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+.