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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
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
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/cm2 to 10mW/cm2
– 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 mm2 and 100 X 100 mm2) results
presented in this work.
An Oscillating Active CMOS
Pixel for Subretinal Stimulation
The Oscillation Circuits: how do they work?
C
V1
The Oscillation Circuits:
how do they work?
Let’s first put in the parasitic capacitances:
INV1
D2
Cp1
T2
V1
INV2
C
T1
D1
Cp2
The Oscillation Circuits:
how do they work?
Understanding Basic Operation of the Circuit
INV1
V1
•
•
•
D2
T1
T2
D1
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 Cp2
The voltage V1 controls the “conductance of T1 and
therefore how fast current will charge Cp1
Cp1 INV2
C
Cp2
The Oscillation Circuits:
how do they work?
Breaking the Feedback Loop
S’
R
INV1
V1
S
T
D2
T1
T2
Cp1 INV2
C
D1
•
•
•
S is the present state of the output
Cp2
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)
The Oscillation Circuits:
how do they work?
Breaking the Feedback Loop
S’
R
INV1
V1
S
T
D2
T1
T2
Cp1 INV2
C
D1
•
•
As current flows through D2 through the “conductance” Cp2
determined by V1 on T1, S’ starts to charge, going high.
The conductance of T1 establishes how fast Cp1 will charge.
The Oscillation Circuits:
how do they work?
Breaking the Feedback Loop
S’
R
INV1
V1
S
T
D2
T1
T2
Cp1 INV2
C
D1
•
If S is high
Cp2
•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
The Oscillation Circuits:
how do they work?
Breaking the Feedback Loop
S’
R
INV1
V1
S
T
D2
T1
T2
Cp1 INV2
C
D1
•
•
As current flows through D1 through the “conductance” Cp2
determined by the light intensity, S’ starts to discharge through
Cp2, going low.
The conductance of T2 (governed by the PIN photodiode
current establishes how fast Cp2 will charge.
The Oscillation Circuits:
how do they work?
Summary of Circuit Operation
S’
R
INV1
V1
S
T
D2
T1
T2
Cp1 INV2
C
D1
•
By breaking the feedback loop, we have established that:Cp2
•
•
•
•
•
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
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/cm2.
– 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?