TI High Accuracy Voltage References REF series, Trimpot? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 21, 2015, 01:32:53 am
content: Im curious about this producthttp://www.voltagestandard.com/DMMCheck_Plus.htmlI happen to own all the TI REF series at whatever the highest accuracy is %.005 in every voltage they make. This product has a bunch of precision resistors too, which I also own. Is it good enough to use these references to calibrate equipment without the trim pot? According to the data sheet I dont see an obvious need for the trim pot. In order to even a trim pot you would need like a high accuracy high resolution bench meter freshly calibrated right?I guess what im asking is for your average UNI-T/FLUKE whatever handheld DMM the reference should be enough for calibration right?

author: singapol
author_rank: Frequent Contributor
timestamp: August 21, 2015, 02:42:08 am
content: Yes and also you can wire these voltage references as current sources.See their respective datasheets.

author: technix
author_rank: Super Contributor
timestamp: August 21, 2015, 08:36:33 am
content: I have three voltage reference at different grades:Coarse check: MC1403 2.5V reference chip, supplied with 5V power from a USB power bank and a Nippon Chemi-Con 4700uF filter cap.Fine check: second-hand ADR02B (drop-in of the REF02 at highest grade) supplied from a 9V batteryPrecision check: second-hand LM399. The 1mA supply current is controlled by a MC1403.Second hand parts have the advantage that those parts have been burned in for an extended period of time, while being cheaper than new.

author: tszaboo
author_rank: Super Contributor
timestamp: August 21, 2015, 08:52:23 am
content: Proper calibration assumes that the reference is at least 4 times better than the device calibrated.You also added an extra 0 also. It is 0.05%. But the question is simple. If you can calibrate the REFxx to 0.01% by connecting it to a higher precision instrument, like a 6.5 digit DMM (about 30PPM or 0.003%) trim it, or write the voltage on it, than you can use this as a transfer standard to calibrate 0.05% class instruments with it. Class A from this series has only like 50-100PPM error/drift/tempco over a realistic range.

author: technix
author_rank: Super Contributor
timestamp: August 21, 2015, 01:04:54 pm
content: Quote from: NANDBlog on August 21, 2015, 08:52:23 amSince you mentioned it, will it benefit me if I choose one of my MC1403, ADR02B and LM399 and send them to a cal lab and write the voltage down on a sticker attached to the module PCB? Will it worth my while calibrating them?

author: ralphd
author_rank: Frequent Contributor
timestamp: August 21, 2015, 01:43:14 pm
content: I recently ordered a couple of 0.05% REF5050s, the same chip used in the model VREF-01 (http://www.voltagestandard.com/Home_Page_JO2U.html). Uncalibrated, you've got a >99% chance it'll be within .02% @25C, and a ~90% chance of 0.01% (see figure 3 in the datasheet). They're pretty cheap too; I found them on aliexpress for $3.50 for a pair.

author: tszaboo
author_rank: Super Contributor
timestamp: August 21, 2015, 01:52:49 pm
content: Quote from: technix on August 21, 2015, 01:04:54 pmI would calibrate the LM399, the other two I'm not so sure.Quote from: ralphd on August 21, 2015, 01:43:14 pmYes, the tolerances they give you in the datasheet are for more than 99.9% confidence.BTW, I've already ordered my PCBs with REF102, the plan is that I will hand assemble a few, burn in for 100+ hours, trim it as much as possible and calibrate it with a 3458A at work. It has battery power, power supply, filter, buffer, so should be a very good reference with reasonable compromises.I expect 3ppm tempco and more than 5 digit accuracy, 8 (and a half) digit written on it.Maybe I even sell it if you guys are interested.

author: technix
author_rank: Super Contributor
timestamp: August 21, 2015, 02:04:44 pm
content: Quote from: NANDBlog on August 21, 2015, 01:52:49 pmADR02 is a drop-in replacement of REF02, a 5.0V reference. My ADR02B in SO-8 package have a guaranteed less than 3ppm/K tempco (1ppm/K typical) and less than 3mV (0.06%) initial accuracy. And since I bought both ADR02B and LM399 second handed, someone have already burned them in for me.So if I send the LM399 or the ADR02B to a cal lab, how many digits should I write down on the sticker?

author: ralphd
author_rank: Frequent Contributor
timestamp: August 21, 2015, 02:10:10 pm
content: Quote from: NANDBlog on August 21, 2015, 01:52:49 pmAnyone seen a similar population-accuracy plot for TL431s?If they are fuse trimmed, I'm guessing they won't have a smooth bell-curve distribution. With 2 fuses, I'd expect 4 peaks.http://www.righto.com/2014/05/reverse-engineering-tl431-most-common.html

author: Macbeth
author_rank: Super Contributor
timestamp: August 21, 2015, 02:20:35 pm
content: I've got a pair of LTZ1000ACH's coming to me from aliexpress. If you're going to buy cheap chinese crap, you may as well go for the best

author: tszaboo
author_rank: Super Contributor
timestamp: August 21, 2015, 02:46:15 pm
content: Quote from: technix on August 21, 2015, 02:04:44 pmThat is up to you. I've seen shunts where the calibration was 500EUR+, done every year. How much are they charging over there for a calibration?Quote from: ralphd on August 21, 2015, 02:10:10 pmI guess you will be measuring the distribution of your resistors, not the 431 in any circuit other than 2.5V

author: technix
author_rank: Super Contributor
timestamp: August 21, 2015, 03:13:02 pm
content: Quote from: NANDBlog on August 21, 2015, 02:46:15 pmI forgot to ask if it makes sense to calibrate ADR02B, and is it as good as LM399 as the primary voltage reference in my DIY DMM design?I have not asked about calibration fee yet but unreliable information says that one-off calibration of hobbyist gear isn't too expensive here.

author: Andreas
author_rank: Super Contributor
timestamp: August 21, 2015, 08:24:20 pm
content: Quote from: technix on August 21, 2015, 03:13:02 pmDepends on how accurate you need it.And no a ADR02B is usually not so stable over time, temperature and humidity as the LM399.It has its reason that all (newer) venerable 6.5 and many 7.5 digit DMMs use the LM399.Quote from: technix on August 21, 2015, 02:04:44 pmI think there are many wrong expectations (in this thread).The datasheet values are not guaranteed when you receive the references (at home).They are values that are valid when the references are adjusted during production.Every time you mistreat the device (e.g. by soldering) the output voltage will shift.Devices in plastic package will also shift when the relative humidity is changeing.And further you will have up to several 100 ppm ageing drift for new devices.For the old LM399: if soldered (mechanical stress) you may start a new ageing curve.(although the ageing of LM399 in metal package is typical much lesser than plastic devices).The tempco is measured at exactly at 3 temperatures.(25 degC, min operating temperature, max operating temperature).Many of the beauty curves in the datasheet are interpolated from these 3 values.The T.C. is also calculated by maximum temperature difference (of the 3 values)divided over the operating temperature range.There is no statement how the reference behaves at a specific temperature.with best regardsAndreas

author: ralphd
author_rank: Frequent Contributor
timestamp: August 21, 2015, 08:50:13 pm
content: Quote from: NANDBlog on August 21, 2015, 02:46:15 pmI guess you will be measuring the distribution of your resistors, not the 431 in any circuit other than 2.5VAnd that's why I'm testing at just 2.5V. With a cheapie uncalibrated multimeter I measured 11 out of a batch of 100 (23-24C ambient):2.510, 2.508, 2.511, 2.511, 2.513, 2.517, 2.509, 2.516, 2.514, 2.509, 2.515

author: nuno
author_rank: Frequent Contributor
timestamp: August 22, 2015, 01:40:06 am
content: Quote from: Andreas on August 21, 2015, 08:24:20 pmIndeed. Without known anything at the time, I remembered to measure the voltage of a few LM4132-1.8V highest grade (0.05%, -/+0.9mV in the 1.8V reference) before and after soldering, and the difference was from -0.7mV to +0.15mV (measured 6 units). Minimum change was +0.1mV.

author: ralphd
author_rank: Frequent Contributor
timestamp: August 22, 2015, 03:02:08 am
content: 2% accuracy for the LM399? Is that correct?http://www.ti.com/product/lm399

author: singapol
author_rank: Frequent Contributor
timestamp: August 22, 2015, 03:17:46 am
content: Quote from: ralphd on August 22, 2015, 03:02:08 amYesgauranteed2% initial tolerance.gauranteed0.5ppm/ degree celciusgauranteed1 ohm maximum dynamic impedanceguaranteed20uV maximum noise.

author: Andreas
author_rank: Super Contributor
timestamp: August 22, 2015, 05:30:49 am
content: Quote from: ralphd on August 22, 2015, 03:02:08 amThe clue is not accuracy.(a target value that is once calibrated at production and then drifting several hundreds of ppm).The clue is precision.(maintaining the output value within some ppm independent of temperature, ageing, humidity).Quote from: singapol on August 22, 2015, 03:17:46 amThe devices that I have measured have around 1-2 ppm driftover a 30 deg C temperature range near room temperature.So I guess the 0.5ppm (average value) are mostly issued bythe temperature extremes of the operating temperature range.with best regardsAndreas

author: nuno
author_rank: Frequent Contributor
timestamp: August 22, 2015, 02:29:52 pm
content: Quote from: unitedatoms on August 22, 2015, 02:07:45 amSo the soldering effect is just, or mainly, mechanical stress?

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 03:32:56 pm
content: Quote from: unitedatoms on August 22, 2015, 03:12:52 pmWhy is this decaying into volt-nuttery?

author: ralphd
author_rank: Frequent Contributor
timestamp: August 23, 2015, 03:34:14 am
content: Quote from: singapol on August 22, 2015, 03:17:46 amSo you need yet another voltage reference to calibrate the LM399?

author: singapol
author_rank: Frequent Contributor
timestamp: August 23, 2015, 03:49:47 am
content: Quote from: ralphd on August 23, 2015, 03:34:14 amNot really...it's how you design a voltage reference around a circuit. Of course Fluke and Keysight formerly HP/agilent are not going to tell you but you can deduce the type of compensation circuitrry they use like very precise resistors/temperature coefficients or auto calibration circuits that are now quite common.

author: tszaboo
author_rank: Super Contributor
timestamp: August 23, 2015, 07:29:23 am
content: Quote from: singapol on August 23, 2015, 03:49:47 amNonsense. Yes you need to calibrate the LM399. No matter what resistors you use the LTZ1000/LM399 circuit initial accuracy is going to be very low. They are very stable.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 26, 2015, 08:20:50 pm
content: Ok so im a bit confused the Uni-T is .1%+5(this means counts?) DC accuracy according to the manual, The Vref is .05% accuracy without a trimpot.....While .05% is not 10x better, it would still be a more reliable calibration than leaving it at factory? I sometimes like working with high resalution ADC/DAC's so the better precision I can get in my circuit and on my meter the better off I am!

author: Andreas
author_rank: Super Contributor
timestamp: August 26, 2015, 09:23:11 pm
content: Quote from: rwgast_lowlevellogicdesin on August 26, 2015, 08:20:50 pmYou have to regard the temperatures:Meters are usually specified for 23 +/- 5 deg CReferences are usually specified at 25 deg C. (and a tempco e.g. 80 ppm/K).With best regardsAndreas

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 27, 2015, 03:52:51 pm
content: Ok so hear is what I am getting at, I have a ton of the .05 REF series IC's across every voltage. The package has been opened but re taped with all the thermal humidity stuff inside. Of course these are plastic and I live in an extreme temperature climate as far as heat is concerned.I have a UniT 61E 22,000 count meter, with probe master probes, I also have some 4000 count ex techs. NI watched mj morton's review of this meter which the DC accuracy was with on .1% specs but it was out on the far side. Will I see any benefit at all from slaping these ref chips on a board and using them to try to get the Uni-T closer to spec? Also Im not understanding if the REF should be burnt in, reading the data sheet the drift is worse over use and time right?I really dont want to spend the money on a voltage standard, but I do work with ADC/DAC's and would like to be fairly accurate out to 0.000 at least.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 27, 2015, 03:56:18 pm
content: suprisingly this thing was highly accurate with resistance, it was dead on a .01 omh 10 watt low drift vishshay minus the probe resistance, and every other precision resistor I tried in multiple ranges, so would it be a good assumption DC current it pretty accurate?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2015, 07:33:59 pm
content: The voltage ranges depend on the voltage reference and the divider ratio in most ranges.The current ranges rely on the voltage reference and the shunt resistors, and possibly some internal amplification.The resistance ranges usually are based on ref. resitors and some divider ratios (often different from the voltage ranges).So DC accuracy / calibration is not really linked to the Ohms ranges. So even if the voltage reference is way of, resitance can be still accurate. Same is possible the other way around.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 28, 2015, 04:57:55 pm
content: Hmmm well I guess the simple yes or no question im asking here is without having a trimpot and an 8 1/2 digit meter.... will calibrating a .1% uni-t to these reference voltages make anything better, or is it a waste of time?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2015, 05:38:52 pm
content: If adjustung the meter makes much snese depends. The first thing would be measuring how good it actually is. So compare a few readings, mainly near the end of the cheap meter.Then you can decide whether it worth opening it. If readings are good enough, I would keep it as is. Adjusting the ref. voltage would change all voltage and current readings the same way, at least the DC readings. Depending in the meter there may be more pots for AC. Be careful not to change things you don't understand - some of the trimmers may be for frequency compensation and are hard to adjust without proper instruments and prceedings.Even if you just write down the correction factors, this might be usefull and enough in the rare cases a accurate absolute reading is needed.