In this thesis, a switched-current sigma-delta modulator (SDM) with op-amp active integrator is proposed. The major study is focused on using the op-amp to reduce the input impedance for high speed and high solution and utilizes the dummy switch to decrease the clock feedthrough (CFT) error. We use a sample-and-hold circuit which consists of an op-amp active memory cell and a dummy switch circuit to implement the integrator. It is applied to the building blocks of SDM. The modulator is a second order sigma-delta modulator. A current comparator transforms the current signal into digital voltage signal. A single-bit digital-to-analog (D/A) feedback circuit is used to convert the one-bit digital output to the SI integrator .The modulator is designed in the current mode technique. The delta-sigma modulator simulates using the parameters of the TSMC 0.35£gm CMOS process. The simulation results show that the signal to noise plus distortion ratio (SNDR) is 72 dB, the sampling rate is 10.24MHz, the oversampling ratio is 128, the power consumption is 21mW, the dynamic range is about 70dB, and the power supply is 3.3V. Furthermore, the circuit is verified by cadence-hspice simulation.