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

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.

• 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.

Section II

Current-Mode Wilkinson ADCDesign 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

• 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

Section III

MeasurementsTest 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

• 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

Thank You

Questions?