AMIS-720639:
600dpi CIS Sensor Chip
7.0 Output Circuits for Converting the Video Signals
Data Sheet
This section discusses the test methods employed to measure the video performance characteristics of the AMIS-720639 image
sensors and serves as a reference for Table 2, Notes 3 and 4. It also serves as an application note for implementing the AMIS-720639
image sensors. The output of each sensor element in the AMIS-720639 image sensor is an emitter of a source. Accordingly, when its
video output line is terminated into a low impedance line, such as the current amplifier in Figure 5, the pulsed video signal currents
proportional to photon integration time are produced. At a high sampling frequency rate, these video current pulse widths are limited to
the pixel sampling time. Hence, the output signal voltage is limited to the signal current pulse time and amplitude. Accordingly, the
usual practice is to integrate this small signal charge instead of using a sensing resistor, RFB, (see Figure 5). In this case, RFB can be
changed to a capacitor with a reset switch, thus converting the circuit to a Miller Integrator. The integrator will convert the charges to
proportional signal voltages. However, the disadvantage to this low-cost application is that the cost is higher than just using a single
amplifier and the implementation is more complex. Another disadvantage to this application is that it will require a signal-inverting
amplifier if a positive-going signal is desired. But, if kept in this simple resistor feedback form and, if the application can accept an
inverted output voltage, this current-to-voltage amplifier can also implement a relatively low cost, simple circuit. Accordingly, it is
introduced and discussed as one of the three amplifier structures that can be used for the video output of the AMIS-720639 device. The
other amplifiers that will be discussed are configured as simple buffer amplifiers.
IOUT
Figure 5: Virtual Ground Amplifier
The first circuit is shown in Figure 5. The signal currents from the photo-site are converted into a voltage signal through its feedback
resistor, while the photo-site output sees a very close approximation to a ground because the input resistor value can be small enough
to render the video line capacitance negligible, hence providing a fast responding video samples. The first method is to use the video
line capacitance as a charge storing capacitance. When the selected sensor’s photo-site outputs its video signal current, the video line
reset switch, SW, is open. After the video is sampled by the host system, SW closes and resets the video line and the photo-site which
are presently under interrogation. Then it opens just prior to the next pixel readout. This reset is active during CP’s high state. The
disadvantage of this circuit is that it has negative-going output and will have pulse shape of the current impulse that decays over a long
period. Hence, it may not be desirable at low clock sample frequencies.
To get around this decaying type of sampling pixels, the second circuit may be more desirable (see Figure 6). This method uses the
video line as a storage medium. It uses a buffer amplifier and buffers the video line with its high input impedance. Hence, the video
line effectively approaches the condition of an open circuit and becomes a capacitance that is proportional to video line length and
geometry. When the photo site produces the signal current, it charges the video line capacitance and converts the output into a voltage
signal. The switch, SW, is a video line reset switch. It resets the video line and the photo-site presently under interrogation, just prior to
the next pixel readout. This reset is active during CP’s high state.
AMI Semiconductor
– May 06, M-20569-001
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