An app note from Analog Devices: Build precise S/H Amps for fast 12 Bit ADCs (PDF!)
S/h amplifier circuits will use one of several standard architectures: closed-loop or open-loop, and inverting or noninverting configurations. No matter what circuit type designers employ, they need to incorporate a high-speed amplifier to buffer the input signal and a switch to connect and disconnect the input signal from the hold capacitor. In addition, a FET-input amplifier is needed to buffer the voltage on the hold capacitor and drive an a-d converter.