1. An Oscillating Active CMOS Pixel for Subretinal Stimulation D. Ziegler, S. Ferazzutti, D. Bertrand, A.M. Ionescu and Ph. Renaud EPFL Microsystems Laboratory (Lausanne, Switzerland) Published at Eurosensors XVI DMS Group Meeting: October 18, 2002

2. An Oscillating Active CMOS Pixel for Subretinal Stimulation This article presents a photodiode circuit whose output is intended to simulate the input necessary to replace biological photoreceptors in stimulating the first-stage processing circuitry in the retina. The circuits are intended to be powered remotely, via RF control, and to become a true “replacement” for failed photoreceptors in the human retina

3. An Oscillating Active CMOS Pixel for Subretinal Stimulation Pieces of the Puzzle –Photoreceptor = PIN photodiode fabricated in a standard 0.35 micron CMOS process sensitivie between 1mW/cm 2 to 10mW/cm 2 –Oscillator circuitry = converts light intensity to a pulse stream, whose frequency is proportional to intensity frequencies between 4 and 400 Hz (low to high intensity) –RF interface = remote powering of photoreceptors, oscillator circuits and other support circuitry. –Single pixel (75 X 75  m 2 and 100 X 100  m 2 ) results presented in this work.

4. An Oscillating Active CMOS Pixel for Subretinal Stimulation The Oscillation Circuits: how do they work? C V1V1

5. Let’s first put in the parasitic capacitances: C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2

6. The Oscillation Circuits: how do they work? Understanding Basic Operation of the Circuit C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 When D1 is on, D2 tends to be off and vice versa The Photodiode controls the “conductance” of T2 and therefore how fast current will charge C p2 The voltage V 1 controls the “conductance of T1 and therefore how fast current will charge C p1

7. The Oscillation Circuits: how do they work? Breaking the Feedback Loop C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 S is the present state of the output S’ is the next stage of the output If S is low D1 is off and T is high R must also be high (for the inverter output S to be low) D2 must be on then if R is high and S is low S’ S T R

8. The Oscillation Circuits: how do they work? Breaking the Feedback Loop C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 As current flows through D2 through the “conductance” determined by V 1 on T1, S’ starts to charge, going high. The conductance of T1 establishes how fast C p1 will charge. S’ S T R

9. The Oscillation Circuits: how do they work? Breaking the Feedback Loop C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 If S is high D1 is on is on and T is low R must also be low (for the inverter output S to be high) D2 must be off then if R is low and S is high S’ S T R

10. The Oscillation Circuits: how do they work? Breaking the Feedback Loop C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 As current flows through D1 through the “conductance” determined by the light intensity, S’ starts to discharge through C p2, going low. The conductance of T2 (governed by the PIN photodiode current establishes how fast C p2 will charge. S’ S T R

11. The Oscillation Circuits: how do they work? Summary of Circuit Operation C V1V1 C p1 C p2 D1 D2 T1 T2 INV1 INV2 By breaking the feedback loop, we have established that: the circuit does indeed oscillate (how exciting) the conductance of T1 controls how fast the output S charges the conductance of T2 controls how fast the output S discharges the higher the conductance on either T1 or T2, the faster the circuit oscillates the ratio of the conductances on T1 and T2 determines the duty cycle of the output S S’ S T R

12. An Oscillating Active CMOS Pixel for Subretinal Stimulation Issues with the Oscillator Circuit –It is very noisy at low light intesn ities (less than 0.5 X 10 -5 W/cm 2. –The duty cycle changes with frequency –Parasitic light limits the minimum duty cycle –Oscillations stop at high light intensities –Other Issues or Advantages to this circuit?