Printed Circuit Board Grounding Considerations
A-2
Grounding Considerations
A.1 Printed Circuit Board Grounding Considerations
When designing analog circuits that share a ground with digital and high
current power supplies, the voltage drop along the high current paths must be
considered. This voltage drop is a result of the current flowing through the
greater than zero resistance of the current path, or high frequency current
transients flowing through a greater than zero inductance of a current path.
If the signal ground is connected to the power supply ground at an improper
location, an excessive voltage drop may occur in the signal ground and
appears as part of the signal, causing an error.
The solution for low frequency analog signals is to establish a single ground
point on the PC board and connect all low frequency grounds to that point. By
using this method, currents flowing along any one path to ground do not
produce error voltages in any other ground path.
Analyzing the current flow paths within the analog section gives an indication
of which components can be lumped together to a common ground path and
which should be separate. One half LSB error with a reference of 4.1 volts
would be approximately 0.5 mV, so the ground trace resistance would have to
be greater than 0.5 ohms with 1 mA of ground current.
When input source signals are low current, a common ground trace may be
appropriate. Higher input current sources, however should always have a
separate ground trace to the most robust ground point location, usually at the
ground entrance to the PCB.
Even though the TLV1544 operating current is low, some high speed current
transients are present, usually caused by output digital switching requiring a
ground plane or wide ground return trace to the central board entry ground for
these signals. All signal paths and their respective ground returns must be
examined to minimize signal loop area.
The power inputs and V
CC
lines must also be analyzed in the same manner
and detail that the ground returns are.
Summary of Contents for TLV1544EVM
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