Maybe video material: positive ground considerations - Page 1
Forum section: electronics / projects

author: robertbaruch
author_rank: Regular Contributor
timestamp: October 10, 2016, 01:10:05 am
content: So I have an old 60's logic board which has a ground, -12v, and +6v connection. Generally the logic on the board switches between 0 and -12v. Now, I'd like to interface that to a tester board that uses at least 5v logic, or possibly 3v3 logic. The signals flow in both directions (but a fixed direction per signal).Converting between two essentially non-overlapping ranges is an extraordinary pain, at least if you want to avoid high quiescent currents (seethis Analog Devices explainer). and slow rise/fall times. It just takes too many mosfets!So, the idea is to provide the old logic board with a positive ground. Thus, the tester circuit can provide 0v, +12v, and +18v, and if I connect +12v to the old board's ground, I get -12v (on the 0v line), 0v (on the +12v line), and +6v (on the +18v line). And so, I can pretty much arbitrarily declare that anything above 3v3 (referenced to the tester's ground, where it's -8.7v referenced to the board-under-test's ground) is a logic 1, so I can either use a zener or maybe a simpler mosfet circuit, because the voltage ranges now overlap.Now, I'm suspicious. I know that ground is whatever you define it to be, but aren't there other requirements for a ground, like it is very low impedance, and the voltage difference between various points along ground must be very low? What are the implications if you flip your power supply to another isolated circuit to make it look like you have a negative supply?I'd sure be interested in a Dave video on this.

author: ajb
author_rank: Super Contributor
timestamp: October 10, 2016, 05:56:11 pm
content: Quote from: robertbaruch on October 10, 2016, 01:10:05 amIf a net must carry high current or critical return currents, then it must be low impedance. Doesn't matter if it's ground or not. "Ground" also doesn't necessarily need to be particularly low impedance, for instance if you were building some really basic circuit it might not need to be any lower impedance than any other net. It all depends on the functional requirements of the circuitry.If your overall design is made easier/simpler/safer/whatever by making some positive voltage your fixed reference point, then you are free to do so.Re: level shifting, you might consider comparators or op-amps as level shifters for unusual (especially high) voltage levels. If your signal voltages are non-overlapping, then digital isolator ICs may be helpful as well.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2016, 06:23:49 pm
content: Be very careful: the ground node must be a low impedance to current travelling in both directions. Positive power supplies and regulators can normally only source current, unless specifically designed to sink as well. This means that the +12V output from the tested board should only be connected to a lower voltage node, otherwise the voltage will rise above +12V, which could cause something bad to happen.If you're certain that the board under test always draws more current from the -12V supply, than the +6V, then it should work but if the +6V supply draws more current, then you have a problem. If you know how much more current the board under test draws from the +6V supply, then the -12V supply, then you could add a resistor between the +12V and GND on the tester, to compensate for it but otherwise you're SOL.

author: robertbaruch
author_rank: Regular Contributor
timestamp: October 11, 2016, 01:35:29 am
content: Quote from: Hero999 on October 10, 2016, 06:23:49 pmNow there's a <dave>trap for young players</dave>!Indeed in my situation the board draws from +6 much less than it will push at the -12v terminal, so the net ground current would be sourcing from the tester.Quote from: ajb on October 10, 2016, 05:56:11 pmYeah, that's what I'd normally use. However, I'd like to shift at frequencies up to 10MHz, and it seems all the comparators and opamps that can do high voltages can't really do that high a frequency. Like, there are no comparators on Digikey with a propagation delay less than 4.5us. Or if they can, they are a lot more expensive than just building a level shifter out of discrete mosfets.

author: SL4P
author_rank: Super Contributor
timestamp: October 11, 2016, 05:01:42 am
content: Keep in mind, that if you want to touch in with a scope or other test gear - the ground and readings must be considered wrt the device and power supply.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2016, 08:04:31 am
content: Quote from: robertbaruch on October 11, 2016, 01:35:29 amThere are level shifter ICs but they're old and a little slower than you need.The HEF4104B might do but at 10MHz it's marginal and at 18V, you're right at the maximum voltage rating.http://www.nxp.com/documents/data_sheet/HEF4104B.pdfThe CD4504B is a bit slower but is specified up to 18V.http://www.ti.com/lit/ds/symlink/cd4504b.pdf