Post on 17-Dec-2015
Current-Mode Multi-Channel Integrating ADC
Electrical Engineering and Computer ScienceAdvisor: Dr. Benjamin J. Blalock
Neena Nambiar16st 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
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.
• 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
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.
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
• 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
Comparators
Ramp GeneratorGray Code Counter
Digital
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
• DNL < 0.5 LSB
• INL > 0.5 LSB
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
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