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PDF AN301 Data sheet ( Hoja de datos )
| Número de pieza | AN301 | |
| Descripción | High-Speed DMOS FET Analog Switches and Switch Arrays | |
| Fabricantes | TEMIC | |
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AN301
High-Speed DMOS FET Analog Switches and Switch Arrays
Jack Armijos
Introduction
This Application Note describes in detail the principle
of operation of the SD210/5000 series of high-speed
analog switches and switch arrays. It contains an
explanation of the most important switch characteristics,
application examples, test data, and other application
hints.
Description
The Siliconix SD210 and SD5000 series are discretes and
quad monolithic arrays, respectively, of single-pole
single-throw analog switches. These switches are
n-channel enhancement-mode silicon field effect
transistors built using double-diffusion MOS (DMOS)
silicon gate technology. Surface-mount versions
(SST211, SD5400 Series) are also available.
This family of devices is designed to handle a wide
variety of video, fast ATE, and telecom analog switching
applications. They are capable of ultrafast switching
speeds (tr = 1 ns, tOFF = 3 ns) and excellent transient
response. Thanks to the reduced parasitic capacitances,
DMOS can handle wideband signals with high
off-isolation and minimum crosstalk.
The SD210 series of single-channel FETs is available
without Zener protection to reduce leakage and in Zener
protected versions to reduce electrostatic discharge
hazards. The SD5000 series is available in 16-lead dual
in-line plastic or sidebraze ceramic packages. Analog
signal voltage ranges up to 10 V and frequencies up to 1
GHz can be controlled.
For surface-mount applications the SST211 series is
offered in the TO-253 (SOT-143) package. The SD5400
series comes in the narrow body gull-wing SO-14
package.
Applications
Fast switching speeds, low on-state resistance, high
channel-to-channel isolation, low capacitance, and low
charge injection make these DMOS devices especially
well suited for a variety of applications.
A few of the many possible application areas for DMOS
analog switches are as follows:
1. Video and RF switching (high speed, high
off-isolation, low crosstalk):
– Multiple video distribution networks
– Sampling scanners for RF systems
2. Audio routing (glitch- and noise-free)
– High-speed switching
– Audio switching systems using digitized remote
control
3. Data acquisition (high speed, low charge injection,
low leakage):
– High-speed sample-and-holds
– Audio and communication A/D converters
4. Other:
– Digital switching
– PCM distribution networks
– UHF Amplifiers
– VHF Modulators and Double-Balanced Mixers
– High-speed inverters/drivers
– Switched capacitor filters
– Choppers
Principle of Operation
Figure 1 depicts an n-channel enhancement-mode device
with an insulated gate and asymmetrical structure. The
gate protection Zener is shown with broken lines to
indicate that, although it is present on the chip, it is not a
main constituent of the fundamental switch structure.
Protection
Zener
Gate
Asymmetrical
Structure
Insulated Gate
Enhancement
Mode Channel
Source
Drain
n-Channel
Body
Figure 1. DMOS Electrical System
Siliconix
22-Jun-94
1
1 page  
AN301
Figure 9 illustrates several typical charge injection
characteristics. Figure 10 shows some of the
corresponding waveforms. The DMOS FETs, because of
their inherent low parasitic capacitances, produce very
low charge injection when compared to other analog
switches (PMOS, CMOS, JFET, BIFET etc.). Still, when
the offsets created are unacceptable, charge injection
compensation techniques exist that eliminate or
minimize them. The solution basically consists of
injecting another charge of equal amplitude but opposite
polarity at the time when the switch turns off.
Off-Isolation and Crosstalk
The dc on-state resistance is typically 30 W and the
off-state resistance is typically 1010 W , which results in
an off-state to on-state resistance ratio in excess of 108.
However, for video and VHF switching applications, the
upper usable frequency limit is determined by how
much of the incoming signal is coupled through the
parasitic capacitances and appears at the switch output
—when ideally no signal should appear there in the off
state.
Off-Isolation is defined by the formula:
Off-Isolation
(dB)
+
20
log
VOUT
VIN
When several analog switches are simultaneously being
used to control high frequency signals, crosstalk becomes
a very important characteristic. For video applications,
the stray signal coupled via parasitic capacitances to the
signal of an adjacent channel can form ghosts and signal
interference. To help obtain high degrees of isolation, it
becomes necessary to exercise careful circuit layout,
reducing parasitic capacitive and inductive couplings,
and to use proper shielding and bypassing techniques.
Figure 11 shows the excellent off-isolation and crosstalk
performance typical of this family of DMOS analog
switches.
VGATE
DV
(a) TOP: 5 V/div
BOT: 50 mV/div
HOR: 0.5 ms/div
POINT (1)
(b) TOP: 5 V/div
BOT: 50 mV/div
HOR: 0.5 ms/div
POINT (2)
Figure 10. Waveforms for Points (1) and (2) of Figure 9
1 Vrms 600 W
600 W
600 W 600 W
160
140
120
Crosstalk
1 Vrms
Crosstalk
600 W
600 W
100
Off Isolation
80
60
Off Isolation
40
1k
10 k 100 k 1 M 10 M 100 M
Frequency (Hz)
Figure 11. SD5000 Crosstalk and Off-Isolation vs Frequency
Siliconix
22-Jun-94
5
5 Page | ||
| Páginas | Total 10 Páginas | |
| PDF Descargar | [ Datasheet AN301.PDF ] | |
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