The TMUX6121, TMUX6122, or TMUX6123 switch is used in conjunction with the voltage holding capacitors (CH) to implement the sample and hold circuit. The basic operation is:
Ideally, the switch delivers only the input signals to the holding capacitors. However, when the switch gets toggled, some amount of charge also gets transferred to the switch output in the form of charge injection, resulting slight sampling error. The TMUX6121, TMUX6122, and TMUX6123 switches have excellent charge injection performance of only 0.51 pC, making them ideal choices for this implementation to minimize sampling error. Due to switch and capacitor leakage current, the voltage on the hold capacitors droops with time. The TMUX6121, TMUX6122, and TMUX6123 minimize the droops due to its ultra-low leakage performance. At 25°C, the TMUX6111, TMUX6112, and TMUX6113 have extremely tiny leakage current at 0.5 pA typical and 20 pA maximum. The TMUX6121, TMUX6122, and TMUX6123 devices also support high voltage capability. The devices support up to ± 16.5 V dual supply operation, making it an ideal solution in this high voltage sample and hold application.
A second switch SW1 is also included to operate in parallel with SW2 to reduce pedestal error during switch toggling. Because both switches are driven at the same potential, they act as common-mode signal to the op-amp, thereby minimizing the charge injection effects caused by the switch toggling action. Compensation network consisting of RC and CC is also added to further reduce the pedestal error, whiling reducing the hold-time glitch and improving the settling time of the circuit.