Power Efficient Continuous-Time Delta-Sigma Modulator Architectures for Wideband Analog to Digital Conversion
Date of Award
Open Access Dissertation
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Robert W. Jackson
Patrick A. Kelly
Electrical and Computer Engineering
This work presents novel continuous-time delta-sigma modulator architectures with low-power consumption and improved signal transfer functions which are suitable for wideband A/D conversion in wireless applications, e.g., 3G and 4G receivers. The research has explored two routes for improving the overall performance of continuous-time delta-sigma modulator. The first part of this work proposes the use of the power efficient Successive-Approximations (SAR) architecture, instead of the conventional Flash ADC, as the internal quantizer of the delta-sigma modulator. The SAR intrinsic latency has been addressed by means of a faster clock for the quantizer as well as full-period delay compensation. The use of SAR quantizer allows for increasing the resolution while reducing the total power consumption and complexity. A higher resolution quantizer, made feasible by the SAR, would allow implementing more aggressive noise shaping to facilitate wideband delta-sigma A/D conversion at lower over-sampling-rates. As proof of concept, a first-order CT delta-sigma modulator with a 5-bit SAR quantizer is designed and implemented in a 130 nm CMOS process which achieves 62 dB dynamic range over 1.92 MHz signal bandwidth meeting the requirements of the WCDMA standard. The prototype modulator draws 3.1 mW from a single 1.2 V supply and occupies 0.36 mm2 of die area.
The second part of this research addresses the issue of out-of-band peaking in the signal-transfer-function (STF) of the widely used feedforward structure. The STF peaking is harmful to the performance of the modulator as it allows an interferer to saturate the quantizer and result in severe harmonic distortion and instability. As a remedy to this problem a general low-pass and peaking-free STF design methodology has been proposed which allows for implementing an all-pole filter in the input signal path for any given NTF. Based on the proposed method, the STF peaking of any feedforward modulator can be eliminated using extra feed-in paths to all the integrator inputs.
A major drawback of the conventional feedforward topology having low-pass STF is the large sensitivity of the STF to the coefficients. In particular, component mismatch, due to random errors in the relative values of individual resistors or capacitors, can significantly degrade the anti-aliasing of the CT modulator and give rise to the unwanted STF peaking. To solve this problem two new architectures, namely dual-feedback and dual-feed-in are proposed which allow us to synthesize a low-pass STF with a smaller number of coefficients than the feedforward structure. The dual-feedback structure which shows significantly lower sensitivity to coefficient mismatch is extensively analyzed and simulated. Also for proof of concept a third-order modulator is implemented in a 130 nm CMOS process which achieves 76 dB dynamic-range over 5 MHz signal bandwidth meeting, for example, the requirements of a DVB-H receiver standard. In addition the modulator shows 77 dB anti-aliasing and less than 0.1 dB worst-case STF peaking. The measured power consumption of the modulator is 6 mW from a single 1.2 V and the die area is 0.56 mm2.
Ranjbar, Mohammad, "Power Efficient Continuous-Time Delta-Sigma Modulator Architectures for Wideband Analog to Digital Conversion" (2012). Open Access Dissertations. 597.