your supply would require a sense line to get higher levels of accuracy.Consider the following example:Setting your output to 10.000V, with a current draw of 1A by your end device. Your supply leads each have 100mOhms of resistance. The point of load is at the other end of your board relative to the entry of the power, and has 100mOhm in trace resistance before the load actually "sees" the supply. In total, we have an equivalent series resistance of 300mOhm between the source and your load. At 1A, that's 300mV of drop. Your 10.000V source looks like 9.700V at the load (assuming fixed current load).Now a Kelvin sense would have an additional set of wires that run in parallel to your power load. They still have the same elemental resistances. Once they hit your board, you have two additional traces for these sense lines, which go all the way to the points of your output. The sense lines do not carry a significant current (usually just leakage and bias currents in the order of a few tens of nano-amps) so they report a very "honest" answer of 9.7V. In this case, this power supply sees that it isn't outputting 10.00V to the Point-of-Load, but 9.7V. The supply's feedback network would then increase the output amplitude until the sense lines report 10.000V.This would be a much more complex design than one I would recommend a novice to take on.For your first power supply, I wouldstronglyurge you to lay out some specs for "good enough." A good reference is required still, but in this case "good enough" would do. I personally am fond of this reference:http://www.analog.com/static/imported-files/data_sheets/AD780.pdf- it's still more expensive than you need, but it's also excellent and pretty robust without being LTZ1000 grade good. It's still made in a DIP package too. If you go for this part, I think you would be fine to get the A grade (±5mV). If that is still too expensive, then the de-facto IC standard is the LM4040 (http://www.ti.com/lit/ds/symlink/lm4040-n.pdf) which comes in a TO-92 package. Note, that it requires a bit more outside of the device to be useful.I also asked the question, "over what load?" which did not get an answer. Here is the thing: most regulators need a load to work well. Without power transistors of some kind following the output of an OpAmp, you can get a pretty stable output with an OpAmp circuit that has no load (it's really loaded, but by the feedback network). Now if you place a power transistor like the 2n3055 or a darlington pair to follow the output of the OpAmp, you need to get a load on the emitter of the new device (bias it). This is actually what is more likely than not, causing "odd" readings with your LM317 (though I hate that device and never use it), it might be worth sticking a 100 ohm load on your output and see what comes of it. If your load is 0-10mA, just use a higher output current op-amp. If your load is 0-100mA, then an OpAmp with a 2n2222 would do fine. 0-1A or more, well then it gets a bit more tricky.Back to accuracy: let's assume you decide to use an AD780 reference, with an OpAmp that has a 2n3055 as a follower, with a cap and some resistive loading to bias it. You'll use the block in a non-inverting configuration, with a 10kOhm, 10-turn pot as the feedback resistor from the output of the emitter to the inverting terminal of the OpAmp. A 1kOhm resistor follows from the inverting terminal to ground. Ideally, you would have an output range of 2.5V to 27.5V assuming you had the source supply to handle this. (Gain range is 1 to 11). Now, let's evaluate the corners of variation just due to the source varying up to 5mV: At minimum: 2.495 < Vout < 2.505. At a maximum: 27.445 < Vout < 27.555. This is an error of 0.2% but in terms of voltage, it's 55mV of error! The nice thing is, since you're using a multi-turn pot in this case, you can dial it in exactly to what you want. Your actual limit is only at the low end.

author: Flump
author_rank: Frequent Contributor
timestamp: July 12, 2014, 02:10:16 pm
content: I dont know what voltage you need or if these are suitable10v -http://www.ebay.co.uk/itm/261441842541?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l26495v -http://www.ebay.co.uk/itm/180753956382?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

author: SeanB
author_rank: Super Contributor
timestamp: July 12, 2014, 02:19:09 pm
content: My first voltage reference is an AD581KH, which is good enough to check most 3 1/2 digit meters on the 20V range to 2 decimal points and to check the 200V range to 1 decimal point. More voltages would be nicer, but it at least is cheap and simple to use.

author: rob77
author_rank: Super Contributor
timestamp: July 12, 2014, 02:53:31 pm
content: actually i'm designing a symmetric"lab" supply for my hobby projects and it's based on LM317 + LM337 + few opamps + TL431 reference + atmega + 2 chan. 12bit external DAC + 8 chan. external 12bit ADC - the short-term stability is good enough for a hobbyist. the designed relosution is 5mV and the accuracy and short term stability of the "analog" part is below the resolution (<5mV fluctuations). so i do expect overall accuracy of 10-25mV short-term.btw... why do you need a precision supply in the first place ? i'm designing mine to build a computerized V-A measurements (matching diodes, transistors....etc).

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 12, 2014, 03:21:41 pm
content: If you have short-term stability, then you can achieve accuracy by running a calibration routine and store the calibration factors in software. This is a common procedure for large automated test setups.I wrote the software for an automated test machine that helped developed the Pentium family of CPU chips. We had a test jig with precision resistors that we used to check and calibrate the test equipment. We sent the test jig off to the calibration lab every month to have them tell us what the exact measurements were so we could calibrate the equipment daily. The "calibration" was just a DOS file with the correction factors (gain, offset) for each source and each meter stored on an 8-inch floppy disk.

author: robrenz
author_rank: Super Contributor
timestamp: July 12, 2014, 03:30:37 pm
content: What happened to the OP ?