Signal Chain, connecting units that each have floating supplies?

[HankMcSpank]

This has been troubling me & I just can't wrap my head around the best way forward.

Two units, each have their own PSU via a wall wart - each have different voltages (it I understand this correctly, in this wall wart scenario, their actual ground isn't *true* ground).

Not that it's particularly relevant - The first unit takes a small analogue signal & amplifies it. The second unit processes the analogue signal.

I want to put in a bit of bespoke circuitry between the two units.

My problem? How to physically connect to the two different units that have different grounds & rails! (ideally I'd like to source the voltage for my little circuit from the second unit which only has one 9V rail)

Maybe it's a simple solution?

The output of the first unit is really just an opamp output, the opamp has dual rails.....+7V & -7V. The unit's output signal will obviously be referenced to the unit's 'pseudo-ground'

The input to the second unit is also an opamp, but this time is a single supply +9V....once again with a pseud-ground.


Is it just a matter of 'commonising' (is there such a word?!) the units' grounds (even though they are in effect different becuase they aren't a true 'common' ground), with a cap in the signal path to DC decouple them?

[Andre Visser]

Yes, your reference voltage should always be tied together and be used as a reference throughout the system.

[HankMcSpank]

Originally posted by Andre Visser
Yes, your reference voltage should always be tied together

I'm not sure I fully understand the way you've written that?

Are you saying I should combine the two separate
grounds together? (one from each unit)

[Andre Visser]

Yes, on the one unit you created +7V, 0V and -7V and the other 0V and +9V. The 0V of each unit should be directly connected.

[HankMcSpank]

Many thanks.

So I take it, the fact that they're not both the same 0V (versus say a true 'common' ground), but each is merely 0V relative to their respective unit's rails, that there's no issue joining them together?

[Andre Visser]

Correct, if not connected these two 0V lines could float to any voltage relative to each other. You must create a common by connecting them together.

[Nico Ras]

Originally posted by Andre Visser
Correct, if not connected these two 0V lines could float to any voltage relative to each other. You must create a common by connecting them together.

I support what Andre wrote, he is correct.