Application: For high SNR and SFDR, such as xDSL and Hi-Fi audio. Preferred architecture: Multi-bit ?SM (Delta-Sigma Modulator). Problem: Nonlinear DAC in the feedback loop degrades the performance.

1
Application For high SNR and SFDR, such as xDSL
and Hi-Fi audio.Preferred architecture
Multi-bit ?SM (Delta-Sigma Modulator).Problem
Nonlinear DAC in the feedback loop degrades the
performance.
A 94dB SFDR 78dB DR 2.2MHz BW Multi-bit
Delta-Sigma Modulator with Noise Shaping DAC
Nonlinear DAC
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 DSM
  is degraded.
• In conclusion The DEM improves SFDR, but
  degrades SNR. In other words, it trades SNR for
  SFDR.

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Proposed Lowpass DSM with Noise Shaping DAC
Quantiazation Noise Q(z)

• 1st -order shaping to DAC noise
• DEM reduces tones
• Noise shaping DAC or NSDEM improves both
  SFDR and SNR

U(z)
Quantizer
V(z)
H(z)
DAC with NSDEM
HI(z)
HD(z)
DEM
5th-order 4-bit Quan. lowpass DSM with NSDEM
DAC Noise D(z)
Accumulator
Differentiator
H(z) loop filter
Quantizer
HI(z)
DAC HD(z)
DEM (PDWA)
3

• No input signal
• DAC thermal noise is shaped
• Reference noise is also shaped

94dB
NSDEM is off
NSDEM is on
Noise floor is limited by switches thermal noise
of input signal path
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 _at_ 3.3V Supply
Technology 0.35µm CMOS

• 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 DSM chip meets the specification for
  ADSL2.