RO0720B

The RO0720B is a high resolution circular solid state scanner designed specifically for applications such as focusing,tracking, angle discrimination, etc.  This monolithic integrated circuit contains a circular array of 720 photodiodes at 0.5°spacing and is available in two aperture versions, .2 and 1.9 mm,  designated the  RO0720BAG and  R00720BJG,respectively.  These arrays are packaged with a ground and polished glass window.The package and pin configuration is shown in Figure 1 and a simplified equivalent circuit is shown in Figure 2.  The actual array is circular with the No. 1 diode at right center (scan is clockwise), and with the last diode physically adjacent to the first diode (Figure 1).  The array may be operated in either a self-start or external start mode.  The self-start mode permits uninterrupted circular scan and is accomplished by buffering the End-of-Scan output with external circuitry and connecting it to the start input. The external mode may be used to obtain longer integration time, much as in a linear array.

Each cell consists of a photosensor with an associated junction capacitance and is connected through an MOS transistor switch to a common video output line.  The switches are turned on and off in sequence by the shift-register multiplexing circuits, thereby periodically recharging each cell to 5V and storing approximately 5 pC on its capacitance.The multiplexing circuits are driven by four clock phases which are easily generated from a TTL master clock which sets the cell-to-cell sampling rate.  During the cycle scan time,the charge on each junction capacitor is gradually removed by the reverse current flowing in the associated photodiode.

In the RO0720B circular scanners the light-sensing area is a long, circular region defined by an aperture in an opaque mask.  Bar-shaped photodiodes extend across the aperture and are connected to the multiplex switches buried under the mask.  The entire aperture is photosensitive; photocurrent generated by light incident between the photodiodes will usually be collected by the nearest diode.  Figure 3 shows simplified aperture geometry along with  an  idealized response function, which would be obtained by scanning a point source of visible light along the length of the aperture.