Unusual Level Shift - Page 1
Forum section: electronics / general

author: AeroETech
author_rank: Contributor
timestamp: February 28, 2023, 09:11:44 am
content: Greetings everyone,Kind of a strange one here. I'm using a very large array of OPA2991 Op-Amps for calibrating a huge array of thermocouples. 256 channels * 2 Op-Amps, to be exact. Because of the Iq, I really want to put channels into shutdown when they're not actively being scanned.The OPA2991S has a shutdown pin, but it's logic level.. sucks..Being that I'm supplying it with +/-18V, it's -18V for enable, and -16V to +18V for disable.If it were just a handful of these, I can think of a dozen different ways to skin the cat of level shifting my 0/+3.3V signal to shut them down, but I'm curious if anyone has any crazy ideas that will get the part count, PCB real estate, and cost down. I'm looking at real amounts of space and money just to turn these damn things on and off.Assume that I have 256 digital outputs at 0/3.3V already, and I need to convert that to 256 outputs of -18V/(-16 to18V).Always curious what someone might be able to come up with when they're not already burdened by their first idea.Thanks!

author: mikerj
author_rank: Super Contributor
timestamp: February 28, 2023, 09:43:51 am
content: A pull down to -18v and a PNP Bipolar or P-channel MOFSET to pull up to the 3v3 rail?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2023, 11:02:39 am
content: The OPA2991 is a bit strange choice for thermocouples, though not impossible.Normally one would use a lower supply for thermocouples. So more like a 5 V or +- 3 V supply - this naturally keeps down the power consumption and I see hardly any need for a high voltage output. Most modern ADCs are more like 3 V or 5 V, rarely more.Another question is if one really needs 2 OP-amps per channel - quite often switching is right at the input.One could consider getting the 256 control signals from a shift register chain (e.g. 4094) and have the shift registers at the negative supply. So the level shifting would only be needed for 3 (maybe 4) signals.

author: AeroETech
author_rank: Contributor
timestamp: February 28, 2023, 11:21:40 am
content: Quote from: mikerj on February 28, 2023, 09:43:51 amYeah, that's pretty much where I'm at. Even in small packages, that number of mosfets eats up quite a bit of real-estate though. Probably can't be avoided.Quote from: Kleinstein on February 28, 2023, 11:02:39 amThere are a lot of unusual reasons for selecting this amp, but for this particular application, it's a good fit. This is a really unusual application.Shift register chain is an interesting idea. I'm actually going to ponder that one. Thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: February 28, 2023, 04:27:28 pm
content: Opto Coupler should work.Best,

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2023, 04:58:02 pm
content: The easiest option would be to run a chain of 32 74HC595 SIPO shift registers with their ground at your -18V rail (use a 78L05 to feed them with its input from Gnd, and its Gnd to -18V) and level shift the SPI bus controlling them. That reduces the problem considerably as you then only need to level-shift three logic signals.