LT6658 Voltage Reference - Page 1
Forum section: electronics / general

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 17, 2019, 10:17:13 am
content: I've been lurking around here for a while and reading up on all the cheap (< $100) voltage references available. I've seen the DMMCheck Plus and it looks pretty good but a visit to the Analog website revealed the existence of theLT6658and it's cheap at around $6 on Digikey. I'm surprised that a search for "LT6658" on the metrology section gets zero hits.What do you guys think of it in comparison to the DMMCheck? If I designed a PCB, we could have a batch made and kick the tires.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 17, 2019, 10:53:14 am
content: Hmmm, I may be answering my own question here but the LT6658, even in the more accurate 'A' version, is only ± 0.05% accurate, the DMMCheckplus is ± 0.007% on volts, almost an order better! The DMMCheckplus also has DC/AC volts, current, frequency, and it has some accurate resistors fitted with a calibration sheet for the whole lot.The LT6658 is called a 'refulator' by the datasheet being designed to provide accurate reference supply to ADCs.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2019, 11:02:35 am
content: The LT6658 is a nice low noise voltage regulator, but not really a long term stable reference. Even for a plastic case the long term drift (120 ppm/1000 hours) is relatively large. The better reference from the series would be LT6655 / LT6657 in the LS8 package.Even than the usual low cost long time stable reference to compare to is the LM399 - though at some 7 V.The initial accuracy is usually not the real problem. For a reference board one would fix this by measuring with a good DMM, unless one really need a reference at a nice round value like 5.00000 V.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 17, 2019, 12:09:26 pm
content: OK, scratch that idea.I just ordered a DMMCheck plus

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: September 17, 2019, 03:21:37 pm
content: QuoteGood choice.With a chip you do not getBurn-in. without this some chips will show excessive drift once you get them into your test setup.Actual calibration. That 0.05% spec is just for show, some chips may be over or under and, if they exceed max or min (how will you know?Isn't this your original problem??) you can return them to the manufacturer and get another (the manuf. will not even be embarrassed), thatis a lot different than a real calibrationFree re-calibration after one year.In addition, the DMMCheck adds Current, resistance and spot frequency checks.

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 25, 2019, 04:34:21 pm
content: My DMMCheck Plus arrived on Monday and I did a quick design for a 3D printed case for it, if anyone wants the .stl file, just ask and I'll post it up here. It's a shallow as I could make it so that the battery sits maybe 1 mm from the bottom.Test indicate that my Keysight 34461A that's about 4 years old is spot on calibration-wise, so too are my Agilent U1252B and U1242B meters. In fact, my $18 Aneng 8008 which I 'calibrated' by comparing it with the 34461A is + 0.1%What a great little board it is.

author: iMo
author_rank: Super Contributor
timestamp: September 25, 2019, 05:54:54 pm
content: Having 40pcs LT1021-10/5-BCN8 here, all of them pretty aged (say 25y+) I somehow doubt that reference is capable to check out an HP34461A or higher..PS: ..as I can see the chip on the DMMCheck board above..

author: Andreas
author_rank: Super Contributor
timestamp: September 25, 2019, 07:30:43 pm
content: Quote from: imo on September 25, 2019, 05:54:54 pmHello,if you keep temperature and humidity constant then the LT1021 (buried zener like LM399/LTZ1000)is rather stable over time after some run in phase.Ageing is then in the same ball park as a LM399. (1-2 ppm/year)Also 1/f noise is comparable to a LM399 (3 uVpp).with best regardsAndreas

author: Gandalf_Sr
author_rank: Super Contributor
timestamp: September 26, 2019, 09:04:28 am
content: Quote from: imo on September 25, 2019, 05:54:54 pmAre you saying that the LT1021 on my DMMcheck plus is 25 years old based off the chip markings? The paperwork claims that my DMMcheck Plus was burned in for 300+ hours and then the readings were taken from their fancy calibrated multimeter

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 26, 2019, 05:52:27 pm
content: I just got my latest rev of DMMCheckPlus as well in past few days... (I had to wait weeks for the new ones to be ready) was happy to see my 34401A measured 5.00000 exactly which is what they tune the 5V to vs. 3458A. Resistance checked out really nicely as well. Which frequencies did you choose for yours? Did you get the two frequency model? Now I feel confident in checking a bunch of old General Resistance 8E16A, Kelvin 0811-1701 .01% resistors I recently acquired and some precision decade boxes I recently got Dekabox DB62, General Resistance RTD-100 (This thing is BIG), OHMITE 3416. They're all from early 70's so well aged already (which could be good or bad I suppose...) The RTD-100 is much older and looks way different than the ones they are selling today...Bill

author: iMo
author_rank: Super Contributor
timestamp: September 26, 2019, 06:42:26 pm
content: Quote from: Gandalf_Sr on September 26, 2019, 09:04:28 amNope, your LT1021 is an youngsterMine are 32+ years old.

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 01:41:24 am
content: For what it's worth my DMMCheck Plus r6 LT1021 has exact same date code as yours does:1807 e3LT1021BCN8-5Bill

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 01:55:31 am
content: Also since the DMM Check Plus came...

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 01:57:09 am
content: Tells me my Datel DVC-350A is doing alright.

author: iMo
author_rank: Super Contributor
timestamp: September 27, 2019, 06:51:22 am
content: The voltage references are as a fine wine - they get better with ageIn order to watch the fluctuation of your references (ie with your 34401A) you have to switch to 6 digits (ie slow 100NPLC) and you have to observe the data coming out the rs232 or HPIB (set the HPIB address to "31" - talk only mode).

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 11:23:12 am
content: Which environment or script would you recommend using with my 82357B GPIB interface to capture the data? Is there a preferred software or script people prefer for metrology? We use labview and test stand at work but is there something easier I could use? Maybe a python script or smaller app?Bill

author: IanJ
author_rank: Unknown
timestamp: September 27, 2019, 11:34:45 am
content: Quote from: notfaded1 on September 27, 2019, 11:23:12 amIf you are running windows and don't want to muck about with scripts etc then my GPIB app is in the Metrology section here.Ian.

author: branadic
author_rank: Super Contributor
timestamp: September 27, 2019, 01:27:23 pm
content: Prepare a Raspberry Pi with Python and you are as flexible as possible. Can share a fully running image for it, if you want.-branadic-

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 02:49:21 pm
content: Yes please branadic I have a raspberry pi 2 and 3 that are waiting for something new to do! The Pi3 is brand new... if your image is all ready to go it'll save me some time setting everything up... awesome! I'll be the first to not want to reinvent the wheel. I also have a windows 10 laptop I like to use for stuff like this a Dell Precision 7530 that's a beast with SSD's and Xeon processor... kinda night and day vs the pi's but it doesn't need to do much I suppose... I've got some nice .01 resistors from early 70's and a collection of resistance decades it'd be neat to check out as well. I envy you guys with 3458A's. I've got a Keysight 82357B USB interface for this.Bill