Bootstrapped Switch

A bootstrapped switch is a sampling-switch circuit that holds its own gate-source voltage constant no matter where the input signal happens to sit, specifically to defeat a signal-dependent on-resistance that a plain switch cannot avoid — a problem invisible in a textbook ideal-switch model, but one that directly limits how linear a real sample-and-hold circuit's output can actually be.

The problem starts with where a simple MOSFET switch's on-resistance actually comes from, and why that resistance is not the fixed quantity a basic circuit diagram implies. A MOSFET's channel resistance in the triode region depends on its gate-source overdrive — the larger V_GS is above threshold, the lower the resistance. In an ordinary sampling switch, the gate is driven to a fixed rail, typically VDD, while the source terminal tracks whatever the input signal is doing during the sampling phase. That means V_GS itself — VDD minus the input voltage — changes continuously as the input signal swings, which means the switch's own on-resistance changes continuously too, lower when the input sits near ground and higher as it approaches VDD.

A signal-dependent on-resistance is not merely a speed inconvenience — it is a deterministic nonlinearity that shows up directly as harmonic distortion, exactly the kind of spur spurious-free dynamic range already covered in this series exists to quantify. The RC time constant governing how completely a sampling capacitor charges during the available sampling window depends on that same on-resistance, so a signal-dependent resistance produces a signal-dependent settling error — the sampled value ends up systematically off by an amount that depends on where in its swing the input signal happened to be at the sampling instant. Because that error correlates with the input signal itself rather than behaving as random noise, it does not average out or show up as a noise floor; it shows up as harmonic distortion tones in the output spectrum, directly degrading SFDR in exactly the way a converter's worst spur, not its average noise, sets its usable performance ceiling.

Bootstrapping solves this by making the switch's gate float relative to the source rather than staying tied to a fixed rail, so the gate tracks the input signal automatically and V_GS stays essentially constant regardless of signal level. The core mechanism uses a capacitor pre-charged to VDD during a setup phase, then connected between the switch's gate and source during the actual sampling phase — because that capacitor's voltage stays fixed at VDD while both its terminals are now free to move together with the source, the gate effectively rides VDD above wherever the input signal happens to be at any given instant. The result is a gate-source voltage held at essentially VDD throughout the entire input swing, producing an on-resistance that stays nearly constant regardless of signal level, precisely the flat curve a plain switch cannot deliver.

Bootstrapping fixes on-resistance linearity specifically, and it is important to be precise about what it does not fix, because the distinction matters for real circuit design. The bootstrapped switch is still an ordinary MOSFET conducting through an inverted channel while it's on, and that channel still holds mobile charge that has to go somewhere the instant the switch turns off — exactly the charge injection mechanism this series has already covered in detail. Bootstrapping addresses the on-resistance-versus-signal-level problem entirely separately from charge injection, and a real high-linearity sampling switch design typically needs both techniques simultaneously: bootstrapping for on-resistance linearity, and dummy-switch cancellation or another charge-injection mitigation for the turn-off transient, because neither technique touches the error mechanism the other one targets.

This combination of techniques is exactly why bootstrapped switches are the standard choice for high-resolution SAR ADC sample-and-hold front ends rather than an exotic option reserved for specialized designs. A successive-approximation converter's input sampling switch sees the full analog input swing directly, and at resolutions of fourteen bits or higher, the harmonic distortion a plain switch's signal-dependent on-resistance would introduce is large enough to directly limit the converter's achievable SFDR and total harmonic distortion specifications — performance numbers the rest of the converter's precision would otherwise be wasted supporting if the front-end sampling switch itself were the actual bottleneck.

A bootstrapped switch is a reminder that a sampling switch's job is not just to connect and disconnect a signal cleanly in time, which charge injection and its cancellation already address, but also to present a genuinely linear impedance while it's conducting, which is an entirely separate requirement with its own entirely separate failure mode. High-resolution converters need both problems solved simultaneously, and neither one's solution substitutes for the other.