DMOS Analog Switch Introduction

High-Speed DMOS FET Analog Switches and Switch Arrays
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 Linear Systems 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. Surfacemount 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
produced 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 inline surface mount 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 offisolation, 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 (highspeed, 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
1
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Principle of Operation
Figure 1 depicts an n-channel enhancementmode 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.
Gate
Asymmetrical
Structure
Protection
Zener
Insulated
Gate
Source
Drain
Enhancement
Mode n-Channel
The double-diffusion process creates a thin selfaligning region of p-type material, isolating the
source from the drain region. The very short
channel length that results between the two
junction depths produces extremely low sourceto-drain and gate-to-drain capacitances at the
same time that it provides good breakdown
voltages.
When the gate potential is equal to or negative
with respect to the source, the switch is off. In
this state, the p-type material in the channel
forms two back-to-back diodes and prevents
channel conduction (Figure 3a). If a voltage is
applied between the S and D regions, only a
small junction leakage current will flow.
G
Body
CGS
Figure 1. DMOS Electrical System
The DMOS field-effect transistor (FET) is
normally off when the gate-to-source voltage
(VGS) is 0 V. The lateral DMOS transistor,
shown in cross-section in Figure 2, has three
terminals (source, gate, and drain) on the top
surface and one (the body or substrate) on the
bottom. A Zener diode with a breakdown
voltage of approximately 40 V is added to
protect the gate against overvoltage and
electrostatic discharges.
Source
Gate
Drain
Oxide
n+
n+
p-
Channel
p
Body
Figure 2. Cross Sectional View of an Idealized
DMOS Structure
S
G
CGD
+
CGS +
rDS(on)
D S
B
CGD
D
B
(a) Off State
(b) On State
Figure 3. Equivalent Circuits
The silicon oxide insulation present between
gate and source forms a small capacitor that
accumulates charge.
If the gate-to-source potential (VGS) is made
positive, the capacitive effect attracts electrons
to the channel area immediately adjacent to gate
oxide. As VGS increases, the electron density in
the channel will exceed the hole density, and the
channel will become an n-type region. As the
channel conductivity is enhanced, the n-n-n
structure becomes a simple silicon resistor
through which current can easily flow in either
direction.
Figure 4 shows typical biasing for ±10 V analog
signal processing. Note that the drain is
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recommended for the output. Since CGD < CGS
this causes less charge injection noise on the
load.
The circuit shown in Figure 4 exhibits the rDS(on)
vs. analog signal voltage relationship shown in
Figure 5.
As can be seen from Figures 3a and 3b, the
body-source and body-drain pn junctions should
be kept reverse biased at all times-otherwise,
signal clipping and even device damage may
occur if unlimited currents are allowed to flow.
Body biasing is conveniently set, in most cases,
by connecting the substrate to V-.
When the analog signal excursion is large (for
example ±10 V) the channel on-resistance
changes as a function of signal level. To
achieve minimum distortion, this channel onresistance modulation should be kept in mind,
and the amount of resistance in series with the
switch should be properly sized. For instance, if
the switch resistance varies between 20 Ω and
30 Ω over the signal range and the switch is in
series with a 200 Ω load, the result will be a
total ∆R = 4.5 %. Whereas, if the load is 100
kΩ, ∆R will only be 0.01 %.
Control
Input
G
S
D
RGEN
Switch Input
VS = ±10 V
Switch
Output
RL
VO
CL
B
-10 V
Figure 4. Normal Switch Configuration for a ±10
V Analog Switch
Main Switch Characteristics
rDS(on)
Channel on-resistance is controlled by the
electric field present across and along the
channel. Channel resistance is mainly
determined by the gate-to-source voltage
difference. When VGS exceeds the threshold
voltage (VGS(th)), the FET starts to turn on.
Numerous applications call for switching a point
to ground. In these cases the source and
substrate are connected to ground and a gate
voltage of 3 to 4 V is sufficient to ensure
switching action.
With a VGS in excess of +5 V, a low resistance
path exists between the source and the drain.
200
rDS(on) (Ω)
20 V = On
-10 V = Off
160
(a)
120
(c)
80
(b)
40
0
-10
-5
0
5
10
15
VS (V)
(a)
(b)
(c)
VBODY = -10 V, VGATE = 20 V
VBODY = -10 V, VGATE = 15 V
VBODY = 0 V, VGATE = 20 V
Figure 5. On Resistance Characteristics
Threshold Voltage
The threshold voltage (VGS(th)) is a parameter
used to describe how much voltage is needed to
initiate channel conduction. Figure 6 shows the
applicable test configuration. In this circuit, it is
worth noting, for instance, that if the device has
a VGS(th) = 0.5 V, when V+ = 0.5 V, the channel
resistance will be:
RCHANNEL =
0.5V
= 500kΩ
1µA
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V+
D
S
VGS(th)
1µA
VSB
1µA
VGS(th)
VGS(th) (V)
3.0
Figure 6. Threshold Voltage Test Configuration
2.5
2.0
1.5
1.0
0.5
Body Effect
As the body voltage increases in the negative
direction, the threshold goes up. Consequently,
if VGS is small, the on-resistance of the channel
can be very high. Figure 8 shows the effects of
VSB and VGS on rDS(on). Therefore, to maintain a
low on-resistance it is preferable to bias the
body to a voltage close to the negative peaks of
VS and use a gate voltage as high as possible.
4
8
12
Charge injection describes that phenomenon by
which a voltage excursion at the gate produces
an injection of electric charges via the gate-todrain and the gate-to-source capacitances into
the analog signal path. Another popular name
for this phenomenon is “switching spikes."
20
Figure 7. Threshold vs Source to Body Voltage
VD
D
1mA
VSB
S
VGS
300
VGS = 4 V
240
180
5V
120
10 V
60
Charge Injection
16
VSB (V)
rDS(on) (Ω)
For a MOSFET with a uniformly doped
substrate, the threshold voltage is proportional
to the square root of the applied source-to-body
voltage. The SD5000 family has a non-uniform
substrate, and the VGS(th) behaves somewhat
differently. Figure 7 shows the typical VGS(th)
variation as a function of the source-to-body
voltage VSB.
0
0
0
4
8
12
16
20
VSB (V)
Figure 8. On Resistance vs Source to Body and
Gate to Source Voltages
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Since these DMOS devices are asymmetrical1,
the charge injected into the S and D terminals is
different. Typical parasitic capacitances are on
the order of 0.2 pF for CDG and 1.5 pF for CSG.
Another factor that influences the amount of
charge injected is the amplitude of the gatevoltage excursion.
This is a directly
proportional relationship: the larger the
excursion, the larger the injected charge. This
can be seen by comparing curves (a) and (c) in
Figure 9. One other variable to consider is the
rate of gate-voltage change. Large amounts of
charge are injected when faster rise and fall
times are present at the gate. This is shown by
curves (a) and (b) in Figure 9.
S
D
∆V
CH
G
impedance tends to produce a rapid decay of the
extra charge introduced in the channel. At turnoff, however, the injected charge might become
stored in a sampling capacitor and create offsets
and errors. These errors will have a magnitude
that is inversely proportional to the magnitude
of the holding capacitance.
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.
Q = CH x ∆V
Off-Isolation and Crosstalk
0
∆Q (pC)
-2
(c)
(b)
-4
(2)
-6
(a)
-8
(1)
-10
-10
-5
0
5
10
VS (V)
(a)
(b)
(c)
VG = 10 V, tf = 0.3 V/µs
VG = 10 V, tf = 0.03 V/µs
VG = 0, -10 V, tf = 0.3 V/µs
The dc on-state resistance is typically 30 Ω and
the off-state resistance is typically 1010 Ω,
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.
Figure 9. SD5000 Charge Injection
Off-Isolation is defined by the formula:
Switching spikes occur at switch turn-on as well
as turn-off time. When the switch turns on, the
charge injection effect is minimized by the
usually low signal-source impedance. This low
1
The chip geometry is such that non-identical behavior
occurs when the source and drain terminals are reversed
in a circuit.
Off - Isolation (dB) = 20log
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
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VGATE
∆V
(a)
TOP: 5 V/div
HOR: 0.5 µs/div
BOT: 50 mV/div
POINT (1)
(b)
TOP: 5 V/div
HOR: 0.5 µs/div
BOT: 50 mV/div
POINT (2)
Figure 10. Waveforms for Points (1) and (2) of Figure 9
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.
Insertion Loss
At low frequencies, the attenuation caused by
the switch is a function of its on-resistance and
the load impedance. They form a simple series
voltage divider network. As an example, for a
600 Ω load impedance the insertion loss for
voice signals (1 Vrms at 3 kHz) is less than 0.3
dB. Thus, the SD5000 series make good audio
crosspoint switches.
1 Vrms
160
600 Ω
600 Ω
600 Ω
600 Ω
140
Crosstalk
120
(dB)
Crosstalk
1 Vrms
100
80
600 Ω
600 Ω
Off Isolation
Off Isolation
60
40
1k
10 k
100 k
1M
10 M 100 M
Frequency (Hz)
Figure 11. SD5000 Crosstalk and Off Isolation vs Frequency
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To Scope
+VDD
+5 V
510 Ω
RGEN
RL V
OUT to
Scope
90%
VIN
50%
10%
0V
td(on) td(off)
+ VDD
VIN
90%
VOUT
50%
10%
0V
51 Ω
tr
tf
Figure 12. Switching Test Circuit
Speed
Because the on-resistance and input capacitance
are low, the DMOS switches are capable of subnanosecond switching speeds. At these speeds
the external circuit rather than the FET itself is
often responsible for the rise and fall times that
can be obtained. Let's consider the switching
test circuit of Figure 12. At turn-on, the fall
time observed at the drain is a function of RG
and of the input pulse amplitude and rise time.
The sooner CGS reaches VGS(th), the sooner turnon will occur, and the lower the rDS(on) reached,
the faster CDS will be discharged.
The turn-off time (or the rise time of VD) is not
as much limited by the velocity at which CGS
can be discharged by the gate control pulse, as it
is by the time it takes to charge up CDS and CDG
via the load resistor RL. Table 1 shows typical
performances obtained. It is important to realize
that stray capacitance and parasitic inductances,
as well as scope probe capacitance, can
seriously affect the rise and fall times (switching
speed).
2
2
VDD
(V)
RL
(Ω)
td(on)
(ns)
tr
(ns)
tOFF
(ns)
5
330
0.6
0.8
4
5
680
0.6
0.7
8
10
680
0.7
0.8
8
15
1k
0.9
1.0
12
tOFF is dependant on RL and does not depend on the
device characteristics.
Table 1. Typical Switching Times
Drivers
The switch driver's function is to translate logic
control levels (either TTL, CMOS, or ECL) into
the appropriate voltages needed at the gate so
that the switch can be turned on or off. The
SD5000 can be operated as an inverter capable
of driving up to 20 V. This high-voltage rating,
together with its high speed, make it an
excellent driver for the other members of the
family. Figure 13 shows several driver circuits.
Since switching times depend on the CGS
charge/discharge times, it is important to note
that the driver's current source/sink capability
plays a very important role in the process.
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15 V
15 V
S
S
D
D
0 to 10 V
±5 V
1 kΩ
G
G
SD210
SD5000
TTL or
CMOS
5V
SD5000
B
SD5000
TTL
1 kΩ
S
D
0 to 10 V
G
15 V
SD210
B
15 V CMOS
D
SD5000
G
TTL
S
-5 V
D
±10 V
15 V
S
G
SD211
-5 V
B
±5 V
-10 V
-10 V
Figure 13. Various DMOS Drivers
High-Speed Multiplexer
In a typical application, the circuit of Figure 14
is used to multiplex and sample-and-hold two
analog signals at a 5-MHz rate. Two of the
switches in an SD5000 are used as level
shifter/drivers to provide the gate drive of the
single-pole-double-throw arrangement formed
by switches 3 and 4. Capacitors C1 and C2
provide charge injection compensation.
Figure 16 illustrates the resulting composite
waveform present at the holding capacitor along
with the gate 3 control signal.
As can be seen, the switching times are about 15
ns, the acquisition time is 80 ns, and the holding
time is about 90 ns. The total sample-and-hold
cycle takes 200 ns.
Even though not
maximized, this speed is faster than what any
other presently available (50 ns) analog switch
products can achieve.
Signal 1 is a 6-V, 156-kHz square wave. Signal
2 is a 2-Vpp, 78-kHz alternating waveform with
a dc offset of -3.4 V (Figure 15).
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The timing and amplitude of gate 2 and gate 3
control-signals can be examined in Figure 17.
Figure 18 shows a single-pole single-throw
configuration used to select one of two AM
modulated 10-MHz signals.
Figure 19
illustrates the two waveforms available at the
output. Table 2 contains typical values of
crosstalk and off-isolation attainable with this
configuration.
FREQ
(Hz)
SIG LVL INS LOSS OFF ISOL XTALK
(dBm)
(dB)
(dB)
(dB)
100 K
0
1.8
80
113
1M
0
1.8
70
92
5M
0
1.9
69
69
10 M
0
2.0
61
65
10 M
6
2.0
61
66
10 M
12
2.0
61
68
Table 2. SPDT Switching Performance
16 V
10 pF
Signal 1
C1
510 Ω
-0 V
16
8.3 V
14
G3
11
G4
13
510 Ω
Signal 2
12 2 9
Signal 1
VOUT
-8 V
120 pF
Signal 2
16 V
0.1 µF
5
510 Ω
8.3 V
6
8
G2
VOUT
1
G1
3
510 Ω
4
Figure 15. The Two Analog Signals to Be
Sampled
10 pF
G3
C2
-8 V
0.1 µF
VOUT
G3
Figure 14. 5-MHz Multiplexer and Sample and
Hold Circuit
Figure 16. Composite Sample and Hold Output
and the Gate 3 Control Signal
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A High-Speed S/H Circuit
Figure 20 shows a fast unity gain input buffer
(Si581) driving an SD5000 switch. One half of
the SD5000 is configured as dummy switches
for charge injection compensation. A JFET
output buffer minimizes droop. Transistors Q1
through Q4 level shift the ECL control input
signals into a voltage (referenced to the analog
signal voltage) used to drive the DMOS FETs.
G2
G3
Figure 17. Gate Control Signals for the SPDT
Switch Configuration
DAC Deglitcher
A very small charge injection makes DMOS
FETs excellent DAC deglitcher switches. Figure
21 illustrates a typical circuit configuration.
SD210
-5 V
0.047 µF
Input 1
10 MHz
Shield
5V
15 V
0.047 µF
Control
TTL
DG413
Output
Channel 1 On
SD210
-5 V
0.047 µF
Input 2
10 MHz
Shield
-5 V
0.047 µF
Channel 2 On
Figure 18. High Frequency SPDT Switch
Figure 19. Two 10-MHZ AM Modulated Outputs
for the SPDT Switch of Figure 18
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-15 V
100 Ω
100 Ω
100 Ω
100 Ω
Q3
Q
ECL
/Q
Q1
Q1 & Q2: LS3250
Q3 & Q4: LS3550
Q4
Q2
SD5000
150 Ω
1 kΩ
240 Ω
1 kΩ
240 Ω
-5.2 V
Output
±3 V
50 Ω
Analog
Signal
Si 581
CH
100 pF
Figure 20. Fast S/H Circuit Achieves Minimum Step Errors
RFB
12 Bit DAC
SD5000
IOUT
9
12
8
5
16
13
-
1
OP-27
+
VOUT
4
T/H
Figure 21. DAC Deglitcher Using DMOS Switches
Part Number
Package
SD210DE
TO-72
SD214DE
TO-72
SD/SST211
TO-72/SOT-143
SD/SST213
TO-72/SOT-143
SD/SST215
TO-72/SOT-143
SD5000N
PDIP
SD5001N
PDIP
SD5000I
CDIP
SD5400CY
SOIC
SD5401CY
SOIC
SD/SST823 3
TO-72/SOT-143
SD/SST824 3
TO-72/SOT-143
3
Future devices available Q1 2003
Type
Zener
Protection
rDS(on) (Ω)
V(BR)DS Min
(V)
Single
Single
Single
Single
Single
Quad
Quad
Quad
Quad
Quad
Single
Single
None
None
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
45
45
45/50
45/50
45/50
70
70
70
75
75
5
5
30
20
30
10
20
20
10
20
20
10
15
20
Table 3. DMOS Device Part Numbers and Packages
11
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