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> Analog
Switches and Multiplexers > APP 638
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> APP 638

Sep 03, 2002

Keywords: CMOS, analog switch, analogue switch,
capacitive gate current ,MUX, multiplexers, on-resistance, RON
flatness, THD, charge injection, ESD protected multiplexers,
fault-protection, ESD protection, force-sense switches

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APPLICATION NOTE 638

Selecting the Right CMOS Analog Switch

Abstract: This
tutorial outlines the basic construction, operation, and
applications of analog switches. On-resistance, flatness, and
charge-injection, each a performance-related specification,
are defined. Application-specific features including
ESD-protection, fault-protection, and force-sense capability
are explained.

Attend
this brief webcast by Maxim on
TechOnline

Also
see application note 4653, "FAQs about analog
switches."

Introduction
First developed about 25 years ago,
integrated analog switches often form the interface between
analog signals and a digital controller.
This tutorial presents the theoretical basis for analog
switches and describes some common applications for them. The
special features of calibration multiplexers (cal-muxes),
fault-protected switches, and force-sense
switches are discussed as well.

In recent years,
integrated analog switches have offered better switching
characteristics, lower supply voltages, and smaller packages.
Because so many performance options and special functions are
available, the well-informed product designer can usually find
the right part for a particular application.

Standard Analog Switches
CMOS
analog switches are easy to use, so most designers take them
for granted. But one should not forget that these switches
solve specific engineering problems. Conventional analog
switches like the early CD4066 or the MAX4066 are now
offered by many semiconductor
manufacturers; their structure is shown in Figure 1.
Maxim also offers devices such as the MAX4610, which is
pin-for-pin compatible with the industry standard but offers
better performance.

Figure
1. The internal construction of a typical analog switch
features parallel n- and p-channel
MOSFETs.

Connecting an n-channel MOSFET in parallel
with a p-channel MOSFET allows signals to pass in either
direction with equal ease. Whether the n- or the p-channel
device carries more signal current depends on the ratio of
input to output voltage. Because the switch has no preferred
direction for current flow, it has no preferred input or
output. The two MOSFETs are switched on and off by internal
inverting and noninverting amplifiers. These amplifiers
level-shift the digital input signal as required, according to
whether the signal is CMOS- or TTL-logic-compatible and
whether the analog supply voltage is single or dual.

Low-Resistance Switches
Taking the p- and n-channel
on-resistances (RON) in parallel (product over sum)
for each level of VIN yields a composite
on-resistance characteristic for the parallel structure
(Figure 2). This plot of RON versus
VIN can be described as linear
if you exclude the effects of temperature,
power-supply voltage, and RON variation with analog
input voltage. You should be aware that these effects
represent disadvantages and that minimizing them is often the
primary purpose of new products. See Table
1.

Figure
2. The n-channel and p-channel on-resistances of Figure 1 form
a low-valued composite
on-resistance.

Table
1. Low-resistance switches

Part Number

Function

RDS(ON)
(Ω max)

ICOM(OFF)/ID(OFF)
(nA max)

RON Match
(Ω max)

RON Flatness
(Ω max

tON/tOFF (ns
max)

Charge Injection (pC typ)

Supply Voltage Range (V)

Pin-
Package

MAX312/MAX313/MAX314

4 SPST NC/NO/NO,NC

10

0.5

1.5

2

225/185

30

±4.5 to ±20

16-DIP|SO

MAX4614/MAX4615/MAX4616

4 SPST NO/NC/NO,NC

10

1

1

1

12/10

6.5

+2 to +5.5

14-DIP/SO/16-QSOP

MAX4617

8X1 Mux

10

1

1

1

15/10

3

+2 to +5.5

16-DIP/SO/QSOP

MAX4618

Dual 4X1 Mux

10

1

1

-

15/10

3

+2 to +5.5

16-DIP/SO|QSOP

MAX4619

3 SPDT

10

1

1

-

15/10

3

+2 to +5.5

16-DIP|SO|6/QSOP

MAX4621

2 SPST NO

5

0.5

0.5

0.5

250/200

80

±4.5 to ±20 or +4.5 to +36

16-DIP/SO

MAX4622

2 SPDT

5

0.5

0.5

0.5

250/200

80

±4.5 to ±20 or +4.5 to +36

16-DIP/SO

MAX4623

2 DPST

5

0.5

0.5

0.5

250/200

80

±4.5 to ±20 or +4.5 to +36

16-DIP/SO

MAX4661/MAX4662/MAX4663

4 SPST NO/NC/NO,NC

2.5

0.5

-

0.4

250/200

120

±4.5 to ±20 or +4.5 to +36

16-DIP/SO|/SSOP

MAX4664/MAX4665/MAX4666

4 SPST NO/NC/NO,NC

4

0.5

0.5

0.5

300/200

300

±4.5 to ±20 or +4.5 to +36

16-DIP|/SO

MAX4667/MAX4668/MAX4669

2 SPST NO/NC/NO,NC

2.5

0.5

0.4

0.4

300/200

450

±4.5 to ±20 or +4.5 to +36

16-DIP/SO

MAX4680/MAX4690/MAX4694

2 SPST NO

1.25

0.5

0.25

0.3

275/200

-60

±4.5 to ±20 or +4.5 to +36

16-DIP/WID
SO/SSOP

MAX4734

1 4:1 MUX

0.8

1

0.2

0.1

25/20

60

+1.6 to +3.6

10-µMAX®
12-TQFN

MAX4741

2 SPST

0.8

1

0.08

0.18

24/16

28

1.6 to +3.6

8-SOT23

MAX4751

4 SPST NO

0.9

2.5

0.12

0.1

30/25

21

+1.6 to +3.6

14-TSSOP
16-QFN

MAX4756

4 SPDT
NO/NC

0.85

3

0.35

0.4

150/40

50

+1.8 to +5.5

36-UCSP™
36-TQFN

MAX4780

4 2:1 MUX

1

1

0.15

0.2

25/10

5

+1.6 to +4.2

16-TQFN
16-TSSOP

MAX4781

1 8:1 MUX

1

2

0.4

0.2

25/15

40

+1.6 to +3.6

16-QFN
16-TSSOP

MAX4855

2 SPST NO/NC

1

2

0.12

0.27

60/40

170

+2 to +5.5

16-TQFN
16-TSSOP

The first
analog switches operated on ±20V supply voltages and had
several hundred ohms of RON. Recent products (the
MAX4601, for
example) achieve 2.5Ω maximum RON with a much lower
supply voltage. Supply voltage has a substantial effect on
RON (Figure 3). The MAX4601 specifies signal
and supply voltages from 4.5V to 36V or from ±4.5V to ±20V. As
you can see, RON increases for lower supply
voltages. The max RON is about 8Ω at 5V, 3Ω at 12V,
and only 2.5Ω at 24V. Many new analog switches specify
low-voltage operation for supply voltages down to 2V. For 5V
supplies, Figure 4 compares the Maxim switches with
older types.

Figure
3. Higher supply voltage causes lower on-resistance.

Figure 4. At +5V supply voltage, later-generation
analog switches have lower on-resistance.

When
selecting switches for single-supply systems, try to choose
from those intended for single-supply use. Such devices save
one pin, because they do not require separate V- and ground
pins. As a result, this economy of pins enables a
single-pole/double-throw (SPDT) switch to fit into a miniscule
6-pin SOT23 package. Similarly, low-voltage dual-supply
applications call for dual-supply switches. These switches
require a V- pin in addition to the ground pin, and typically
specify a logic interface with standard CMOS and TTL
levels. The SPST MAX4529, for example, is also available in a
6-pin SOT23 package.

Many high-performance analog
systems still rely on higher-level bipolar supplies such as
±15V or ±12V. The interface to these voltages requires an
additional supply pin commonly labeled VL (shown in
the MAX318 data
sheet). The VL supply connects to the system logic
voltage, which is usually 5V or 3.3V. Having the input logic
signals referenced to the actual logic levels increases the
noise margin and prevents excessive power
dissipation.

Often misunderstood is the analog-switch
concept pertaining to input logic levels and their effect on
supply current. If the logic inputs are at ground or
VCC (or VL when available), analog
switches have essentially no supply current. Applying TTL
levels to a 5V switch, however, can cause the supply current
to increase more than 1000 times. To avoid unnecessary power
consumption, you should avoid TTL levels, which are simply a
legacy of the 1980s.

Signal Handling
Figure 3 also shows the value of
RON versus signal voltage. These curves fall within
the specified range of supply voltage, because analog switches
can only handle analog-signal levels between the supply
voltages. Under- or overvoltage inputs can permanently damage
an protected switch by producing uncontrolled currents through
internal diode
networks. Normally, these diodes protect the switch against
short-duration electrostatic
discharge (ESD) as high as ±2kV.

RON
for a typical CMOS analog switch causes a linear reduction of
signal voltage that is proportional to current passing through
the switch. This might not be a disadvantage for modest levels
of current or if the design accounts for RON
effects. However, if you accept a certain level of
RON, then channel matching and RON
flatness can interest you. Channel matching describes the
variation of RON for the channels of one device;
RON flatness describes the variation of
RON versus signal range for a single channel.
Typical values for these parameters are 2Ω to 5Ω, and only
0.5Ω maximum for very low RON switches (MAX4601,
for instance). The smaller the ratio of
matching/RON or flatness/RON is, the
more accurate the switch.

In most applications, you can
avoid excessive switch current by modifying the circuit
design. To change the gain of an op
amp by switching between different feedback resistances,
for example, choose a configuration that places the switch in
series with a high-impedance input (Figure 5a). Because
switch currents are insignificant, you can ignore the value of
RON and its temperature coefficient. Switch current
in the alternative design (Figure 5b) can be
substantial, because it depends on the output
voltage.

Figure 5. Gain-control circuits are good (a) or bad (b)
depending on the amount of current through the
switch.

Break-Before-Make
Turn-on and turn-off times
(tON and tOFF) for most analog switches
vary from below 60ns to as high as 1µs. For Maxim's
"clickless" audio switches, tON and tOFF
are in the millisecond range to eliminate the audible clicks
otherwise present when switching audio signals. The relative
magnitudes are also important: tON >
tOFF yields break-before-make
action, and tOFF > tON yields make-before-break.
This distinction is critical for some
applications.

Figure 5a shows that you must take care
in switching between the two gains. One switch is normally
closed in a typical make-before-break application. In changing
gain you must avoid opening both switches at once; that is,
the second switch must close before the first switch opens.
Otherwise, the op amp applies open-loop gain and drives its
output to the rails. The opposite configuration
(break-before-make) is also useful in switching among
different input signals to a single op amp. To avoid short
circuits between the input channels, a given connection must
be switched off before the next one is switched
on.

When a changing signal level modulates the
on-resistance, causing a variation in the insertion loss,
analog switches generate total
harmonic distortion (THD). Consider a 100Ω switch with 10Ω
RON flatness, for example. Loading this switch with
a 600Ω termination produces 0.24% of THD. Consequently, for
low THD you should avoid loading the output of an analog
switch.

Charge-Injection Effects
As mentioned above, low
RON is not necessary in all applications. Lower
RON requires greater chip area. The result is a
greater input capacitance whose charge and discharge currents
dissipate more power in every switching cycle. Based on the
time constant t = RC, this charging time depends on load
resistance (R) and capacitance (C). It normally lasts a few
tens of nanoseconds, but low-RON switches have
longer-duration on and off periods. High-RON
switches are faster.

Maxim offers both types of
switches, each with the same pinout in the same miniature
SOT23 package. The MAX4501 and the MAX4502 specify higher
on-resistance but shorter on/off times. The MAX4514 and the
MAX4515 have lower on-resistance but longer switching times.
Another negative consequence of low on-resistance can be the
higher charge injection caused by higher levels of capacitive
gate current. A certain amount of charge is added to or
subtracted from the analog channel with every on or off
transition of the switch (Figure 6). For switches
connected to high-impedance outputs, this action can cause
significant changes in the expected output signal. A small
parasitic capacitor (CL) with no other load adds a
variation of ΔVOUT, so charge injection can be
calculated as Q = ΔVOUTCL.

Figure
6. Charge injection from the switch-control signal causes a
voltage error at the analog output.

A track/hold amplifier,
which maintains a constant analog output during conversion by
an A/D
converter, offers a good example of this (Figure
7). Closing S1 charges the small buffer capacitor (C) to
the input voltage (VS). The value of C is only a
few picofarads, and VS remains stored on C when S1
opens. The held voltage (VH) is applied to the
buffer by closing S2 at the beginning of a conversion. The
high-impedance buffer then maintains VH constant
over the ADC's conversion time. For short acquisition times,
the track/hold's capacitor must be small and S1's
on-resistance must be low. On the other hand, charge injection
can cause VH to change by ±ΔVOUT (a few
millivolts), thereby affecting the accuracy of the following
ADC.

Figure
7. A typical track/hold function requires precise control of
the analog switches.

Having reviewed these
fundamentals, we now focus on innovative switches for special
applications.

T-Switches for Higher Frequencies
The T-switch is
suitable for video and other frequencies above 10MHz. It
consists of two analog switches in series, with a third switch
connected between ground and their joining node. This
arrangement provides higher off-isolation than a single
switch. The capacitive
crosstalk for a T-switch turned off typically rises with
frequency due to the parasitic capacitances in parallel with
each of the series switches (Figure 8). The problem in
operating a high-frequency switch does not lie in turning it
on, but in turning it off.

When the T-switch is turned
on, S1 and S3 are closed and S2 is open. In the off state, S1
and S3 are open and S2 is closed. In that case (the off state)
the signal tries to couple through the off-capacitance of the
series MOSFETs, but is shunted to ground by S3. If you compare
the off isolation at 10MHz for a video T-switch (MAX4545) and
a standard analog switch (MAX312), the result is dramatic:
-80dB versus -36dB for the standard switch.

Figure
8. The T-switch configuration attenuates RF frequencies that
couple through the stray capacitance between the source and
the drain of an open (off) switch.

Smaller Packages
Other advantages for CMOS analog
switches include small packages, such as the 6-pin SOT23, and
no mechanical parts (unlike reed relays). Maxim offers a small
video switch (MAX4529) as well as a standard, low-voltage SPDT
switch (MAX4544). Both come in 6-pin SOT23 packages and
operate from supply voltages in the 2.7V to 12V range. The
MAX4544 is the smallest SPDT analog switch currently
available. See Table 2.

Table 2. Small
packages

Part Number

Function

RDS(ON)
(Ω max)

ICOM(OFF)
/ID(OFF)
(nA
max)

RON Flatness
(Ω
max)

tON/tOFF (ns
max)

Charge Injection (pC max)

Off-Isolation (dB max)/Frequency (MHz)

Supply Voltage Range (V)

Pin-
Package

MAX4501/MAX4502

SPST NO/NC

250

1

-

75/50

10

-100/0.1

+2 to +12

5-SOT23-5|8-DIP/SO

MAX4503/MAX4504

SPST NO/NC

250

1

-

150/100

10

-90/0.1

±1 to ±6

5-SOT23-5|8-DIP/SO

MAX4514/MAX4515

SPST NO/NC

20

1

3

150/100

10

-90/0.1

+2 to +12

5-SOT23-5|8-DIP/SO

MAX4516/MAX4517

SPST NO/NC

20

1

4

100/150

20

-86/0.1

±1 to ±6

5-/SOT23-5|8-DIP/SO

MAX4529

SPST NC

70

1

10

75/75

10

-80/10

±2.7 to ±6 or +2.7 to +12

6-SOT23-6|8-DIP/SO//µMAX

MAX4544

SPDT

60

0.1

6

100/75

5

-76/-90

+2.7 to +12

6-SOT23-6|8-DIP/SO/µMAX

MAX4707

1 SPDT NO

3

1

0.85

20/15

5

-82/1

+1.8 to +5.5

5-SC70
6-SC70

MAX4719

2 SPDT

20

0.5

1.2

80/40

18

-80/1

+1.8 to +5.5

10-µMAX
12-USCP

MAX4723

2 SPDT NO/NC

4.5

0.5

1.2

80/40

18

-80/1

+1.8 to +5.5

8-µMAX
9-USCP

MAX4730

1 SPDT

5.5

-

0.95

45/26

3

-67/1

+1.8 to +5.5

6-SC70

MAX4733

2 SPDT NO/NC

50

0.1

9

150/60

7.5

-72/1

+2 to +11

8-µMAX
9-/USCP

As
mentioned earlier, Maxim offers many variations of popular
analog switches like the CD4066, including a family of
low-cost quad analog switches (MAX4610–MAX4612). The MAX4610
is a pin-compatible improvement over the industry-standard
4066, but with lower supply voltage (as low as 2V) and higher
accuracy: channel matching to within 4Ω maximum and channel
flatness to within 18Ω maximum. This family of parts offers
three switch configurations, and their lower on-resistance
(< 100Ω at 5V) suits low-voltage applications. A tiny
14-pin TSSOP
package (6.5 x 5.1 x 1.1mm ³ max) solves the problem of board
space.

ESD-Protected Switches
Based on the success of Maxim's
ESD-protected interface products, ±15kV ESD
protection was added to some of the more recent analog
switches (Table 3). Maxim now offers the first switches
with ±15kV ESD protection per IEC 1000-4-2 Level 4 (the
highest level). All analog inputs are ESD-tested using the Human
Body Model, as well as the Contact and Air-Gap Discharge
methods specified in IEC 1000-4-2. The MAX4551/MAX4552/MAX4553
switches are pin-compatible with many standard quad-switch
families such as the DG201/211 and the MAX391 types. To
augment standard multiplexer
families like the 74HC4051 and the MAX4581, Maxim also
released ESD-protected multiplexers. You no longer need to use
costly TransZorbs® to protect your analog
inputs.

Table 3. ±15k ESD per IEC 1000-4-2, in
16-pin DIP,
SO, and QSOP
packages

Part Number

Function

RDS(ON)
(Ω max)

ICOM(OFF)
/ID(OFF)
(nA
max)

RON Match
(Ω max)

RON Flatness
(Ω
max)

tON/tOFF (ns
max)

Charge Injection (pC typ)

100kHz OFF-Isolation/Crosstalk (dB
typ)

Supply Voltage Range (V)

MAX4551

4 SPST NO

120

±1

4

8

110/90

2

-90/-90

±2 to ±6 or +2 to +12

MAX4552

4 SPST NC

120

±1

4

8

110/90

2

-90/-90

±2 to ±6 or +2 to +12

MAX4553

4 SPST NO, NC

120

±1

4

8

110/90

2

-90/-90

±2 to ±6 or +2 to +12

MAX4558

8 x 1 Mux

160

±1

6

8

150/120

2.4

-96/-93

±2 to ±6 or +2 to +12

MAX4559

Dual 4 x 1 Mux

160

±1

6

8

150/120

2.4

-96/-93

±2 to ±6 or +2 to +12

MAX4560

3 SPDT

160

±1

6

8

150/120

2.4

-96/-93

±2 to ±6 or +2 to +12

MAX4569

1 SPST NO

70

±0.5

2

4

150/80

6

-75

±2 to ±6
+2 to +12

MAX4577

2 SPST

70

±0.5

2

4

150/80

4

-75/90

±2 to ±6
+2 to +12

MAX4620

4 SPST NO

70

±0.5

2

4

150/80

5

-75/90

±2 to ±6
+2 to +12

Fault-Protected Switches
As mentioned under "Signal
Handling" above, the supply-voltage rails for an analog switch
restrict the allowed range for input signal voltage. Normally
this restriction is not a problem, but in some cases the
supply voltage can be turned off with analog signals still
present. That condition can permanently damage the switch, as
can transients outside the normal range of the power supply.
Maxim's fault-protected switches and multiplexers guarantee overvoltage
protection of ±25V and power-down protection of ±40V,
along with rail-to-rail signal handling and the low
on-resistance of a normal switch (Figure 9). The input
pin, moreover, assumes a high impedance
during fault conditions regardless of the switch state or load
resistance. Only nanoamperes of leakage current can flow from
the source.

Figure
9. This internal structure shows the special circuitry in a
fault-protected analog switch.

If the switch (P2 or
N2) is on, the COM output is clamped to the supply by two
internal 'booster' FETs. Thus, the COM output remains within
the supply rails and delivers a maximum of ±13mA depending on
the load, but without a significant current at the NO/NC pin.
The fault-protected switches, MAX4511/MAX4512/MAX4513, are
pin-compatible with the DG411–DG413 and DG201/DG202/DG213
types (Table 4). Note that signals pass equally well in
either direction through an ESD- and fault-protected switch,
but these protections apply only to the input side.

Table 4. Fault protection with
rail-to-rail signal swings

Part Number

Function

RDS(ON)
(Ω max)

ICOM(OFF)
/ID(OFF)
(nA
MAX)

RON Match
(Ω max)

tON/tOFF
(ns
max)

Overvoltage Supplies
ON/OFF
(V)

Charge Injection
(pC max)

Supply Voltage
Range(V)

Pin-
Package

MAX4505

1 Line Protector

100

±0.5

-

-

±36/±40

-

±8 to ±18 or +9 to +36

5-SOT|8-µMAX

MAX4506

3 Line Protector

100

±0.5

7

-

±36/±40

-

±8 to ±18 or +9 to +36

8-DIP/SO/CERDIP

MAX4507

8 Line Protector

100

±0.5

7

-

±36/±40

-

±8 to ±18 or +9 to +36

18-/DIP/SO/20-SSOP

MAX4508

8 x 1 Mux

400

±0.5

15

275/200

±25/±40

10

±4.5 to ±20 or +9 to +36

16-DIP/SO

MAX4509

4 x 1 Mux

400

±0.5

15

275/200

±25/±40

10

±4.5 to ±20 or +9 to +36

16-DIP/SO

MAX4511

4 SPST NO

160

±0.5

6

500/400

±36/±40

5

±8 to ±18 or +9 to +36

16-DIP/SO/CERDIP

MAX4512

4 SPST NC

160

±0.5

6

500/400

±36/±40

5

±8 to ±18 or +9 to +36

16/-DIP|16/SO|16/CERDIP

MAX4513

4 SPST NO, NC

160

±0.5

6

500/400

±36/±40

5

±8 to ±18 or +9 to +36

16-DIP/SO/CERDIP

MAX4533

4 SPDT

175

±0.5

6

250/150

±25/±40

5

±4.5 to ±20 or +9 to +36

20-Wide SO/SSOP

MAX4631

2 SPST NO

85

±0.5

6

150/100

±36/±40

10

±4.5 to ±20 or +9 to +36

16-DIP/SO/CERDIP

MAX4632

2 SPDT

85

±0.5

6

150/100

±25/±40

10

±4.5 to ±20 or +9 to +36

16-DIP/SO/CERDIP

MAX4633

2 DPST NO

85

±0.5

6

150/100

±36/±40

10

±4.5 to ±20 or +9 to +36

16-DIP/SO/CERDIP

MAX4510

1 SPST NC

160

±0.5

-

500/175

±36/±40

5

+9 to +36
±4.5 to ±20

6-/SOT23
8-µMAX

MAX4520

2 SPDT NO

160

±0.5

-

500/175

±36/±40

5

+9 to +36
±4.5 to ±20

6-SOT23
8-µMAX

MAX4534

1 2:1MUX

275

±2

10

275/200

±25/±40

10

+9 to +36
±4.5 to ±20

14-TSSOP/SO/DIP

MAX4535

2 2:1MUX

275

±2

10

275/200

±25/±40

10

+9 to +36
±4.5 to ±20

14-TSSOP/SO/DIP

MAX4711

4 SPST NC

25

±0.5

1

125/80

±7/±12

25

+2.7 to +11
±2.7 to ±5.5

8-µMAX
9-UCSP

Force-Sense Switches
Maxim offers a family of analog
switches with different switch types residing in the same
package. The MAX4554/MAX4555/MAX4556 devices, for instance,
are configured as force-sense switches for Kelvin sensing in
automated test equipment (ATE). Each part contains
low-resistance high-current switches for forcing current and
higher-resistance switches for sensing voltage or switching
guard signals. On-resistance for the current switches is only
6Ω, and for the sensing switches is 60Ω at ±15V supply
voltages. The MAX4556 contains three SPDT switches with
break-before-make action.

Typical force-sense
applications are found in high-accuracy systems and in
measurement systems that involve long distances (Figure
10). For 4-wire measurements, two wires force a voltage or
current to the load, and two other wires connected directly to
the load sense the load voltage.

Figure
10. With the 4-wire technique, two wires force and two other
wires sense the measured voltage.

Alternatively, a
2-wire system senses load voltage at the ends of the force
wires opposite the load. Load voltage is lower than the source
voltage, because the forcing voltage or current causes a
voltage drop along the wires. Longer distance between source
and load, larger load current, and higher conductor resistance
all contribute for this degradation. The resulting signal
reduction can be overcome by using a 4-wire technique in which
the two additional voltage-sensing conductors carry negligible
current.

Force-sense switches simplify many
applications, such as switching between one source and two
loads in a 4-wire system. They are suitable for use in
high-accuracy measurement systems, such as nanovoltmeters and
femtoammeters, and for 8- or 12-wire force-and-sense
measurements using the guard wires of triax cables. For more
information, please see the MAX4554/MAX4555/MAX4556 data
sheet.

Multiplexers
In addition to switches, Maxim makes a
number of multiplexers (muxes). A mux is a special version of
a switch in which two or more inputs are selectively connected
to a single output. A mux can be as simple as an SPDT switch
or come in 4:1, 8:1, 16:1, or even dual 4:1 and 8:1
combinations. The digital control for these higher order muxes
is similar to a binary decoder with three digital inputs
required to select the appropriate channel.

A
demultiplexer is basically a mux used backwards. That is, one
input connects to two or more outputs based on the decoded
address data.

There are, finally, cross-point switches.
A cross-point switch is usually an M x N device, whereby any
or all of M inputs may be connected to any or all of N outputs
(and vice versa).

Calibration Multiplexers
Calibration multiplexers
(cal-muxes) are used in precision ADCs and other
self-monitoring systems. They combine different components in
one package: analog switches for generating accurate voltage
ratios from an input reference voltage; internal precision
resistor-dividers; and a multiplexer for selecting between
different inputs. Maxim introduced this combination of
functions in a single package.

Two of these devices
(MAX4539 and MAX4540) can balance two major errors associated
with an ADC system: offset and gain
error. Using the internal precision voltage-dividers,
these devices measure gain and offset in a few steps,
controlled trough the serial
interface of a microcontroller. The reference ratios
15/4096 and 4081/4096 (with respect to the external reference
voltage) are accurate to 15 bits. The ratios (5/8)(V+ - V-)
and V+/2 are accurate to 8 bits.

The cal-mux first
applies one-half the supply voltage to verify that power is
present. The system then measures zero offset and gain error,
and forms an equation to correct the subsequent readings. Zero
input voltage, for example, should produce a digital zero
output. The cal-mux calibrates for offset error by applying a
very small input voltage of 15/4096 referred to
(VEFHI - REFLO). For a 12-bit ADC with
4.096V reference, 15/4096 equals 15mV and also 15 LSBs. The
digital output therefore should be binary 000000001111. To
measure offset error, the microcontroller simply records the
difference between binary 000000001111 and the ADC's actual
output.

To measure gain error, the cal-mux applies a
voltage of 4081/4096 referred to (VREFHI -
VREFLO). The microcontroller then records the
difference between binary 111111110000 and the ADC's digital
output. Knowing the ADC's offset and gain error, the system
software constructs calibration factors that adjust the
subsequent outputs to produce correct readings. The cal-mux
then serves as a conventional multiplexer, but with the
ability to recalibrate the system periodically.

USB 2.0 Switches
A universal
serial bus (USB) is a high-speed interface for handheld
devices to communicate with computers. Multiple USB devices
can be connected to a computer, and analog switches are used
to route the USB signal to different devices. The USB 2.0 is a
high-speed signal that requires a high-bandwidth and
low-capacitance analog switch.

Maxim offers a good
selection of USB 2.0-compliant switches ideal for USB 2.0
high-speed applications. Table 5 shows a few examples
of USB 2.0 switches.

Table 5. USB 2.0 switches

Part

Functions

RON (O, max)

RON Match (O, max)

tON/tOFF (ns,
max)

ICOM(OFF) (nA, max)

CON (pF, typ)

COFF (pF, typ)

Charge Injection (pC)

BW (MHz)

Supply Voltage Range (V)

MAX4906F

2 SPDT

7

1.2

60/30

1000

6

2

5

1000

3.0 to 3.6

MAX4907F

2 SPST NO

7

1.2

60/30

1000

4

2

5

1000

3.0 to 3.6

MAX4906EF

2 SPDT

7

0.8

1.4/35

1000

10

9

20

500

2.7 to 3.6

MAX4899E

3 x 1 MUX

5

0.8

—

1000

15

10.2

25

425

2.7 to 3.6

MAX4899AE

4 x 1 MUX

5

0.8

—

1000

15

10.5

25

425

2.7 to 3.6

High-Voltage Switches
In ultrasound applications,
high-voltage pulses (±100V) are applied to transducers to
generate ultrasonic waves. Analog switches are required for
routing these high-voltage signals between the transducers and
the main systems, so the switches must be able to handle
high-voltage signals.

Maxim offers a good selection of
high-voltage analog switches that are ideal for ultrasound
medical applications. Table 6 shows a few
examples.

Table 6. High-voltage switches

Part

Functions

VSUPPLY (Single, V)

VSUPPLY (Single, V)

VSUPPLY (Dual, ±V)

VSUPPLY (Dual, ±V)

BW (MHz)

IL(OFF) (nA)

tON (ns)

tOFF (ns)

CON (pF)

COFF (pF)

min

max

min

max

MAX4800A/MAX4802A

8 SPST NO

40

200

40

100

20

2000

5000

5000

36

11

MAX14800–MAX14803

16 SPST NO

40

200

40

100

20

2000

3500

3500

36

11

µMAX
is a registered trademark of Maxim Integrated Products,
Inc.

TRANSZORB is a
registered trademark of Vishay General Semiconductor,
LLC.

UCSP is a trademark of Maxim Integrated Products,
Inc.

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Download,
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APP 638: Sep 03, 2002

AN638, AN 638, APP638,
Appnote638, Appnote 638

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