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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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4.14 The Analog/digital Interface Problem

Analog and digital circuits must often be located on the same board. Analog circuits can be sensitive to interference signals as small as 10 µV. Analog circuits can often rectify out-of-band signals and this introduces offset errors. An example of interference occurs in very accurate A/D converters. If the signal sample includes noise, the A/D converter generates an incorrect result.

It is often suggested that an analog circuit should have a separate ground plane. This solution still requires a connection between the two grounds. This solution forces any interfering currents to concentrate near the point of connection. This interference field, in turn, couples interference into the A/D converter. This approach is thus not recommended.

The easiest way to limit interference coupling is to make sure that the fields of the analog and digital signal processing do not share the same physical space. The rules that provide this separation are as follows:

Analog components should not mix with digital components.
Analog traces should not mix with digital traces.
Connections through a connector should be separated by function. This includes grounds and power. The circuits should not share the same power decoupling or power leads.
The only thing the analog and digital circuits can share is the ground plane. If this is done then there will not be an interference problem. The A/D converter should be oriented so that the analog input terminals face ...

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