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I'm using a level shifter to go from 3.3V to 5V (digital/analog). According to the datasheet, if I put 1 ground on the left side of the IC and the other on the right, it would tie the grounds together inside the IC.

This would be bad since I'm trying to use a single point star ground with separate ground planes and it seems like this ruins it. If I want to keep using the same part, whats the best approach? I'm thinking tie both sides of the level shifter to the input ground and have the output current take a long path through the single point ground? or vice versa?

http://www.ti.com/lit/ds/symlink/sn74lvc8t245.pdf

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  • \$\begingroup\$ Which pin is input ground and which is output ground? And what GND is the third pin? The datasheet doesn't differentiate... \$\endgroup\$ – Huisman Apr 9 at 6:46
  • \$\begingroup\$ @Huisman right the data sheet doesn't differentiate because I think it ties them all together internally. I naively tied the grounds on the left side, where all the inputs are, to the input ground, and everything on the right to the output. \$\endgroup\$ – Gswffye Apr 9 at 15:08
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I'm thinking tie both sides of the level shifter to the input ground and have the output current take a long path through the single point ground?

I would do the opposite, so keep the loop at the output short because that's where the current flows.

The inputs are high impedance so almost no current flows. For noise reasons the argument could be made that the inputs should have the better ground connection but since we're dealing with digital input signals, I would not be too concerned about noise at the inputs.

I think in the end there isn't that much difference between ground at input side or ground at output side. This isn't RF, not very sensitive (using low voltages) and also not high current.

If you still have doubts you could consider to connect the levelshifter's grounds to a local ground plane and then connect that plane via a link / zero ohm resistor / blob of solder to the output side or the input side.

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