How good are these parts if I build a voltage reference out of it? - Page 1
Forum section: electronics / testgear

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 09:00:33 am
content: I am thinking about building a voltage reference, using TL431 as the voltage standard and OP07C as the precision amplifier. How good will that be?And since I don't have those sexy decade switches, would using reed relays affect the precision? How to select the resistors and trim the voltage standard? Will the microcontroller controlling the relays affect the output, since the control to the reed relays obviously have to come from a string of 74HC595s on the microcontroller's SPI bus? Is the "dial in a voltage on a keypad and hit go" a good user experience?One step further, since a microcontroller is called for already, does it make sense to make it computer controlled? Is there any existing protocol available?

author: mzzj
author_rank: Super Contributor
timestamp: May 05, 2015, 09:46:36 am
content: Quote from: technix on May 05, 2015, 09:00:33 amGood or not depends what you need. TL431 would be about same level of accuracy as voltage standards 150 years agoThere is probably about a metric ton of better voltage references than TL431.

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 12:05:06 pm
content: Quote from: mzzj on May 05, 2015, 09:46:36 amPlease suggest a part. I have checked recently that even from Shenzhen suppliers selling parts salvaged from other boards, LM399 and LTZ1000 are expensive like hell, costing US$10 and US$50 each.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 05, 2015, 12:29:58 pm
content: Quote from: technix on May 05, 2015, 12:05:06 pmThe whole point of having a voltage reference to use as a standard is stability. You are not going to get anything standard-worthy out of a 431. Unfortunately, better things usually come with a heftier price tag. There are reasons why the 399 and LTZ are such well-established go-to reference parts.

author: mzzj
author_rank: Super Contributor
timestamp: May 05, 2015, 01:17:42 pm
content: Quote from: technix on May 05, 2015, 12:05:06 pmREF02 is pretty common and you should be able to find some for 1-2usd/piece or less. Even brand new should be like 3-5 usd.It's nowhere as good as LM399 or LTZ but much much better than TL431Some others that you might find: ADR4550, REF192, MAX6050

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 01:22:44 pm
content: Quote from: mzzj on May 05, 2015, 01:17:42 pmHow good is MC1403? The datasheet quotes a 10ppm tempco.

author: c4757p
author_rank: Super Contributor
timestamp: May 05, 2015, 01:29:38 pm
content: If you can't spend $10 on an LM399, you probably don't need a voltage reference at all.

author: mzzj
author_rank: Super Contributor
timestamp: May 05, 2015, 01:34:36 pm
content: Quote from: technix on May 05, 2015, 01:22:44 pmWell its something like 10 to 100 times better than TL431 so a definite improvement.Probably good enough to start playing around. If I remember correctly your workbench didn't include fancy 8,5 digit meters or anything like that.

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 02:02:51 pm
content: Quote from: c4757p on May 05, 2015, 01:29:38 pmI don't have a 8.5 digit DMM to justify that - I just need something that can be used to calibrate my 3.5 digit meters, sort of a crude version of a transfer standard that I will send to a calibration lab.

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 02:06:51 pm
content: Quote from: mzzj on May 05, 2015, 01:34:36 pmNo all I have is 3.5 digit old ones. If I am to send one down to a calibration lab, should I finalize the design and send down that digitally controlled voltage standard assembly, or just the reference chip itself? It is just a working standard I will be used to calibrate my gear against.

author: mzzj
author_rank: Super Contributor
timestamp: May 05, 2015, 06:18:50 pm
content: Quote from: technix on May 05, 2015, 02:06:51 pmMany of the listed references are available with 0,02% accuracy. That should be more than enough to check your meters even without sending to calibration lab.

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 08:00:35 pm
content: Quote from: mzzj on May 05, 2015, 06:18:50 pmI want to test the low voltage ranges too so I will need that accuracy and I probably will have to build an adjustable supply out of a reference chip, and that (as well as my old meters) will have to take a trip to the cal lab.

author: ebastler
author_rank: Super Contributor
timestamp: May 05, 2015, 08:14:50 pm
content: Quote from: technix on May 05, 2015, 08:00:35 pmIf you feel that you need calibration (repeatedly, if you are really serious about it?), the few extra $ for a better part should be negligible compared to the overall cost of the module and calibration service, shouldn't they?

author: technix
author_rank: Super Contributor
timestamp: May 05, 2015, 08:22:39 pm
content: Quote from: ebastler on May 05, 2015, 08:14:50 pmCalibrating a piece of gear with LTZ1000 at its heart here in Shanghai at a government-operated cal lab (cheapest option actually) cost the same as the LTZ1000 itself. If I used MC1403 probably it won't worth a trip down a cal lab, but I don't have equipment that justifies a purchase of LM399 or LTZ1000.Maybe when I decided to get a 8.5-digit DMM I will need that. However before that, are you suggesting that the accuracy of MC1403 as posted in the datasheet is more than good enough for me to calibrate my gear against?

author: ebastler
author_rank: Super Contributor
timestamp: May 05, 2015, 08:40:53 pm
content: Quote from: technix on May 05, 2015, 08:22:39 pmThe MC1403 is only specified at +- 1% absolute accuracy, it seems. So if you want an "absolute" standard, rather than just checking your DMMs for drift, that may not be enough for you. Some of the other references mentioned in the thread fare a factor of 10 better in this respect, e.g. the REF192 or the MAX6050 at 0.1% and 0.2%, respectively. At first glance, I have not seen one with the 0.02% accuracy mentioned by mzzj, however.

author: macboy
author_rank: Super Contributor
timestamp: May 05, 2015, 11:01:30 pm
content: Quote from: ebastler on May 05, 2015, 08:40:53 pmREF102 is 0.025% and has very good stability over temperature and time. It is a good candidate to build a 3.5 digit standard that doesn't need any initial calibration, just build and use. If you add trimming capability and calibrate it, you can get much better accuracy that should hold true for a long time.

author: edavid
author_rank: Super Contributor
timestamp: May 05, 2015, 11:45:29 pm
content: How about an AD588AQ? Judging from eBay, there are plenty of them in China.

author: technix
author_rank: Super Contributor
timestamp: May 06, 2015, 04:39:55 am
content: Quote from: ebastler on May 05, 2015, 08:40:53 pmThat is why I said I was thinking about sending a MC1403 down to a cal lab - to get an accurate initial reading. Then the MC1403 will have a good enough drift and tempco to allow me continuously calibrate my gear against.

author: technix
author_rank: Super Contributor
timestamp: May 06, 2015, 09:01:38 am
content: Quote from: macboy on May 05, 2015, 11:01:30 pmIf I want more accuracy how many digits can I squeeze out of a REF102 with calibration capabilities? And how to design calibratable relay operated decade switches?

author: icon
author_rank: Regular Contributor
timestamp: May 06, 2015, 05:51:05 pm
content: Have a look at this chap's channel on Youtube - might be useful.https://www.youtube.com/channel/UCDqryeq1kMDSEQwltWqASrAJohn

author: Andreas
author_rank: Super Contributor
timestamp: May 06, 2015, 07:22:37 pm
content: isl21009bfb850 : 0.01% @ 5Vmax6350: 0.02% @ 5Vref102c: 0.025% @ 10VWith best regardsAndreas

author: technix
author_rank: Super Contributor
timestamp: May 06, 2015, 07:56:42 pm
content: Quote from: Andreas on May 06, 2015, 07:22:37 pmPart #Nominal voltageInitial accuracyTempcoDriftPrice (from Shenzhen, unit: CNY)ISL21009BFB8252.5V+/- 0.5mV (+/- 0.02%)3ppm50ppm/1000hrs15/eachREF02AU5V+/- 0.015V (+/- 0.3%)4ppm100ppm/1000hrs5/eachMC14032.5V+/- 25mV (+/- 1%)10ppmNo data4.25/each (8.5/pack of 2)MAX6350CPA5V+/- 1mV (+/- 0.02%)1ppm30ppm/1000hr30/eachMaybe MC1403 is a bad buy...I think I will get a few REF02 and start building from that. Hope OP07C is a good enough op amp for instrumentation purposes and I am still worried about the reed relays and a long string of 74HC595 used as decade switch replacement. What kind of power supply should I use? Is a maximum rating of 20V enough? Is punching in a voltage on a keypad and hit go a good user interface?

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 07:10:41 am
content: Quote from: macboy on May 05, 2015, 11:01:30 pmScored a few used LM399 at US$2.5/each, end of discussion. I am going to design my standard project around these.

author: ebastler
author_rank: Super Contributor
timestamp: May 07, 2015, 12:14:42 pm
content: Quote from: technix on May 07, 2015, 07:10:41 amCongratulations -- good to see that your project is making progress. With that choice it will make sense to have your voltage reference calibrated once it's built, since the LM399 has great stability, but a rather large 2% initial tolerance.

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 12:50:55 pm
content: Quote from: ebastler on May 07, 2015, 12:14:42 pmSo your suggestion here is go ahead and build the voltage reference out of LM399, and send it on a trip to the cal lab? Will its status being a used part affect its accuracy?

author: macboy
author_rank: Super Contributor
timestamp: May 07, 2015, 02:31:37 pm
content: Quote from: technix on May 07, 2015, 12:50:55 pmYes, the status as used affects it... for the better. It is well known that these heated zeners drift the most in the first few hundred hours of use, then level off to a much better long-term stability. A used device is actually best, otherwise you need to 'burn in' the new device for several months in order to get over that initial period of drift.The LM399 has excellent stability, but poor initial tolerance. That means that it any two of them could be at significantly (a few %) different voltages, but they will each be extremely stable, over a range of temperature, and in the short term and long term. After building your device, you will need to get it calibrated so that it is putting out exactly some value that you choose. Then (if you did it right) it should remain quite stable over time.Only you can decide what amount of accuracy is needed, and you need to decide this before starting the design of the project. After you go beyond about 0.01% (100 ppm), you get into volt-nut territory where everything matters. Choice of components becomes critical: low drift, precision DC op-amps, resistors need to be special expensive low tempco ones. And the layout of the PCB becomes important too.If you want to make it adjustable, you might try building a couple of decades of Kelvin-Varley dividers. You could use relays in place of the switches, but I'd suggest using latching relays (less self-heating) and make sure that they are signal (or telecom type) not power relays. Depending on your specifications, you might look for ones with low thermoelectric contacts.There are various threads here on eevblog and other forums discussing building precision references. Search and read, and try not to lose your mind in all the volt-nuttery.

author: technix
author_rank: Super Contributor
timestamp: May 07, 2015, 03:15:10 pm
content: Quote from: macboy on May 07, 2015, 02:31:37 pmI am creating a LM399 "VoltCard" (voltage reference with a 5-pin card edge connector) and whatever precision I can get out of that is going to be my target precision. The op amp is OP07C and my PCB house have limitations, so no volt nutting allowed here.

author: slurry
author_rank: Regular Contributor
timestamp: June 01, 2015, 03:18:14 pm
content: I found two new LM399H so i decided to make a PCB for them, as i'm no volt nut the PCB is rather simple.I used OP77 as buffer, i can add the link to OSH park PCB when i get home if anyone is interested in a PCB?