In high precision circuits much effort is spent in compensating charge injection effects and usually, the resulting errors are determined by simulation only. However, little attention is paid on a detailed analysis of the charge flow although even simple and basic calculations can provide useful insight. In this paper we consider as example a switched capacitor sample-and-hold-circuit that is frequently used in high-resolution, high-speed analog-to-digital converters. As result of our detailed analysis we obtain analytical expressions that are used to derive rules for increasing the precision of the circuit without reducing its operation speed significantly.