Output Circuits for Converting the Video Signals
This section discusses the test methods employed to measure the PI3039 image sensors
video performance characteristics and serves as a reference for Table 2, notes 3 & 4. It also
serves as an application note for implementing the PI3039 image sensors. The output of each
sensor element in PI3039 image sensor is an emitter of a source follows. Accordingly, when
its video output line is terminated into low impedance line, such as current amplifier in Figure
4A, the pulsed video signal currents proportional to photon integration time are produced. At
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, in Figure 4A. 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 is in low-cost
application, the cost is higher than just using single amplifier, as well as, the complexity for its
implementation. Not to mention, that it will require a signal-inverting amplifier if a positive
going signal is desired. But, 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 and 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 PI3039 device. The
other amplifiers that will be discussed are configured as simple buffer amplifiers.
From all
sensor video
Video
Output
0
RIN
RFB
IOUT
RESET
SW
SWITCH
OP-AMP
Figure 4A. Virtual Ground Amplifier
The first circuit is shown in Figure 4A. Virtual Ground Amplifier. The signal currents from the
photo site is converted into voltage signal through its feedback resistor, while the photo-site
output see 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. Then after the video is sampled by host system, SW
closes and resets the video line and the photo-site that is presently under interrogation. Then
PAGE 8 OF 10, PI3039, 12/2/02
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