Cmos differential amplifier for a delta sigma modulator applicable for

Abstract

An analog-digital converter includes a .DELTA..SIGMA. modulator, a digital filter, a high-pass filter and a multiplier which are connected in series. Analog input is converted into serial-bit strings by the .DELTA..SIGMA. modulator, for which gain `1/A` is set. The digital filter extracts low-frequency components, corresponding to the analog input, from the serial-bit strings, so the low-frequency components are converted into parallel-bit digital data. The high-pass filter removes DC offset component from output of the digital filter; and then, output thereof is multiplied by scaling gain `A` by the multiplier so that digital output is produced. The .DELTA..SIGMA. modulator includes at least three switched-capacitor integrators and a one-bit quantizer, which are connected in series, as well as a one-sample delay circuit. One-bit output, produced by the one-bit quantizer, is delayed by the one-sample delay circuit, whose output is delivered to each switched-capacitor integrator. Each switched-capacitor integrator is configured using a CMOS differential amplifier which is configured by a CMOS operational amplifier and at least one amplitude-limiting circuit. The amplitude-limiting circuit is configured by two PMOS transistors and two NMOS transistors which are connected in parallel in a diode-connection manner; and this circuit is provided to limit amplitude in output of the CMOS differential amplifier by stabilizing its operating point.