This third circuit is desirable for high-speed application, specifically, above 5.0 MHz. Again, a
buffer amplifier is employed. It uses the same buffer stage as in the above second circuit. It
has a positive-going output buffer amplifier, but instead of applying the video directly to the
input of amplifier, it uses a small shunt-sensing resistor to ground. See Figure 4C, Signal
Current To Voltage Converter. In this case, a small 50 ohms resistor load, low enough in
impedance to allow the image sensor to effectively see a virtual ground, is employed. This
low impedance minimizes the effect of video line capacitance. The signal is pulsed out as an
impulse current. Accordingly, this signal current produces a fast rising signal voltage across
the resistor, then the signal decays at a slightly slower rate. Accordingly, at high clock rates,
the time duration is short enough for the impulse current to develop an approximated square
wave voltage across the resistor. See Figure 3A, Timing of the PI3039 Sensor. Iout, the
signal current across the 50 ohms is exemplified as a very fast rising and falling signal voltage
pulse. The advantage of this circuit is its a positive going output signal, which it eliminates the
need for the second inverter stage. In addition, although it is not recommend for low
frequency operation, its low impedance video line lends to high-speed operation, above 5.0
MHz. Accordingly, this circuit offers the high-speed performance, in addition to, the cost and
implementation advantage. Disadvantage is that since it senses the output on a 50W resistor,
the signal-to-noise slightly less than the circuit that stores the signal charges on the video line.
From all
sensor video
Video
Output
IOUT
OP-AMP
SW
50 ohms
RFB
RIN
Figure 4C. Signal Current to Voltage Converter
Printed in USA. All rights reserved. Specifications
©2002 Peripheral Imaging Corporation.
are subject to change without notice. Contents may not be reproduced in whole or in part without
the express prior written permission of Peripheral Imaging Corporation. Information furnished
herein is believed to be accurate and reliable. However, no responsibility is assumed by Peripheral
Imaging Corporation for its use nor for any infringement of patents or other rights granted by
implication or otherwise under any patent or patent rights of Peripheral Imaging Corporation.
PAGE 10 OF 10, PI3039, 12/2/02
AMI [ AMI SEMICONDUCTOR ]
