Define tracking and hold mode

A sample-and-hold (S/H) circuit has an electronic switch, a hold capacitor $C$ and buffer amplifiers. It works in two modes set by a logic control signal.

 Vin --[A1]--o/ o--+--[A2]--> Vout
                   |
                  === C
                   |
                  GND

Tracking (sample) mode

• The switch is closed.
• The capacitor charges through the low-impedance input buffer and the output follows the input: $V_{out}(t) \approx V_{in}(t)$.
• The time taken for the output to settle to the input value is the acquisition time.

Hold mode

• The switch is opened at the sampling instant.
• The capacitor keeps the voltage it had at that instant, and the high-impedance output buffer prevents discharge, so $V_{out}$ stays constant at $V_{in}(kT)$.
• The A/D converter converts this constant voltage during the hold time.
• Small errors in this mode: droop (slow fall of voltage due to leakage) and feedthrough.