typical ground plane voltage differences? forcing ground voltage via op-amp - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: January 28, 2014, 05:10:01 am
content: Is there some kind of table that would show typical ground plane voltage differences in different copper thicknesses?Just a rough rule of thumb that would ignore large power components and such for quick design reference.Is it typically microvolts? Nanovolts?I am curious because I thought that maybe if I was to expect a ground loop voltage problem In my PCB I could add a ground force circuit :http://cds.linear.com/docs/en/datasheet/1150fb.pdf* bottom of page 11I might just include the pad in my PCB to see what happens, I wonder if it will improve DC performance any on my analog board. I am guessing I could just measure the ground plane with a regular voltmeter to get the measurements, as the voltmeters bias current should not effect such a low impedance path....I would put the lt1010 at the ground entrance to the circuit and the ground pull point o nthe other side of the board. Or wherever the distance is greatest (obviously not to a isolated trace but to some point multiple components are connected.So long the difference is more then the best the LT1010 can do it should improve things right? Are there any downsides to this circuit?

author: Rerouter
author_rank: Super Contributor
timestamp: January 28, 2014, 07:06:21 am
content: depending on the current it can be in 10's of millivolts, (remembering op amps can pull quick surges)Now i could be entirely wrong, but an old memory is saying for 2 points in a continuous plane the effective resistance is 1.414 times less than the straight line resistance between the 2 points.

author: poorchava
author_rank: Super Contributor
timestamp: January 28, 2014, 07:13:16 am
content: I am not really convinced that copper thickness makes much difference for opamps. For high current stuff - sure, but not for opamps.I think if you separate opamp power rails from high current ones and place a low impedance bulk cap nearby the opamp, the drop can be neglected.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2014, 06:21:08 pm
content: Ground planes will be on the order of hundreds of microohms, I think...Contact resistance dominates, because the formula has a logarithmic term in it relating to the size (diameter) of the pad/via making the connection.What are you doing that requires amplifying a signal that crosses a ground plane? Can't you place the signal and amplifier directly adjacent so the ground drop is minimal?You can also cut the ground plane around the local circuitry, to separate the local area from outside currents. Do this carefully, and don't have traces crossing the gaps.If precision on the output is also required, you can use kelvin connections, so an "output" and "ground" lead come off from the circuit, to be sensed differentially at a later location. Alternately, you can move the op-amp's output ground sense point to the "ground" lead, in which case it becomes an output/reference pair -- same thing, just grounding it at one end or the other.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: January 28, 2014, 07:48:31 pm
content: But would it make any sense to force a ground using a chip like that to try and improve performance of a DC board? What are the criteria for using a force ground circuit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2014, 08:11:11 pm
content: In my opinion it's a stupid idea. Why waste an undefined amount of current (could take 100mA+?) when the problem is completely and utterly avoidable. An autorouter I think could be convinced to do a board successfully; if your layout is worse than an autorouter, you have bigger problems than the layout alone!I think the only case where such a thing would be applicable is existing wiring between equipment which cannot be modified to a suitable alternative (differential at least, or four-wire kelvin if necessary). Then it becomes necessary to "fix" ground loop problems. But it's still only applicable if the circuits share non-isolated power supply rails: otherwise there's nothing to "push" against. And if you have to add wires, well...Tim