A study of a low-voltage, low-power, current-mode ΣΔ modulator circuit

The possibility of realizing a low-voltage and low-power audio-band Sigma Delta modulator has been studied. The current-mode circuit technique has been adopted to enable the modulator to operate at less than 3 V supply voltage. The modulator to uses analog integrators instead of switched-capacitor-type integrators to avoid the use of analog switches, which allows low-voltage operation of the modulator circuit. A newly designed voltage-to-current converter is used to form an analog integrator. The designed circuit was actually fabricated by using a CMOS 0.6 mu m process and its characteristics were evaluated. A signal-to-noise ratio of 68 dB in a 20 kHz bandwidth for the first-order modulator construction was measured from the 2.5 V supply voltage at an oversampling ratio of 256. However, only 72 dB of the signal-to-noise ratio and 65 dB of distortion were measured for the second-order modulator construction, although operation of the oversampling ratio of 48 was successfully confirmed. We concluded that low-voltage and low-power operation with less than 3 V of supply voltage is possible for the audio-band Sigma Delta modulator, although signal-to-noise ratio and distortion issues in the 2nd-order configuration are left for future study. (C) 1998 Scripts Technica, Elects Eng Jpn, 124(1): 24-32, 1998.