In this paper the transfer function of a system with windowed current integration is discussed. This kind of integration is usually used in a sampling mixer and the current is generated by a transconductance amplifier (TA). The parasitic capacitance (C_p) and the output resistance of the TA (R_o,TA) before the sampling mixer heavily affect the performance. Calculations based on a model including the parasitic capacitance and the output resistance of the TA is carried out. Calculation results show that due to the parasitic capacitance, a notch at the sampling frequency appears, which is very harmful because it causes the gain near the sampling frequency to decrease greatly. The output resistance of the TA makes the depth of the notches shallow and decreases the gain near the sampling frequency. To suppress the effect of C_p and R_o,TA, an operational amplifier is introduced in parallel with the sampling capacitance (C_s). Simulation results show that there is a 17dB gain increase while C_s is 1pF, g_m is 9mS, N is 8 with a clock rate of 800MHz.