Design of a High Precision Comparator for Implementation of a WDR Sensor

1. Miriam Pekar Alex Liberchuk Supervisors: Dr. Alexander Fish Mr. Arthur Spivak Design of a High Precision Comparator for Implementation of a WDR Sensor P-2011-130 10/2011

2. What is an Image Sensor? • An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices. • The two most popular kinds of image sensors are: • Charge-coupled device (CCD). • Complementary Metal–Oxide–Semiconductor (CMOS).

3. Why CMOS and not CCD? • CMOS is implemented using less components. • CMOS sensors consume less power. This is important in portable devices. • Provides faster readout. • Cheaper to manufacture. CMOS Drawbacks: • CMOS sensors, traditionally, are more susceptible to noise. • Light sensitivity of a CMOS chip tends to be lower because several transistors are located next to each photodiode. • CMOS sensors tend to have Low Dynamic Range.

4. Effects of Low Dynamic Range Imaging: Low DR Imaging Wide DR Imaging Dynamic Range quantifies the ability of a sensor to image highlights and shadows. Goal of Our Project: Improve the Dynamic Range of the CMOS Sensor

5. What is a CMOS Sensor? • It is an image sensor produced by a CMOS semiconductor process. • It consists of a photodiode and extra circuitry next to each photodiode converting the light energy to a voltage, later the voltage is converted to a digital signal.

6. What is a Comparator? a comparator is a device that compares two voltages and switches its output to indicate which is larger. • A good comparator implementation can be an Operational Amplifier connected in open loop.

7. The Use of the Comparator in a WDR Sensor: • If a pixel value exceeds the threshold - i.e. the pixel is expected to be saturated at the end of the exposure time - the reset is given at that time to that pixel. The binary information concerning the reset (i.e., if it is applied or not) is saved in a digital storage for later calculation of the scaling factor. Thus, we can represent the pixel output in the following floating- point format: M⋅2EXP. Here, the mantissa (M) represents the digitized pixel value, and the exponent (EXP) represents the scaling factor. • This way, the maximal signal value the sensor can process is raised – higher DR.

8. Project Process Flow Specifications Choose Suitable Comparator Topologies Design Procedures Set-up to determent W/L (each Topology) Full SPECTRA simulation Remaining Tasks

9. Our Project: Design a High Precision Comparator to Implement a WDR Sensor • Technology - TOWER 180nm • The Comparator’s Design Requirements: • Gain = 1000 • Bandwidth = 1 - 2 MHz • Slew Rate > 1.8 V/µsec • Power Dissipation < 100nW • CLoad= 150 fF • 0V < Vout < 3.3V • 0.2V < Vin < 2V GBW = 1-2 GHz

10. Project Process Flow Specifications Choose Suitable Comparator Topologies Design Procedures Set-up to determent W/L (each Topology) Full SPECTRA simulation Remaining Tasks

11. Comparator Topologies • Simple One-Stage • Two-Stage • Folded Cascode • Gain Boosted Folded Cascode

12. Project Process Flow Specifications Choose Suitable Comparator Topologies Design Procedures Set-up to determent W/L (each Topology) Full SPECTRA simulation Remaining Tasks

13. Simple One-Stage Comparator • The topology resulted in poor performance, due to poor gain and bandwidth

14. Two-Stage Comparator Active Load Bias Current Differential Pair Enable Switch Common Source Amplifier Current Mirror

15. ENABLE=ON ENABLE=OFF Two-Stage Comparator cont. • Results: Gain, BW Power Dissipation Slew Rate GBW = Gain*BW= (62.03dB)*1.4MHz = 1.769GHz All the design requirements were met!

16. Folded Cascode Comparator Current Source Cascode Transistors Differential Pair Common Source Amplifier Bias Circuit Current Mirror

17. Folded Cascode Comparator cont. ENABLE = ON ENABLE=OFF Power Dissipation Slew Rate Gain, BW GBW = Gain*BW= (60.12dB)*1.36MHz = 1.379GHz Results: All the design requirements were met!

18. Project Process Flow Specifications Choose Suitable Comparator Topologies Design Procedures Set-up to determent W/L (each Topology) Full SPECTRA simulation Remaining Tasks

19. Full SPECTRA simulation • DC analysis – make sure all transistors are in saturation mode • AC analysis – find a suitable W/L for the desired Gain, BW and GBW. • Transient analysis – checks the Slew Rate, and Power Dissipation. • Now, Corners were checked.

20. Project Process Flow Specifications Choose Suitable Comparator Topologies Design Procedures Set-up to determent W/L (each Topology) Full SPECTRA simulation Remaining Tasks

21. Remaining Tasks • Create and check Gain Boosted Folded Cascode topology. • Comparison of all topologies designed in this project. • Layout Implementation of the best topology and post layout simulations.

22. Questions תודה רבה!