We propose an anti-piracy security technique based on locking for analog and mixed-signal circuits designed in FD-SOI. We show that obfuscating the body-bias voltages of tunable transistors is an effective way for inducing high functionality corruption. The obfuscation is achieved by constituting a secret key from the concatenation of the input digital codes of the body-bias generators that produce the correct body-bias voltages. We also propose a slight modification of the bodybias generator that increases prohibitively the time complexity of counter-attacks aiming at finding an approximate key. The proposed locking scheme is demonstrated on a Σ∆ modulator used in highly-digitized RF receiver architectures.