Effects of the sampling pulse width on the frequency characteristics of a sample-and-hold circuit

Frequency characteristics, especially at high frequency, are important in applications such as sample-and-hold circuits for LCD driver ICs where the required signal bandwidth is much wider than the Nyquist rate. The frequency characteristics of a sample-and-hold circuit using analogue switches have hitherto been estimated from the frequency characteristics of a simple RC circuit comprising a storage capacitor and the on-resistance of the analogue switch. However, the effects of sampling pulse width and storage capacitor resetting are not taken into account. By giving an exact frequency-domain analysis of sample-and-hold circuits, the paper clarifies the limitations of conventional RC estimates. It also reveals the effects of storage capacitor resetting. A narrower pulse causes more ripples in the frequency characteristics. Storage capacitor resetting alleviates these ripples although the DC gain deteriorates for a narrow sampling pulse. This analysis was verified by simple experiments. A design for an LCD driver IC is also described as an application example