AN201 Vishay Intertechnology, AN201 Datasheet - Page 6

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

This versatility allows the same advantages at the “back end”
of the system—that is, a single wire can be used to carry all the
control signals to the remote site. Then the control signals may
be converted back to analog form and demultiplexed for
controlling the system.
Logic Compatibility
The compatibility of the multiplexer is a measure of how easy
it is to interface with other system components, such as
transducers,
environment, etc.
The DG408/409 has many features which make this
interfacing as easy as possible. For example, a regulator has
been included on the chip to provide stability against power
supply and temperature variations. The regulator maintains
TTL compatibility over power supply variations, while the
dynamic specifications can be met over the full military
temperature range. The regulation also guarantees a low
current consumption of "75 mA, making it suitable for
battery/portable applications.
Application Enhancements
The following examples of applications using the DG408/409
are intended to highlight some specific improvements over
their predecessors.
Sample-and-hold circuits using analog multiplexers are widely
used in analog signal processing and data conversion
systems to store analog voltages accurately over time periods
ranging from nanoseconds to several minutes. This ability
finds many applications, including data distribution systems,
www.vishay.com
6
AN201
Vishay Siliconix
IN
IN
1
2
A/D
DG408
converters,
power
FIGURE 10. High Performance Sample/Hold Circuit
+
–
supplies,
Amplifier
the
simultaneous
oscilloscopes, digital volt meters, signal reconstruction filters,
and analog computational circuits. Although they are
theoretically simple, these high-speed, high-accuracy circuits
need careful consideration in their design. Figure 10 shows
the DG408 in a sample-and-hold circuit. During the sample
phase, one switch in the analog multiplexer is closed and the
hold capacitor is charged to the input voltage via the on-state
switch. Once the capacitor is charged to its final value, the hold
mode is entered by opening the switch.
During the hold mode, the capacitor voltage can be examined
via the low-leakage buffer. By repeating this with other
switches in the multiplexer, many analog inputs can be
sequentially examined.
In a high-speed system, an important specification for the
circuit designer is the acquisition time of the sample-and-hold
system—that is, the time delay between the sample command
and the capacitor reaching its final value.
Figure NO TAG shows that the acquisition time is dependent
on two factors. The first factor is the time from when the sample
command is given to when the switch is turned on (i.e., the t
of the switch). For the DG408, the t
maximum at 25_C; this translates to a 4 times improvement
over existing pin-compatible devices.
The second contribution to acquisition time is the time taken
for the hold capacitor to charge to its final value. The charging
time is determined by the source impedance of the analog
source, the on-resistance of the switch, and the hold capacitor.
Hence, for low-impedance analog sources, the on-resistance
of the switch will play an important part in determining the time
constant. The on-resistance of the DG408/409 is guaranteed
at 100 W over the whole analog voltage range, making a 500%
improvement over existing pin-compatible parts.
C
H
+
–
Low Leakage
Buffer
sample-and-hold
Converter
A/D
on
is guaranteed as 250 ns
designs,
Document Number: 70600
To
Processor
sampling
05-Aug -99
on