Transcript Current Mode Multichannel Wilkinson ADC

Current-Mode Multi-Channel
Integrating ADC
Electrical Engineering and Computer Science
Advisor: Dr. Benjamin J. Blalock
Neena Nambiar
16st April 2009
Outline
• Background
– Analog-to-Digital Converters
– Motivation
• Current mode ADC
– Current Ramp Generator
– Current Comparator
– Gray Code Counter
• Measurement
– Test Setup
– Test Results
• Conclusion
Section I
Background
Analog to Digital Converters (ADC)
• Convert analog information from the real world
to digital for information processing
• E.g.: temperature, pressure, voice, color, light,
etc.
• Important parameters include:
–
–
–
–
Sampling rate
Input signal range
Resolution
Power dissipation
• Types of ADCs
– Sensor data
– Communication systems
Motivation
• A multi-channel ADC to convert multiple sensor signals
to digital simultaneously.
• Voltage mode ADC converts voltage to
digital. Eg. Temperature sensor
• Sensors with current signals.
• Electrodes
• Photosensors provide current output signals
– Transimpedance amplifiers are implemented to convert current
to voltage.
– Bandwidth, offset, linearity of the amplifier play a key role in
achievable accuracy.
• Multi-channel current-mode Wilkinson used directly
– Reduction in power dissipation
– Improves area efficiency
Research Goals
• A current-mode ADC to support current output sensors
and photodiodes.
• Gray code counter
–
To support a high rate of counting
• Design to support 12-bit resolution
–
–
current-ramp generator
comparator
• The ADC should be tested for functionality and the
parameters of the ADC are to be measured.
Section II
Current-Mode Wilkinson ADC
Design Blocks
Multi-channel Wilkinson ADC
• Building blocks of the Multichannel Wilkinson ADC
– Single Ramp Generator
– Counter
– Number of comparators = number of channels
• ADC operation:
– Phase 1: comparators
sample the input and ramp
generator resets.
– Phase 2:
• ramp changes from 0 to
IREF
• counter counts from 0 to
4096 (e.g., 12 bit).
• channel comparator
changes state, storing
value of count
Current Ramp Generator
• Ramp Generator provides a linearly increasing current
with respect to time
• VDS modulation of Mp2 causing nonlinearity is dealt
with using a CG stage Mp1CG.
• Offset current due to OTA2 offset removed by using
C2, Mn1 and S2.
• Improved matching using active current mirrors
Current Comparator
• Current comparator compares two currents and
changes the output state depending on the comparison
• Minimum bias current maintained in signal path
transistors
• OTA5 (p-input differential pair) used instead of inverter
to reduce power dissipation.
• Lower capacitance penalty at input to OTA5
• Reduced coupling between input and output
Gray Code Counter
• Binary counters have
multiple bits changing
simultaneously.
• Gray code counters
have single bit
changing at a time.
– Avoids missing codes
and glitches in the
output due to multiple
bits toggling.
Decimal
Binary
Gray
0
00
00
1
01
01
2
10
11
3
11
10
Novel Gray Counter with Carry Look Ahead
• A new 12-bit Gray-code counter.
– Includes carry look ahead technique
– Clock skew problem is reduced
– No feedback from MSB to LSB, hence improved
frequency of operation
Chip Photograph and Layout
Ramp GeneratorGray Code
Counter
Comparators Digital
• 4-channel ADC implemented in AMI 0.5-µm process
• Total area including pad frame: 1.5 mm x 1.5 mm.
• Integrating capacitor off chip due to space constraint
Section III
Measurements
Test Setup
Test Results
Test Board
Differential Nonlinearity (DNL)
Integral Nonlinearity (INL)
• Monotonicity of the
ADC.
• Units in LSB (Least
significant bit)
• DNL < 0.5 LSB for
optimum performance
• INL net effect of the
DNL
• INL < 0.5 LSB for
optimum performance.
Results for DNL and INL
Histogram techniques used to measure DNL and INL. Four million
samples collected with an input sine wave, digitized output
compared with ideal sine wave histogram
• DNL < 0.5 LSB
• INL > 0.5 LSB
Ramp Generator Analysis
• Plot of the two inputs of OTA shows
– P-input linearity >12 bits (Blue)
– N-input linearity <12 bits (Green)
• A possible new OTA with higher linearity can be
used in place of the simple differential pair.
Original Contributions
• Novel 12-bit Multi-channel Current-Mode
Wilkinson ADC architecture
• Temperature independent Ramp signal
generator
• Gray-code counter without feedback or
clock skew effects
• Improved current comparator design
Conclusions
• A new Wilkinson-style (integrating) multi-channel
current-mode ADC architecture is described.
• The design blocks of the ADC were described.
– Ramp Generator
– Current Comparator
– Gray Code Counter
•
•
•
•
Test setup was described
Parameters of the ADC
Test results
Suggested improvements
Thank You
Questions?