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Due to the heavily doped substrate material in modern MOS technologies, digital interference couples to sensitive analog nodes much more easily than in technologies with p/sup -/ substrates. The guard ring method of isolating sensitive nodes is no longer as effective as before. In this contribution we investigate the isolation of analog circuits by etching a gap between analog and digital circuits from the back of the wafer. Experiments show that interference coupling 35 dB above noise floor is completely removed when a gap is etched around the analog circuit.