AN201 Vishay Intertechnology, AN201 Datasheet - Page 5

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

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
where
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.
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
V = Q
Q
C is the load capacitance at the output.
i
Address
is the injected charge in picocoulombs
V
R
i
V
/C
SOURCE
SOURCE
O
V
R
FIGURE 7. Charge Injection Test Circuit
SOURCE
SW Off
SOURCE
= 0 V
= 0 W
Switch On
DG408
SW Off
10 nF
V
O
DV
O
the greater the number of channels, the slower the speed due
to additional capacitance at the common output node.
The DG408/409 switching speed (t
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
DG408/409 by t
dependent on the application.
Assuming a t
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 t
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.
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.
Processor
Output
Digital
From
Channel 1
Channel 2
FIGURE 9. Using the DG408 as a Demultiplexer
SAMPLE
FIGURE 8. Channel Switching Rate
SAMPLE
t
ON
Converter
ON
D/A
, t
of 1.2 ms, the maximum switching rate for
t
SAMPLE
OFF
time becomes much smaller. For
, and t
t
OFF
Vishay Siliconix
SAMPLE
TRANS
8) is defined for the
DG408
) is 250 ns maximum
, where t
AN201
www.vishay.com
SAMPLE
OUT
OUT
1
8
is
5