1. Current-Mode Multi-Channel Integrating ADC Electrical Engineering and Computer Science Advisor: Dr. Benjamin J. Blalock Neena Nambiar 16 st April 2009

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

3. Section I Background

4. 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

5. 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

6. 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.

7. Section II Current-Mode Wilkinson ADC Design Blocks

8. 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 I REF counter counts from 0 to 4096 (e.g., 12 bit). channel comparator changes state, storing value of count

9. Current Ramp Generator Ramp Generator provides a linearly increasing current with respect to time V DS 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

10. 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

11. 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. DecimalBinaryGray 000 101 21011 3 10

12. 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

13. 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 Generator Gray Code Counter Digital

14. Section III Measurements Test Setup Test Results

15. Test Board

16. 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.

17. 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

18. 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.

19. 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

20. 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

21. Thank You Questions?