A basic sample-and-hold circuit consists of a voltage source (DAC in this case), a switch, a capacitor, and a buffer. As the name implies, this circuit has two modes of operation: sample and hold. In sample mode, the switch is closed connecting the DAC output to the hold capacitor, C_H. In hold mode, the switch opens, disconnecting the DAC output from C_H. Thus, the final output is held to the sampled value because of the charge stored on hold capacitor C_H. The output buffer is needed for delivering the required current. In a practical circuit, the switch leakage and the amplifier bias current make the capacitor drift from the stored value. Therefore, the sample-and-hold circuit must be refreshed, even if the DAC value does not change. The key design parameters of a sample-and-hold circuit are charge injection and voltage droop.