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PDF TA201 Data sheet ( Hoja de datos )
| Número de pieza | TA201 | |
| Descripción | High-performance Multiplexing | |
| Fabricantes | Temic | |
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TA201
High-performance Multiplexing with the DG408
Steve Davies
The DG408 and DG409, new multiplexers from
Siliconix, represent a new generation of
high-performance multiplexers and demultiplexers with
many specific improvements over existing products
available today. Built with the company’ s high-voltage
silicon-gate technology, these new ICs offer significantly
reduced on-resistance (<100
W), leakage
currents (IS(OFF) < 0.5 nA), power dissipation
(2.25 mW), and much faster switching (250 ns) over older
industry standards. These improved specifications allow
designers to greatly reduce system errors and improve
system performance.
sampling time of a particular channel and results in lower
multiplexer efficiency. The DG408 has switching times
(250 ns) guaranteed to be more than four times faster than
previously available (1 ms) multiplexers. Its guaranteed
break-before-make time (10 ns) prevents crosstalk during
switching transitions.
IN1
DG408
The DG408 and DG409 will enhance two primary
multiplexer and demultiplexer applications:
communications and telemetry . Important multiplexer
specifications depend on the application and the accuracy
required by the system. For example, in communications,
switching speed is important; whereas, in telemetry ,
on-resistance, charge injection, and output capacitance
are critical because they determine the accuracy of the
system. This article will present examples of these types
of applications and discuss the benefits that these new
multiplexers bring to their system performance.
Communications
DG408
OUT
The digital telephone exchange is a communication
multiplexed system. In this type of system (see Figure 1),
a number of telephone channels carrying speech are
sequentially switched (i.e., multiplexed) for fixed periods
of time into an analog-to-digital converter
. Once
converted to a digital form, the dif ferent speech signals
can be processed and routed within the exchange.
DG408
A typical specification for the voice bandwidth in a
telephone exchange is 3.3 kHz. For this bandwidth, an
8-kHz sampling rate is suf ficient (i.e., sampling rate > 2
times the bandwidth). Therefore, each sampling period is
125m s, during which time, each of the 32 channels of the
multiplexer must be addressed. This means that each
channel will be turned on for 3.906 ms (125 ms/32). This
figure is ideal, since the multiplexer cannot switch in zero
time. Depending on the particular multiplexer used, there
will either be an overlap between sampling pulses (i.e.,
make-before-break switching), which leads to crosstalk
between channels, or a separation between samples (i.e.,
break-before-make switching), which reduces the
DG408
IN32
Address Bus
Figure 1. 32-Channel Multiplexed System
Siliconix
1
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TA201
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. T ransients
(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
The DG408/409 switching speed (t TRANS) 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 ef ficiency 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.
V = Qi/C
where
Qi is the injected charge in picocoulombs
C is the load capacitance at the output.
Channel 1
tSAMPLE
tON tOFF
Channel 2
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
Figure 8. Channel Switching Rate
Assuming a t SAMPLE 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. T echniques are available to improve the
switching rate, and an example using the DG408/409 and
DG400 will be shown later.
Versatility
VO
Address SW Off
Switch On
DV
O
SW Off
Figure 7. Charge Injection Test Circuit
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 giventime. Fast switching
operation is often dif ficult to achieve using lar ger
multiplexing devices. That is, the greater the number of
channels, the slower the speed due to additional
capacitance at the common output node.
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.
Digital
Output
From
Processor
D/A
Converter
DG408
OUT1
OUT8
Figure 9. Using the DG408 as a Demultiplexer
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.
Siliconix
5
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| PDF Descargar | [ Datasheet TA201.PDF ] | |
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