Space-efficient way to shift 12 signals to negative levels? - Page 1
Forum section: electronics / projects

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 02:23:31 pm
content: Hopefully someone else has had to do this and has an idea. I need to shift 12 slow, digital signals from 0V/5V to -5V/5V. (Inverting or not, I don't care.) Currently I am using 12 PMOS and 12 load resistors to the -5V rail, but this is already a tight PCB and I'd like to keep it small. 12 SOT-23 transistors actually take up a relatively large amount of space. SC-70 is only a bit better, and those seem to be a bit expensive for P-channel.Are there any chips out there that can do level-shifting to a negative rail? I was hoping to find something with just a high-side output (keeping the load resistors - I can do 0603 or 0402, so I don't really mind them), but despite the regular availability of sink-only devices, source-only devices seem to be a bit of a rarity (and the ones that do exist are mostly power switches, a bit excessive for 12 logic signals).

author: Crazy Ape
author_rank: Regular Contributor
timestamp: August 31, 2013, 04:14:07 pm
content: Would your requirements fit within the specifications of a simple 4066 (or three in your use case).Tie one side of switch to -5v, put a pull-up resistor to +5v on the other side of the switch. 0v / 5v on the control should give you -5v / +5v on the pin with the pull-up.If you want to invert it, just change the voltage signs in the above (and pull-up becomes pull-down).http://www.natalnet.br/~aroca/afron/CD4066BC.pdf

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 04:33:59 pm
content: If I power the 4066 across the -5V and 5V rails, then a 0V control input is an undefined logic level. And if I power it across the 0V and 5V rails, it won't accept a negative voltage on the switch, right? Or am I missing something?

author: sync
author_rank: Frequent Contributor
timestamp: August 31, 2013, 04:45:41 pm
content: Quote from: c4757p on August 31, 2013, 04:33:59 pmLook at the 4053. It has a extra pin (Vss) for control input reference.

author: Crazy Ape
author_rank: Regular Contributor
timestamp: August 31, 2013, 04:53:12 pm
content: Quote from: c4757p on August 31, 2013, 04:33:59 pmOoops, yeah, scratch that idea.A 4316 should work though, has built in Logic-Level Translation.http://www.ti.com/lit/ds/symlink/cd74hc4316.pdf

author: nctnico
author_rank: Super Contributor
timestamp: August 31, 2013, 05:06:04 pm
content: A quad rail-to-rail opamp in a small package. Set the reference level at 2.5V and off you go. Be aware though that opamps used in this way may show weird effects on their inputs because of the overdrive situation. They may need more 'drive' then expected so the 2.5V may need to be 'stiffer' than expected.

author: Rufus
author_rank: Super Contributor
timestamp: August 31, 2013, 05:21:27 pm
content: Quote from: c4757p on August 31, 2013, 02:23:31 pmFDMA3023PZ is a dual P channel MOSFET in a 2mm square package.NX3008PBKV is a dual in SOT666 which is also about 2mm square.

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 05:38:44 pm
content: I like the op amp idea. They can be found in quite small packages and I'll only need three quads.I forgot about the 4316!Thanks. That's another option.Quote from: Rufus on August 31, 2013, 05:21:27 pmI like these, I'll have to add them to my list of "standard" parts. But I think they're still a bit expensive for shifting twelve signals...

author: c4757p
author_rank: Super Contributor
timestamp: August 31, 2013, 05:56:46 pm
content: I'm going to go with an "op amp".TS3704: quad comparator with a direct CMOS output, TSSOP. Thanks for the help!