AN201 Vishay Intertechnology, AN201 Datasheet - Page 8

• Manufacturer Part Number: AN201
• Description: High-performance Multiplexing With The DG408
• Manufacturer: Vishay Intertechnology

High-Speed Switching
In large segments of the electronics industry, speed is a
primary concern in product design. Computer graphics, video
equipment, and medical electronics are a few examples where
high-speed switching is required. The activation frequency of
a multiplexer (i.e., the frequency at which the switch can be
toggled) is directly related to the switching speed of the device
(i.e., the faster the switching speed, the higher the activation
frequency). The DG408 has guaranteed maximum of 250 ns
switching speed, thus making it theoretically possible to toggle
the switch up to 2 MHz. Figure 12 shows how a two-level
multiplexer can be used to increase the data transmission
rates in an analog multiplexed system.
Use of the two-level system gives improved performance over
a single-level system. Examples of these improvements are
listed below.
Output capacitance results only from the second-level device
and not from the sum of the first-level devices.
The leakage currents at the output node will be reduced from
the sum of the second-level devices to that of the second-level
device.
In a design where one multiplexer is sampled while the other is
switched, the switching speed of the system is increased to
that of the DG400.
Crosstalk and isolation are improved because one-half of the
system will have two off switches in series to the output node at
any given time.
Differential Multiplexing of Low-Level Signals
When multiplexing low-level signals, careful choice of a
multiplexer and handling of the system layout helps avoid
signal masking by ac noise pick-up and dc voltages generated
by thermocouple effects. To transmit the signal effectively,
several factors must be considered.
D Single-ended or differential signal paths
D Low-level transmission or preamplification
D The type of conductor
The choice between a single-ended (DG408) or differential
(DG409) multiplexer is really a function of the system. Cost will
dictate a single-ended connection, and for a high signal level,
the shorter distance should provide a stable-environment
system. For a system that has a significant signal path length
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AN201
Vishay Siliconix
and is potentially being routed in a noisy environment (e.g.,
motors), common-mode signals can be picked up. For
low-level signal transmission, this common-mode signal will
provide a significant error. To minimize the effect of the
common-mode signals, a twisted pair of wires feed a
differential multiplexer, such as the DG409, which is buffered
by a differential amplifier that rejects the unwanted
common-mode portion of the signal (see Figure 13).
FIGURE 12. Two-Level Multiplexing with the DG408
IN
IN
32
1
–V
+V
DG408
+V
–V
DG411
Document Number: 70600
05-Aug -99