Low-Area 4th-Order Shared-Amplifier Sigma-Delta Modulator

Analog-to-digital converters provide a vital interface in mixed-signal electronic systems. One of the typical approaches to implement high-resolution sigma-delta modulators often involves the choice of high-order loop-filters, which imply the use of a large number of integrators. As in common topologies each integrator is implemented by one operational amplifier, high-order modulators can demand a high number of operational amplifiers, increasing the total circuit area and power consumption. In order to overcome this feature, a shared-opamp technique is explored in this work. It is a 4-order sigma-delta modulator, with the first stage being implemented by one op-amp, and the last 3 stages being shared by the same op-amp. This fact makes possible not only to reduce the total power consumption of the integrators, but also reduces the total number of op-amps by half, which lead to area reduction. Simulations results show a peak SNR of 100.1 dB and a THD of -80 dB were achieved for the audio band from 20 to 20 kHz, with a total power consumption of 9.83 mW, using the XFAB 1.8 V CMOS 0.18 technology.