Sample-and-Hold Amplifier (SHA)

Aperture jitter is the sample-to-sample variation in the precise instant the SHA switch opens. This timing uncertainty modulates the phase of the digitized waveform, creating a unique, clock-related fingerprint.

• Mechanism: Noise in the sampling clock or switch driver causes the hold command to arrive slightly early or late.
• Impact: The voltage error is proportional to the signal's slew rate at the sampling instant. Higher frequency inputs suffer greater amplitude errors.
• Fingerprint: The resulting phase noise sidebands and non-uniform sampling artifacts are specific to the clock generation and distribution circuitry of a particular device.

Pedestal error is an undesired DC voltage offset introduced at the output during the transition from sample mode to hold mode. It is primarily caused by charge injection and clock feedthrough from the sampling switch.

• Charge Injection: When the MOSFET switch turns off, the mobile charge in its channel is dumped onto the hold capacitor, causing a voltage step.
• Clock Feedthrough: The gate-drive clock signal couples through the gate-drain overlap capacitance directly to the hold capacitor.
• Fingerprint: This error is a function of the switch's exact physical dimensions, threshold voltage, and layout parasitics, making it a strong, static, device-specific DC bias.

Droop is the gradual decrease in the held output voltage over time during the hold mode. It is caused by leakage currents discharging the hold capacitor.

• Sources: Leakage through the off-state sampling switch, the input bias current of the output buffer amplifier, and the intrinsic dielectric absorption of the capacitor itself.
• Fingerprint: The droop rate (dV/dt) is highly dependent on temperature, semiconductor process variations, and the physical quality of the capacitor. This creates a time-dependent signature that can be measured over the ADC's conversion cycle.

Dielectric absorption (or soakage) is a memory effect in the hold capacitor where a residual charge persists after a rapid voltage change, causing the held voltage to creep toward a previous value.

• Mechanism: Dipoles in the capacitor's dielectric material align slowly with the applied field and do not relax instantly when the field changes.
• Fingerprint: This introduces a history-dependent error, where the current held sample is contaminated by the voltage of prior samples. This dynamic non-linearity creates a complex, signal-correlated signature that is extremely difficult to clone or model without the exact physical component.

Acquisition time is the finite interval required for the hold capacitor to charge to the input voltage and settle to within a specified error band. Non-linear slewing during this period creates distortion.

• Slewing: If the input buffer cannot supply enough current, the capacitor voltage changes at a fixed maximum rate (slew rate), causing a non-linear tracking error for large, fast input steps.
• Fingerprint: The specific slew-rate limit and the shape of the small-signal settling tail (often an underdamped ringing) are determined by the amplifier's compensation and parasitic poles, providing a unique transient signature.

Kickback noise is a transient disturbance injected back into the input signal source when the sampling switch closes. The internal charge redistribution creates a high-frequency glitch.

• Mechanism: The sudden connection of the discharged hold capacitor to the input node causes a rapid current draw, which interacts with the source impedance to create a voltage spike.
• Fingerprint: The amplitude and spectral content of this glitch depend on the switch's on-resistance, the capacitor value, and the exact parasitic routing. This signature can be detected by analyzing the input signal's disturbance during the sampling instant.