Low-Power CMOS ΣΔ modulators for speech coding
A ΣΔ modulator with continuous-time loopfilter has some important advantages compared to its discrete-time counterpart. Bandwidth requirements to the active elements of the loopfilter are relaxed, so that power consumption is reduced. Furthermore, aliasing is reduced, eliminating the need for an anti-aliasing filter at the modulator input. A 4th order, 64 times oversampling ΣΔ modulator with microphone input was designed and shows 80 dB dynamic range over the 300–3400 Hz voice bandwidth. THD is −72dB for a 40mVRMS maximum input signal at 95 μA current consumption from a 2.2V supply voltage. The active die area of the modulator is 0.5 mm2 in a standard 0.5μm CMOS process.
KeywordsSpeech Code Versus Supply Voltage Input Transistor Clock Jitter Switch Capacitor Filter
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