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Signal Chain,  connecting units that each have floating supplies?
Signal Chain,  connecting units that each have floating supplies?
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Old 27th December 2008, 10:16 AM   #1
HankMcSpank is offline HankMcSpank  United Kingdom
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Default Signal Chain, connecting units that each have floating supplies?

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?

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Old 27th December 2008, 10:35 AM   #2
Andre Visser is offline Andre Visser  South Africa
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Yes, your reference voltage should always be tied together and be used as a reference throughout the system.
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Old 27th December 2008, 01:18 PM   #3
HankMcSpank is offline HankMcSpank  United Kingdom
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Quote:
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)
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Old 27th December 2008, 02:00 PM   #4
Andre Visser is offline Andre Visser  South Africa
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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.
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Old 27th December 2008, 05:23 PM   #5
HankMcSpank is offline HankMcSpank  United Kingdom
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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?
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Old 27th December 2008, 05:38 PM   #6
Andre Visser is offline Andre Visser  South Africa
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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.
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Old 27th December 2008, 10:30 PM   #7
Nico Ras is offline Nico Ras  South Africa
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Quote:
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.
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