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Application: For high SNR and SFDR, such as xDSL and Hi-Fi audio.
Preferred architecture: Multi-bit ∆ΣM (Delta-Sigma Modulator).
Problem: Nonlinear DAC in the feedback loop degrades the performance.
A 94dB SFDR 78dB DR 2.2MHz BW Multi-bit A 94dB SFDR 78dB DR 2.2MHz BW Multi-bit Delta-Sigma Modulator with Noise Shaping DACDelta-Sigma Modulator with Noise Shaping DAC
A 94dB SFDR 78dB DR 2.2MHz BW Multi-bit A 94dB SFDR 78dB DR 2.2MHz BW Multi-bit Delta-Sigma Modulator with Noise Shaping DACDelta-Sigma Modulator with Noise Shaping DAC
Input Output Multi-level Quantizer
Multibit DAC
Loop Filter
Existing solution
Pros: DEM randomizes the mismatch in DAC and spread the energy of the toned noise to the entire band. Thus, the nonlinearity is improved and the SFDR is increased.
Cons: The spread noise increases the noise floor, and hence the in-band noise power. SNR of the M is degraded.
In conclusion: The DEM improves SFDR, but degrades SNR. In other words, it trades SNR for SFDR.
Nonlinear DAC
1st -order shaping to DAC noise DEM reduces tones Noise shaping DAC or NSDEM improves
both SFDR and SNR
Proposed Lowpass Proposed Lowpass M with Noise Shaping DACM with Noise Shaping DACProposed Lowpass Proposed Lowpass M with Noise Shaping DACM with Noise Shaping DAC
Differentiator Accumulator
U(z) V(z)
HD(z)
Quantizer
DAC with NSDEM
DAC Noise D(z)
Quantiazation Noise Q(z)
H(z)
DEM HI(z)
21Cdu
Vr+
Vr- 1D
1D
2d1dVin+
Csu
16x Csu
Cdu
2d
1d
12
2d
1d 2
2d
1d
1 12
2d
1d 2
2d
1d
1
1d1
2d1d2d1d2d1d
2d2a
16xControl Logic
4-b Quantizer
169Barrel Shifer
16
16
Overflow Logic
Barrel Shifer
8Pointer Selection
84-b Accumulator
<2j>DFF
2n
Barrel Shifer
8Pointer Selection
8
<2j+1>DFF
2n
DFF
2n
1
DFF
2n
16
2d1d
Q
2d1d
4-b DAC and Differentiator
16 16
PDWA
16x
Encoder4
Vout
1
1
5th Order Loop Filter
Qn
DAC + HD(z)
5th-order 4-bit Quan. lowpass M with NSDEM
HI(z) DEM (PDWA)
Quantizer
H(z) – loop filter
94dB
Signal Bandwidth 2.2MHz
Clock Frequency 35.2MHz
SFDR / DR 94dB / 78dB
Peak SNR / SNDR 77dB / 69dB
Input Range 5.04Vpp (differential)
Power Consumption 62mW @ 3.3V Supply
Technology 0.35μm CMOS
Accumulator, DEM, Control
Logic Quantizer
1st I
nteg
rato
r D
ifer
enti
ator
CLK Gen
Output Buffer
2n
d I
nteg
rato
r3rd
Int
egra
tor
4th I
nteg
rato
r
5th I
nteg
rato
r
Gai
n St
age
Noise floor is limited by switches thermal noise of input signal path
No input signal DAC thermal noise is shaped Reference noise is also shaped
NSDEM is off
NSDEM is on
Unlike most of the existing DEMs that trade SNR for SFDR. NSDEM improves both DAC SFDR and SNR. NSDEM shapes the inherent DAC thermal noise. Fabricated M chip meets the specification for ADSL2+.