AN201 Vishay Intertechnology, AN201 Datasheet - Page 2

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

Telemetry
Telemetry offers many applications for multiplexer and
demultiplexer combinations. A telemetry system uses
transducers (a device which converts a physical variable, such
as pressure, flow, temperature, etc. to an electrical equivalent)
to measure variables, which are fed back via a multiplexer,
monitored, and acted upon if necessary.
Figure
high-performance,
transducer output generally produces an analog output (which
may need preamplification and filtering prior to multiplexing).
With a wide variety of transducer types available, the inputs to
the
high-frequency, dc, high-level, low-level, voltage, current, and
differential signals. Whether a signal requires preconditioning
before being multiplexed depends on the total accuracy
required of the system.
Because the multiplexer follows the transducer output, the
multiplexer specification will have a significant bearing on the
system accuracy. For example, a low-level signal can,
potentially, require preamplification. A primary source of error
with a low-level signal may be the switching transients present
in the multiplexer. These transients are the result of charge
injection from the switches, producing an error voltage (usually
positive for a CMOS switch) which appears at the multiplexer
output. Hence, the lower the signal level, the greater the error
introduced by the charge injection of the switch.
For example, the DG408 with its 20-pC (typical) charge
injection will create a 20-mV offset error when switching into
www.vishay.com
2
AN201
Vishay Siliconix
Transducers
multiplexer
2 shows the position of a multiplexer in a
may
closed-loop telemetry system. The
Multiplexer
DG408
take
many
FIGURE 2. Position of the Multiplexer in a Telemetry System
forms,
Converter
A/D
including
Link
a 1000-pF load. For low-level signals, this offset may be
excessive. Using a differential multiplexer, such as the DG409,
will provide at least an order of magnitude improvement in the
total offset error.
High-level signals become a potential problem at different
values, depending on the technology used to manufacture the
multiplexer. For a CMOS multiplexer, high-level signals
greater than the positive and negative supplies must be
avoided to prevent permanent damage to the device. If the
supplies are exceeded by the analog signal, the inherent
source/drain-to-supply diodes (Figure 3) will become forward
biased.
When the expected overloads have a short duration, usually
a couple of switching diodes used in series with the supply
leads will prevent permanent damage by blocking the flow of
reverse current in the power supply loads.
Differential signals can be generated by bridge-type
transducers. These devices will produce a signal of two
components: a common-mode signal which is large and a
small difference signal. It is the difference component that
conveys the measurement information. Figure 4 shows the
DG409 differential multiplexer being buffered by a full
differential
common-mode voltage. The amplifier output consists solely of
the differential signal. The ability of the differential multiplexer
to reject unwanted common-mode voltages makes it
especially useful in systems where pick-up of electrical noise
is a concern.
Processor
amplifier
Converter
D/A
which
rejects
Demultiplexer
DG408
Document Number: 70600
the
unwanted
05-Aug -99