# Tie a pin to board ground with no current flow

Discussion in 'General Electronics Chat' started by Vinnie90, Jul 19, 2016.

1. ### Vinnie90 Thread Starter New Member

Jul 7, 2016
14
0
Hey Guys,

I've been trying to figure out a way to tie a floating pin to my board ground. This is in principle very easy, HOWEVER for the specifications I have there must be no current flow between the floating point and the ground. I have tried to use an inverting amplifier with R1 and 2 = 0 but it doesn't seem to work. Any Idea?

Thanks for the help
Vinnie

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
Current cannot be "forced" into an input of any kind.the only invitation an electron understands is voltage.

Does the pin have a voltage when floating? If so, current will flow until that voltage reaches zero.

3. ### Vinnie90 Thread Starter New Member

Jul 7, 2016
14
0
Maybe I'll be a little bit more specific: I have a conductive surface (imagine a piece of conductive carbon paper) and I put some electrodes on the outer frame of this surface (see attached image). You can also think of it as a resistor network, in which I set a current and then I read the voltage between a reference point and other nodes. All the pins are connected to four analog MUXs 16-1: with MUX 1 and 2 I select the pins where the current has to flow (I impose the current via an indipendent current source) while via MUX 3 and 4 I acquire the differential voltage between a reference voltage pin (set by MUX 4) and all the other pins (MUX 3). For this project I have to assure that the current flows only through two pins (the ones I set by the first two MUXs) and that there's no current flow in the others (leakage current in order of nA is ok).

The differential signal I read is correct as long as I have very small currents. When I go for higher currents I get wrong values. The reason for that is that the multiplexers saturate. The single ended signals that can go into the COM of the MUX can swing between V+ and V- (in my case +/- 5 V). Being floating these signal are very high when referenced to the ground of the multiplexers.

So my idea was to tie the reference pin that is given by MUX 4 to ground and assuring at the same time that no current flows in that pin, by putting a buffer inverting amplifier, with the non inverting pin to the MUXs ground. However this solution is not really working

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4. ### Papabravo Expert

Feb 24, 2006
11,739
2,483
Floating is something that happens to a high impedance input. Regardless of the floating voltage there is no current flow into or out of that high impedance input. If you ground that input there is still no current flow into or out of that input. It is still a high impedance input. It doesn't change it's behavior just because it it no longer floating.

5. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,077
2,001
Tying a point in a floating circuit to ground and having any significant current in that ground connection are two very different things. This may be more semantics than devices, but if a circuit truly is floating with respect to ground, then grounding some part of it should not cause currents large enough to disrupt your measurements. Am I missing something?

ak

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
Just keep powerful equipment grounds, like motors, separate from your grounding scheme for logic components

Those two ground pathways should have only one common point, ideally placed at the battery connection.