AN201
Vishay Siliconix
Document Number: 70600
05-Aug -99
www.vishay.com
1
High-Performance Multiplexing with the DG408
The DG408 and DG409, new multiplexers from Vishay Silico-
nix, represent a new generation of high-performance
multiplexers and demultiplexers with many specific improve-
ments over existing products available today. Built with the
company’s high-voltage silicon-gate technology, these new
ICs offer significantly reduced on-resistance (<100C0032C0087),
leakage currents (I
S(OFF)
< 0.5 nA), power dissipation
(2.25 mW), and much faster switching (250 ns) over older
industry standards. These improved specifications allow
designers to greatly reduce system errors and improve system
performance.
The DG408 and DG409 will enhance two primary multiplexer
and demultiplexer applications: communications and teleme-
try. Important multiplexer specifications depend on the
application and the accuracy required by the system. For
example, in communications, switching speed is important;
whereas, in telemetry, on-resistance, charge injection, and
output capacitance are critical because they determine the
accuracy of the system. This article will present examples of
these types of applications and discuss the benefits that these
new multiplexers bring to their system performance.
Communications
The digital telephone exchange is a communication
multiplexed system. In this type of system (see Figure 1), a
number of telephone channels carrying speech are
sequentially switched (i.e., multiplexed) for fixed periods of
time into an analog-to-digital converter. Once converted to a
digital form, the different speech signals can be processed and
routed within the exchange.
A typical specification for the voice bandwidth in a telephone
exchange is 3.3 kHz. For this bandwidth, an 8-kHz sampling
rate is sufficient (i.e., sampling rate > 2 times the bandwidth).
Therefore, each sampling period is 125 C0109s, during which time,
each of the 32 channels of the multiplexer must be addressed.
This means that each channel will be turned on for 3.906 C0109s
(125 C0109s/32). This figure is ideal, since the multiplexer cannot
switch in zero time. Depending on the particular multiplexer
used, there will either be an overlap between sampling pulses
(i.e., make-before-break switching), which leads to crosstalk
between channels, or a separation between samples (i.e.,
break-before-make switching), which reduces the sampling
time of a particular channel and results in lower multiplexer
efficiency. The DG408 has switching times (250 ns) guaran-
teed to be more than four times faster than previously available
(1 C0109s) multiplexers. Its guaranteed break-before-make time
(10 ns) prevents crosstalk during switching transitions.
OUT
IN
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IN
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FIGURE 1. 32-Channel Multiplexed System
Address Bus
DG408
DG408
DG408
DG408