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From: Mario Trams
Subject: Re: Still confused over supply/signal grounds
Date: Tue, 10 Dec 2002 20:29:29 +0100
Organization: Chemnitz University of Technology
NNTP-Posting-Date: 10 Dec 2002 19:26:19 GMT
> I have read some old threads about supply/signal grounds but I still
> have a few questions regarding that issue.
> If for example I have a analogue device and digital device sharing a
> double-sided pcb with the analogue device (with signal & supply
> ground) using a +-12 supply and the digital using +5 only. Both these
> devices are using separate power supplies with the digital device
> providing a timing signal to the analogue device.
> 1.Is the supply ground equivalent to the digital ground?
Basically yes. The "Ground" is where all voltage levels are
referenced to. Some devices have multiple ground pins that might
be labeled differently (i.e. AGND or DGND) in order to achieve better
electrical behaviour of the chip. All these grounds have to be connected
together (except in this case described below or when the manufacturer's
datasheet tells something else).
> 2.For example, if I were to assign the top-side of the pcb as the
> signal ground and the bottom-side of the pcb as the digital ground, is
> it advisable to have the digital ground plane close to the analogue
Usually, one makes two separate ground planes that are connected
directly under the interface between the digital and the analog
section (i.e. directly beneth a AD or DA converter).
Separate digital and analog sections visibly! Separating both
avoids digital crosstalk to be induced into analog signals.
> 3. According to the datasheet, the supply ground should be connected
> to the signal ground as close as possible to the analogue device. If
> that's the case, then the supply ground from the power supply should
> also connect to that same point. Please correct me if I'm wrong.
Grounds are always connected, finally. Except when you have galvanic
decoupling devices such as optocouplers or inductive couplers, which
is not the case in your design, I guess. One purpose of such devices
is to have separate grounds.
Hope this helps,
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