AN207 Vishay Intertechnology, AN207 Datasheet - Page 8

• Manufacturer Part Number: AN207
• Description: 15-ns DG611 Switch Family Combines Benefits of CMOS And Dmos
• Manufacturer: Vishay Intertechnology

AN207
Vishay Siliconix
Thanks to its high speed, low r
DG611 family makes it possible to implement SCFs with cutoff
frequencies in the 1-MHz range. These higher frequencies are
suitable for applications in the radio IF range (400-500 kHz).
Using high-speed Op Amps and chip resistors it is possible to
implement these SCFs in small hybrids.
The advantage of an RF SCF is that the filter’s characteristics
can be digitally controlled to adapt to the instantaneous
bandwidth of the desired signal.
Figure
frequency response
implementation that uses an inverting switched-capacitor
integrator approach.
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14(a) illustrates a simple low pass filter and its
FIGURE 13. Programmable Low-Pass Filter
400
300
Analog
Input
DG613
14(b). Figure
DS(on)
FIGURE 12. Fast and Accurate Closed-Loop Sample-and-Hold
and low capacitances, the
LF356A
f
14(c) is an SCF
–3dB
100 pF
400
300
2 RC
1
V
DG613
S/H
OUT
SW3
SW2
SW1
Video Overlaying
An inexpensive NTSC video titler can be implemented by
superimposing the output of a character generator on a
standard composite video background. A high-speed switch
like the DG613 is used to select certain pixel groups from one
of the two available sources during each horizontal line sweep.
The same principle can be used to combine two video signals
into a single image (i.e. blue background.) Both video sources
must be sync-locked.
An advantage of the DG61X family is that its low charge
injection eliminates switching noise that could appear as halos
or shadows around the superimposed image. It is worth noting
that if you disregard the propagation delay in the driver stages,
the effective switching transition time from one video signal to
the other is well below 5 ns!
Figure 15 illustrates a basic pixel-rate switch circuit. Each
incoming signal arrives to its own 75-
The DG613 is configured for a single-pole double-throw
function and selects one of the two signals at a time. The
CLC410 amplifier drives a 75- back- terminated output line.
A back-terminated line provides impedance matching and
eliminates any reflections caused by possible impedance
disturbances on the transmission line. Connectors, transitions
from coax cable to PC board traces, and transitions from board
traces to IC package lead will cause slight impedance
mismatches.
The output amplifier must have a gain of 2 (6 dB) since the
back-termination resistor will introduce a 6-dB loss.
C
H
Document Number: 70605
termination resistor.
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03-Aug-99