AN201 View Datasheet(PDF) - Vishay Semiconductors

Part Name

Description

Manufacturer

AN201

High-Performance Multiplexing with the DG408

Vishay Semiconductors 

AN201

Vishay Siliconix

This typical offset (at 25_C) should be compared with the

signal level to determine whether the error introduced by the

offset is acceptable.

Another source of error that may be introduced by the switch

occurs when the switch changes state. Transients (due to

capacitive coupling between the drivers and drain) can be

manifested as an error voltage appearing on the output node.

The effect of charge injection is measured in volts and is given

by

V = Qi/C

where

Qi is the injected charge in picocoulombs

C is the load capacitance at the output.

The DG408/409 devices have been internally compensated to

minimize the effects of the injection. This is achieved by

including compensation capacitors on the output switch.

These capacitors are sized to produce an equal and opposite

transient which tends to cancel out the effect of the switch

injection. Typical charge injection for the DG408/409 is 20 pC

for the test configuration shown in Figure 7.

RSOURCE

VSOURCE

VSOURCE = 0 V

RSOURCE = 0 W

DG408

VO

10 nF

the greater the number of channels, the slower the speed due

to additional capacitance at the common output node.

The DG408/409 switching speed (tTRANS) is 250 ns maximum

at room temperature with a 10-ns minimum break-before-

make time. While this break-before-make time prevents

overlap or “alias” between channels, it reduces multiplexer

efficiency and, therefore, is kept as short as possible.

A channel-switching rate (Figure 8) is defined for the

DG408/409 by tON, tOFF, and tSAMPLE, where tSAMPLE is

dependent on the application.

Channel 1

tON

tSAMPLE

tOFF

Channel 2

FIGURE 8. Channel Switching Rate

Assuming a tSAMPLE of 1.2 ms, the maximum switching rate for

the DG408/409 (with no pulse-edge overlap) is once every

1.5 ms or a frequency of 666 kHz. This example shows that the

switching speed of the DG408/409 is not a significant factor

unless the tSAMPLE time becomes much smaller. For

multi-channel systems, if the sampling theorem is obeyed, the

maximum switching rate will limit the number of channels

and/or the maximum frequency components of any of the

channel inputs. Techniques are available to improve the

switching rate, and an example using the DG408/409 and

DG400 will be shown later.

VO

Address SW Off

Switch On

DVO

SW Off

FIGURE 7. Charge Injection Test Circuit

Versatility

With CMOS switches, signal conduction is the same in either

direction. Therefore, as shown in Figure 9, it’s possible to use

the DG408 as a demultiplexer with one input from the

digital-to-analog converter and 8 outputs.

Switching Speed

Multiplexers operate in real time (i.e., samples are taken

sequentially and represent the analog input signal). Obviously,

the quicker a multiplexer changes state, the more samples can

be taken in a given time. Fast switching operation is often

difficult to achieve using larger multiplexing devices. That is,

Document Number: 70600

05-Aug -99

Digital

Output

From

Processor

D/A

Converter

DG408

OUT1

OUT8

FIGURE 9. Using the DG408 as a Demultiplexer

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