CurrentMode MultiChannel Integrating ADC

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Current-Mode Multi-Channel Integrating ADC

• Electrical Engineering and Computer Science
• Advisor Dr. Benjamin J. Blalock
• Neena Nambiar
• 16st April 2009

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Outline

• Background
• Analog-to-Digital Converters
• Motivation
• Current mode ADC
• Current Ramp Generator
• Current Comparator
• Gray Code Counter
• Measurement
• Test Setup
• Test Results
• Conclusion

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Section IBackground
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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

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Motivation

• A multi-channel ADC to convert multiple sensor
  signals to digital simultaneously.
• 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

• Voltage mode ADC converts voltage to digital. Eg.
  Temperature sensor
• Sensors with current signals.
• Electrodes

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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.

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Section IICurrent-Mode Wilkinson ADCDesign
Blocks
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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

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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

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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

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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.

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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

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Chip Photograph and Layout

• 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

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Section IIIMeasurementsTest SetupTest Results

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Test Board
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Differential Nonlinearity (DNL)Integral
Nonlinearity (INL)

• Monotonicity of the ADC.
• Units in LSB (Least significant bit)
• DNL lt 0.5 LSB for optimum performance
• INL net effect of the DNL
• INL lt 0.5 LSB for optimum performance.

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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 lt 0.5 LSB
• INL gt 0.5 LSB

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Ramp Generator Analysis

• Plot of the two inputs of OTA shows
• P-input linearity gt12 bits (Blue)
• N-input linearity lt12 bits (Green)
• A possible new OTA with higher linearity can be
  used in place of the simple differential pair.

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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

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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

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Thank You

• Questions?