AN207 View Datasheet(PDF) - Vishay Semiconductors
Part Name
Description
Manufacturer
AN207 Datasheet PDF : 12 Pages
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AN207
Vishay Siliconix
SW2
Analog
Input
SW3
SW1
DG613
CH
S/H
FIGURE 12. Fast and Accurate Closed-Loop Sample-and-Hold
VOUT
Thanks to its high speed, low rDS(on) and low capacitances, the
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 14(a) illustrates a simple low pass filter and its
frequency response 14(b). Figure 14(c) is an SCF
implementation that uses an inverting switched-capacitor
integrator approach.
fSELECT
DG613
400 W
VIN
300 W
100 pF
400 W
300 W
LF356A
VOUT
f–3dB
+
1
2pRC
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-Ω termination resistor.
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-Ω 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.
FIGURE 13. Programmable Low-Pass Filter
www.vishay.com S FaxBack 408-970-5600
6-8
The output amplifier must have a gain of 2 (6 dB) since the
back-termination resistor will introduce a 6-dB loss.
Document Number: 70605
03-Aug-99
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