Fluke 28 II VREF output - Page 1
Forum section: electronics / testgear

author: Bobson
author_rank: Regular Contributor
timestamp: December 02, 2023, 11:56:31 am
content: https://youtu.be/CiXD6wo3cLc?si=bJQ7H2PEl54es9-t&t=1442You can see 2.497v coming out between COM and V sockets. My 28 II generates exactly the same voltage.Probably it is just LT1790 VREF voltage connected to these sockets.Wonder how stable is it and if does it work with 87 V and is it the same with different samples.

author: Bobson
author_rank: Regular Contributor
timestamp: December 02, 2023, 07:41:55 pm
content: Anybody with Fluke 87 DMMs could report results?

author: AVGresponding
author_rank: Super Contributor
timestamp: December 02, 2023, 08:46:19 pm
content: That's not from the LT1790, unless there's a serious fault. More likely it's stray voltage from the diode test mode you have to switch past. Try selecting DC while in current mode then going back to AC. You should now get a reading of zero, or near enough.

author: Bobson
author_rank: Regular Contributor
timestamp: December 03, 2023, 08:31:09 am
content: Quote from: AVGresponding on December 02, 2023, 08:46:19 pmWrong. Diode mode has +2.375V, while in AC mA it has -2.497V. After switching to DC mA 0.0V, and returns to -2.4975V after going to AC mA back. So it is definitely not a charge after Diode, it stays stable.

author: J-R
author_rank: Super Contributor
timestamp: December 03, 2023, 10:36:30 am
content: Quote from: Bobson on December 02, 2023, 07:41:55 pm87V: 2.0912V using V jack in combination with any of the other jacks.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 03, 2023, 10:46:50 am
content: Quote from: Bobson on December 03, 2023, 08:31:09 amAfter a night's sleep to sober up, and a coffee, looking at this again with a fresh head it looks like there is maybe a DC offset being introduced to the ACI measurement (in order to make it easier to measure around the ZCP for small signals?) from the current source and via the ACI divider using part of the V front end. The manual is quite brief on the subject and doesn't go into great detail; fair enough really, if you have a faulty U1 ASIC, you aren't going to desolder it and put a new one in, you're just throwing it away and getting a new meter.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 03, 2023, 10:53:00 am
content: Quote from: J-R on December 03, 2023, 10:36:30 amAbout -2.117VDC wrt Common on mine. You'd expect the Common to be at more or less the same potential as the uA/mA and A jacks in this scenario, it's basically shorted to them via the shunt.

author: Bobson
author_rank: Regular Contributor
timestamp: December 03, 2023, 11:21:03 am
content: Quote from: J-R on December 03, 2023, 10:36:30 am28 ii also has the same voltage between V jack and any other in mA AC mode.DC offset before ADC is possible, but why output it to the V-jack?In any case, it gives DMM one more calibrated output voltage value and I like it.

author: AVGresponding
author_rank: Super Contributor
timestamp: December 03, 2023, 12:46:57 pm
content: Quote from: Bobson on December 03, 2023, 11:21:03 amBecause they are using part of the V front end in the mA divider. I would check the voltage with different loads before you try to use it as a reference. Say, 1kΩ, 10kΩ etc, to see how much the load affects it.

author: Bobson
author_rank: Regular Contributor
timestamp: December 03, 2023, 04:27:48 pm
content: It is stable loaded with several Megohms, so no problems here. Would be used to test low V range of DMMs in the field.

author: J-R
author_rank: Super Contributor
timestamp: December 03, 2023, 11:34:32 pm
content: Interesting discovery, but I don't see how it's useful. I checked my 87V again today and now it's 2.0855V, so not particularly reliable. Also, I'd want 5V for 5k/50k count DMMs or 1.9V for 2k/20k count DMMs. For sanity checks, just having two DMMs measuring the same thing would be more foolproof, rather than relying on this output. Just my opinion on it.