An approach to tackle quantization noise folding in double-sampling ΣΔ modulation A/D converters

Abstract

61-modulation is a proven method to realize high-and very high-resolution analog-to-digital converters. A particularly efficient way to implement such a modulator uses double-sampling where the circuit operates during both clock phases of the master-clock. Hence, the sampling frequency is twice the master-clock frequency. Unfortunately, path mismatch between both sampling branches causes a part of the quantization noise to fold from the Nyquist frequency back in the signal band. Therefore, the performance is severely degraded. In this paper, we show that the problem is reduced but not eliminated by employing multibit quantization. Next, we present an indepth solution for the problem. The approach consists of modifying the quantization noise transfer function of the overall modulator to have one or several zeros at the Nyquist frequency. This way the effect of noise folding can nearly be eliminated. It is shown that this can be implemented by a simple modification of one of the integrators of the overall modulator circuit. Finally, several design examples of single-bit and multibit modulators are discussed.

A delay-based complex double-sampled resonator for use in fs/4 quadrature bandpass .SIGMA..DELTA. modulators

“…Since amplitude modulation of v(n) with (-1) n corresponds in the Z-domain to a substitution z → -z [1], the Z-transform of v I out and v Q out are equal to:…”

“…One of such techniques is the modified NTF and is developed separately in the context of QBP [3] and doublesampling [1]. Here, the NTF is modified such that it has one zeros in the image band, next to those in the signal band of interest.…”

“…In-creasing the sample frequency, normally leads to the requirement of faster circuitry. In this regard, double-sampling is interesting because it allows switched capacitor (SC) circuits to update their output during both clock phases [1]. This way, a sampling frequency of twice the clock frequency is achieved without requiring a faster opamp [2].…”

Efficient double-sampled cascaded .SIGMA..DELTA. modulator topologies for low OSRs

“…The double-sampling resonator with a unit delay can be implemented using only one opamp as proposed in [5]. So, only two opamps like the the proposed second-order double-sampling modulator, and (c) the proposed double-sampling cascade 2-2 modulator topology.…”

“…This is the main drawback of double sampling modulators. Several techniques have been proposed to alleviate the quantization noise folding in doubled-sampled modulators such as employing the fully floating method in the feedback DAC paths and/or placing a zero at the half of the sampling frequency (f s /2) of the NTF, etc [3,4,5]. However, both techniques affect the modulator's NTF and signal transfer function (STF) and hence the modulator's noise shaping ability.…”