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.

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

IOUT

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OP-AMP

SW

RESET

SWITCH

From all

sensor video

Video

Output

RFB

RIN

0