1. Contemporary Design of High-
Speed, High-Performance, Analog
to Digital Converters in Deep
Submicron CMOS
,Mikko Waltari
San Diego, CA
May 2,2, 2012
May 2012 1
2. Outline
• Motivation
• Deep Submicron Challenges for ADC Design
• ADC Front-End
• Linearity Calibration
• Time Interleaving
• 14-bit 200MS/s ADC core
May 2, 2012 2
3. Motivation
• Demand for high-speed high-resolution (>12 bits, >100MS/s)
ADCs increasing.
– Main applications in wired and wireless communications.
• The architecture most suitable for meeting these
specifications is the pipeline ADC.
• Most commercially available products are implemented in
SiGe or 0.18µm CMOS.
• Deep sub-micron CMOS offers many attractive benefits
– Low power consumption
– High sampling rate
– Higher level of integration
• Multi-channel products, SoC integration, utilization of DSP for increased
performance, integration of digital support functions (decimation, IQ
mismatch correction, down conversion, etc)
May 2, 2012 3
4. Deep Sub-Micron Challenges for ADC
Design
• Low supply voltage (< 1.2V).
– Limited analog voltage swing.
– Many traditionally used analog circuit structures
unfeasible.
• Low intrinsic gain of short channel devices.
– Difficult to design high gain amplifiers, which are a key
building blocks in pipelined ADCs.
• Deep sub-micron is good for digital
– Small size, low power consumption.
-> use of digital techniques to enhance the analog
performance.
May 2, 2012 4
5. ADC Front-End
• The front-end largely
determines the ADC
linearity at high 70dB SNDR
frequencies.
• The most demanding
50dB
application is the direct
IF sampling.
– The use of sub-sampling F in
(signal frequency > Fs/2) Typical Behavior
increases the challenge.
May 2, 2012 5
6. Front-End Driver
• ADC input has a ADC
switched sampling
capacitor.
– Difficult to drive with ADC
good linearity.
• Many SiGe/BiCMOS
designs use integrated • Alternative is to make
buffer amplifier. the capacitor easier to
– Designing such an
amplifier in CMOS drive.
impractical. – Eliminate nonlinear
charge kick-back.
May 2, 2012 6
7. Sampling Switch
• Largely determines the ADC linearity at high signal
frequencies.
• Simplest switch is a single NMOS transistor or a
transmission gate.
– Resistance depends on the signal voltage -> poor linearity.
• Technique called bootstrapping makes the transistor
gate voltage follow the input signal -> signal
independent on-resistance.
– Utilizing deep n-well provides further improvement.
May 2, 2012 7
8. Switch Circuits
CLK
S in g le tra n s is to r s w itc h
C LK CLK CLK CLK
CLK
B o o ts tra p p e d s w itc h CLK
w ith d e e p N - w e ll
May 2, 2012 8
9. Linearity Calibration
• Capacitor mismatch and low amplifier gain produce INL
(integral non-linearity) errors -> poor SNDR.
• Can be improved by increasing device sizes -> larger area,
higher power consumption.
• Digital Calibration is a better solution
– Correction coefficient added to every ADC output sample.
• Foreground calibration at power up with a help of a
calibration DAC. Doesn’t track temperature and voltage
variations.
• Continuous background calibration utilizing the statistics of
the input signal to determine the calibration coefficients.
May 2, 2012 9
11. Time-Interleaving
• ADC sampling
rate can be ADC1
increased by C LK
In O ut
time
interleaving two ADC2
or more ADC
cores.
C L K /2
May 2, 2012 11
12. Time-Interleaving Errors
• Gain mismatch, voltage offset, and clock skew
between the ADC cores create spectral
artifacts.
• The effect of timing skew is signal frequency
dependent, limiting the high frequency
performance of time-interleaved ADC.
May 2, 2012 12
13. Time-Interleaving Calibration
• Gain mismatch and voltage offset relatively easy to
calibrate.
• Timing skew calibration is more difficult.
• Calibration is divided into two tasks:
1. Error Detection
2. Error Correction
• Correction can be done digitally or by feeding back
an analog correction signal using a DAC
• Detection is the more difficult of the two tasks.
May 2, 2012 13
14. Clock Skew Calibration Using Out of
Band Test Signal
• A low-level narrow-
band test signal is
injected into the ADC
Test Signal
Generation
input outside the DAC
signal band.
– A known test signal Analog ADC1 Digital
Out
In
makes the error
MUX
detection robust and ADC2
Mismatch Error
relatively fast.
ge
Parameter te Signal
Estimation Generation
• Error correction done
with digital filters.
May 2, 2012 14
15. Time Interleaving Calibration
• Multi-tone
pattern shows
image tones
before
calibration.
• Calibration
reduces the
artifacts by
more than
40dB.
May 2, 2012 15
16. bit 200MS/s ADC core in 65nm-14
CMOS
• 65nm CMOS process, 1.2V supply voltage
• High linearity input sampling: >78dB SFDR up
to 366MHz input frequency.
• Background linearity calibration
• Low power consumption (130mW)
• 2x time interleaving to extend the sampling
rate to 400MS/s
May 2, 2012 16
