A built-in circuit includes a semiconductor substrate (SB) with a bottom part (PSB) and upper layer (CSB) strongly doped that the bottom part. A first block (BC3) and a second block (BC1) are formed in the upper part of the substrate. An isolating device is arranged closed to the second block (BC1) and includes an isolating circuit (CRS) linked to the bottom part of the substrate (PSB). A mass connection (PTMD1) provide a minimal impedance at a given frequency.
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