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A 0.6V CMOS Image Sensor with In-Pixel BiphasicCurrent Driver for Biomedical Application

Chin-Lin Lee and Chih-Cheng HsiehSignal Sensing and Application Laboratory

National Tsing Hua University Department of Electrical EngineeringHsinchu, Taiwan, R.O.C

Email: g9761571@oz.nthu.edu.tw

Abstract A 0.6V pulse frequency modulation (PFM) CMOSImage Sensor (CIS) array with in-pixel biphasic current pulsedriver is presented in this paper. It achieves a photon-to-biphasic current conversion for biomedical applications likeartificial 2-D vision recovery. The photon-to-biphasic-currentconversion gain, the biphasic pulse width, polarity, and outputrate are all tunable depends on applications and environments.A 32x32 pixel array with 30x30 um 2 pixel size has been designedand fabricated in 0.18um CMOS technology providing the fillfactor of 24.5%. Measurement results show a 0.63Hz/luxconversion gain of PFM sensor within 25hz~5kHz output withpower consumption as 2uW~55uW depends on illumination.The maximum driving capability of biphasic neural stimulationcurrent pulse is 20A with a 10k electrode model.

I. I NTRODUCTION IOMEDICAL applications have been one of the mostresearched field in recent years. A lot of people have eye

diseases in the world. Blindness diseases, such as retinitis pigmentosa (RP) or age-related macular degeneration (AMD),cause progressive degeneration of rods, cods and cones in theretina [1]. In previous works, MOS image sensors have beenapplied for helping blind people [2]. The Optacon, or optical-to-tactile converter, is probably the first use of a solid-stateimage sensor for blind people [3]. Nowadays, many studieshave been carried out considering a number of implantationways such as epi-retinal space (Epi.), sub-retinal space (Sub.)and suprachoroidal transretinal stimulation (STS).

Low power circuit is an important issue and critical goal in portable and implantable products. In this work, we propose alow power pulse frequency modulation image sensor suitable

for biomedical application. The pulse frequency modulation(PFM) photo sensor [4]-[7] has been used in high dynamicrange (HDR) and retinal prosthesis. It has many advantagessuch as asynchronous operation, digital output, wide dynamicrange and low voltage operation. A digital output of the PFMsensor eliminates the effects of column-related noise onanalog performance [8] and each pixel in PFM sensor canindividually produce an output pulse without other systemclock, that is, asynchronously [2]. In spite of these advantages,

the major limitation of this pixel architecture is its complex pixel structure which leads to a larger pixel size and a potentially higher pixel wise fixed-pattern noise (FPN) [8].

A 32 32 pixels PFM CIS array with in-pixel biphasiccurrent output has been proposed to replace the function of damaged retinal cell. The block diagram of the pixel cell isshown in Fig. 1. The photo sensor is a pn-junction photodiode(PD) implemented by n-diffusion and p-sub. Whenilluminated, the photon excited electron-hole pairs areaccumulated at the capacitance of pn-junction. The PFMcircuit converts the slope of the voltage difference on sensor node to frequency output which is proportional to the inputlight intensity. A frequency-to-voltage (F-V) converter isdesigned to convert the output pulse to a voltage which is thenused to generate biphasic current pulses [9]-[11].

The outline of this paper is as follows. In Section II, thearchitecture of the pixel is described. Then, the simulation andmeasurement results are summarized in Section III. Finally,the conclusions are drawn in Section IV.

Fig. 1 Block diagram of the pixel cell

II. P IXEL ARCHITECTURE AND CIRCUIT The proposed pixel contains two components: PFM photo

sensor and biphasic pulse generator which includes afrequency-to-voltage converter.

B

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A. PFM photosensor Unlike the conventional CMOS image sensor, the main

components of PFM image sensor are digital circuits that canoperate at low supply voltage, and carry out analog-to-digitalconversion within the pixel. The circuit structure of the

proposed PFM photo sensor cell is as shown in Fig. 2. It iscomposed of a photodiode, a Schmitt trigger with tunablethreshold, a reset transistor Mr, and a feedback loop with adelay chain (inverter chain). Before the exposure starts, thesensor node V pd is reset to V DD through the device Mr. Withlight illumination, the voltage at PD as V pd is discharged bythe photon-generated carriers and decreases. When V pd islower than V th, the threshold voltage of the Schmitt trigger, theoutput V out changes to HIGH and is feedback through a delaychain to reset the sensor node. Then, V out is pulled to LOWagain. It results in a pulse generation behavior and thefrequency is proportional to the light intensity. Since the full-well of sensor node is self-reset and re-used through thefeedback loop, the dynamic range of PFM sensor is much

higher than the conventional image sensor. The delay chain isused to implement the necessary reset pulse width andmaintain the oscillation condition of the loop. The outputfrequency f shows a linear function of the input light intensityand can be expressed as Equation (1)

)( thdd pd

pd

V V C

I f

(1)

Vpd

Delay Chain

VDD

VoutPD

Mr

IpdCpd

Fig. 2 Basic circuit of a PFM photosensor

Fig. 3 The transient response of the sensor node V pd and the pulse output of Schmitt trigger

The transient response of V pd and pulse output of Schmitttrigger are shown in Fig 3. To reduce power consumption and

pixel size, a six-transistor (6T) Schmitt trigger with tunablethreshold is implemented instead of using the conventionalanalog comparator. As shown in Fig. 4, the V thH and V thL arethe upper and lower thresholds of the Schmitt trigger which

can be varied by changing external bias voltage V H and V L respectively. The photon-generated pulse frequency is tunable

by adjusting the thresholds, which results in a front-stagetunable gain. With this front-stage tunable gain, the dynamicrange and sensor SNR can be optimized based on applications.

Fig. 4 Schematic of the Schmitt trigger

Fig.5 Schematic of the F-V converter

Fig.6 Schematic of the biphasic current generator

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B. Biphasic current generator For the artificial retina application, the image sensing chip

needs to generate an in-pixel biphasic current for neuralstimulation. An in-pixel frequency-to-voltage (F-V) converter followed by a voltage-to-biphasic-current generator is

proposed to provide the electric stimulus. As shown in Fig. 5,the F-V converter is implemented by a switching current I sw and an integration capacitor C int for low operating voltageavailability and minimized circuit complexity. Within acertain exposure period controlled by reset, the voltage onC int is proportional to the frequency output of PFM sensor.Through the level shifter (M psf ), the converted voltage is thenfed to the voltage controlled current source as shown in Fig. 6.The generated current is then mirrored to as a current sink anda source of electrode, respectively. In this design, the biphasic

pulse width, polarity, and output rate are adjustable bycontrolling the timing of the 1 and 2. The impedance of electrode and a tissue is modeled by the equivalent circuit asshown in Fig. 6. (10k in this work)

The biphasic current pulse is used for generating theelectric stimulus to a nerve cell [12]. An optimized matching

between the negative and positive current level is important todeliver a good charge balance [13]. The switch SW ref ,controlled by 3, is used to remove the charge residue on theelectrode after every stimulation cycle. It can avoid the neuronmemory effect and possible physiological damage.

III. PROTOTYPE CHIP AND MEASUREMENT R ESULT The microphotograph of the fabricated chip (1.8 mm 1.8

mm) in TSMC 0.18 m CMOS technology is shown in Fig. 7.The pixel layout is shown in Fig. 8. Each 30x30 m2 pixel sizecontains a 220.5 m2 n-diffusion/p-substrate photodiode resultsin a 24.5% fill factor. An in-pixel electrode pad window isembedded for further electrode formation.

Fig.7 Microphotograph of the fabricated chip

Figure 9 shows the measured sensitivity of the PFM photosensor. It shows a linear response between input light intensity(lux) and output frequency (Hz) as expectation. The measuredconversion gain (frequency/illumination) is 0.63 (Hz/lux).

Figure 10 shows the output waveform of this chip with anelectrode model (10k ) under illumination. The measuredmaximum PFM output frequency is 5.3 kHz at 0.6V supply.The maximum driving capability of biphasic neuralstimulation current pulse is 20A limited by 0.6V supply and10k loading.

Fig. 8 Pixel layout

PFM output response

Illumination (lux)

0 2000 4000 6000 8000 10000

P u

l s e

F r e q u e n c y

( H z

)

0

1000

2000

3000

4000

5000

6000

Fig. 9 Sensitivity of the PFM photosensor

Fig. 10 The measured PFM sensor output, biphasic currentoutput and F-to-V output waveforms.

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Four test patterns have been applied to do the imagereconstruction verification. The reconstructed sample imagesfrom biphasic current output level is shown in Fig. 11. Itshows that an English letter A, an arrow, five circles and anumber 7 have been successfully rebuilt and recognized. Themeasured results of the fabricated prototype chip are

summarized in Table I.

Fig. 11 The captured image from prototype chip

TABLE I

Specifications

Technology TSMC CMOS 0.18-um

Supply voltage 0.6V

Number of pixels 32x32

Pixel size 30 m 30 m

Fill factor 24.5%

Frame rate 20 Hz

PFM output Frequency 25Hz~5kHz

Power consumption 2uW~55uW(without current driver)

PFM conversion gain 0.63Hz/lux

Biphasic current20A

(with 10k loading)

IV. CONCLUSION The design and test of a 1024 pixels PFM CMOS image

sensor array with in-pixel biphasic current pulse driver has been presented. The main work is a photon-to-biphasic currentconversion system on chip operated at 0.6V supply voltage. Atunable front-gain by adjusting the threshold voltages of

Schmitt trigger is implemented. The resulting biphasic pulsewidth, polarity, and output rate are all tunable depends onapplications and environments. It is verified that the patternscan be reconstructed and recognized from the photon-to-

biphasic current conversion of the proposed sensor chip. Itshows a low-voltage and low-power solution for retinal

implantation.

ACKNOWLEDGMENT

The authors would like to thank Prof. L.S. Fan, Prof. K.T.Tang, S.F. Yeh, and M.T. Chung for supporting this work.

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