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AN201

High-Performance Multiplexing with the DG408

Vishay Semiconductors 

AN201

Vishay Siliconix

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 2 shows the position of a multiplexer in a

high-performance, closed-loop telemetry system. The

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 multiplexer may take many forms, including

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

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 amplifier which rejects the unwanted

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.

Transducers

Multiplexer

DG408

Link

A/D

Converter

Processor

Demultiplexer

DG408

D/A

Converter

www.vishay.com

2

FIGURE 2. Position of the Multiplexer in a Telemetry System

Document Number: 70600

05-Aug -99

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