MAX4588CPI Datasheet MAX4588CWI+T, MAX4588EAI+, MAX4588CPI | P13-P15

Detailed Description

Logic-Level Translators

The MAX4588 is constructed of high-frequency “T”

switches, as shown in Figure 8. The logic-level inputs

are translated by amplifier A1 into a V+ to V- logic sig-

nal that drives amplifier A2. Amplifier A2 drives the

gates of N-channel MOSFETs N1 and N2 from V+ to V-,

turning them fully on or off. The same signal drives

inverter A3 (which drives the P-channel MOSFETs P1

and P2, turning them fully on or off) from V+ to V-, and

turns the N-channel MOSFET N3 on and off. The logic-

level threshold is determined by V

and GND.

Switch On Condition

When the switch is on, MOSFETs N1, N2, P1, and P2

are on and MOSFET N3 is off (Figure 8). The signal

path is COM_ to NO_, and because both N-channel

and P-channel MOSFETs act as pure resistances, it is

symmetrical (i.e., signals may pass in either direction).

The off MOSFET, N3, has no DC conduction, but has a

small amount of capacitance to GND. The four on

MOSFETs also have capacitance to ground that,

together with the series resistance, forms a lowpass fil-

ter. All of these capacitances are distributed evenly

along the series resistance, so they act as a transmis-

sion line rather than a simple R-C filter. The MAX4588’s

construction allows an exceptional 180MHz bandwidth

when the switches are on.

Typical attenuation in 75Ω systems is 2.5dB and is rea-

sonably flat up to 50MHz. Higher-impedance circuits

show even lower attenuation (and vice versa), but

slightly lower bandwidth due to the increased effect of

the internal and external capacitance and the switch’s

internal resistance.

MAX4588

Low-Voltage, High-Isolation,

Dual 4-Channel RF/Video Multiplexer

______________________________________________________________________________________ 13

A1 A2 A3

V-

GND

INPUT

V+

COM_ NO_

NORMALLY OPEN SWITCH CONSTRUCTION

ESD DIODES

ON GND, NO_,

AND COM_

CSS

SCLK

DIN

DOUT

NOTE: ALL INPUT SIGNALS ARE SPECIFIED WITH t

AND t

< 10ns.

TIMING IS MEASURED FROM 50% OF DIGITAL SIGNAL.

A2 DISABLE

CSH

MAX4588

Figure 7. Serial Timing Diagram

Figure 8. T-Switch Construction

MAX4588

The MAX4588 is optimized for ±5V operation. Using

lower supply voltages or a single supply increases

switching time, on-resistance (and therefore on-state

attenuation), and nonlinearity.

Switch Off Condition

When the switch is off, MOSFETs N1, N2, P1, and P2

are off and MOSFET N3 is on (Figure 8). The signal

path is through the parasitic off-capacitances of the

series MOSFETs, but it is shunted to ground by N3.

This forms a highpass filter whose exact characteristics

are dependent on the source and load impedances. In

75Ω systems, and below 10MHz, the attenuation can

exceed 80dB. This value decreases with increasing fre-

quency and increasing circuit impedances. External

capacitance and board layout have a major role in

determining overall performance.

Applications Information

Power-Supply Considerations

Overview

The MAX4588 construction is typical of many CMOS

analog switches. It has four supply pins: V+, V-, V

, and

GND. V+ and V- are used to drive the internal CMOS

switches and set the limits of the analog voltage on any

switch. Reverse ESD-protection diodes are internally

connected between each analog signal pin and both

V+ and V-. If the voltage on any pin exceeds V+ or V-,

one of these diodes will conduct. During normal opera-

tion these reverse-biased ESD diodes leak, forming the

only current drawn from V- and V+.

Virtually all the analog leakage current is through the

ESD diodes. Although the ESD diodes on a given sig-

nal pin are identical, and therefore fairly well balanced,

they are reverse-biased differently. Each is biased by

either V+ or V- and the analog signal. This means their

leakages vary as the signal varies. The difference in the

two diode leakages from the signal path to the V+ and

V- pins constitutes the analog signal-path leakage cur-

rent. All analog leakage current flows to the supply ter-

minals, not to the other switch terminal. This explains

how both sides of a given switch can show leakage

currents of either the same or opposite polarity.

There is no connection between the analog signal

paths and GND. The analog signal paths consist of an

N-channel and P-channel MOSFET with their sources

and drains paralleled and their gates driven out of

phase with V+ and V- by the logic-level translators.

and GND power the internal logic and logic-level

translators, and set the input logic thresholds. The

logic-level translators convert the logic levels to

switched V+ and V- signals to drive the gates of the

analog switches. This drive signal is the only connec-

tion between the logic supplies and the analog sup-

plies.

Bipolar-Supply Operation

The MAX4588 operates with bipolar supplies between

±2.7V and ±6V. The V+ and V- supplies are not required

to be symmetrical, but their sum cannot exceed the

absolute maximum rating of 13.0V. Do not connect the

MAX4588 V+ pin to +3V and connect the logic-level

input pins to +5V logic-level signals. This level

exceeds the absolute maximum ratings, and may

cause damage to the part and/or external circuits.

CAUTION: The absolute maximum V+ to V- differen-

tial voltage is 13.0V. Typical “±6-Volt” or “12-Volt”

supplies with ±10% tolerances can be as high as

13.2V. This voltage can damage the MAX4588. Even

±5% tolerance supplies may have overshoot or

noise spikes that exceed 13.0V.

Single-Supply Operation

The MAX4588 operates from a single supply between

+2.7V and +12V when V- is connected to GND.

Observe all of the precautions listed in the

Bipolar-

Supply Operation

section. Note, however, that these

parts are optimized for ±5V operation, and AC and DC

characteristics are degraded significantly when operat-

ing at less than ±5V. As the overall supply voltage (V+

to V-) is reduced, switching speed, on-resistance, off-

isolation, and distortion are degraded (see

Typical

Operating Characteristics

Single-supply operation also limits signal levels and

interferes with grounded signals. When V- = 0, AC sig-

nals are limited to -0.3V. Voltages below -0.3V can be

clipped by the internal ESD-protection diodes, and the

parts can be damaged if excessive current flows.

Power Off

When power to the MAX4588 is off (i.e., V+ = 0 and V-

= 0), the

Absolute Maximum Ratings

still apply. This

means that none of the MAX4588 pins can exceed

±0.3V. Voltages beyond ±0.3V cause the internal ESD-

protection diodes to conduct, with potentially cata-

strophic consequences.

Power-Supply Sequencing

When applying power to the MAX4588, follow this

sequence: V+, V- (if biased to potential other than

ground), V

, then logic inputs. Apply signals on the

analog NO_ and COM_ pins any time after V+, V-, and

GND voltages are set. Turning on all pins simultaneous-

ly is acceptable only if the circuit design guarantees

concurrent power-up.

Low-Voltage, High-Isolation,

Dual 4-Channel RF/Video Multiplexer

14 ______________________________________________________________________________________

The power-down sequence is the opposite of the

power-up sequence. That is, the V

and logic inputs

must go to zero potential before (or simultaneously

with) the V- then V+ supplies. The

Absolute Maximum

Ratings

must always be observed in order to ensure

proper operation.

Grounding

DC Ground Considerations

Satisfactory high-frequency operation requires that

careful consideration be given to grounding. For most

applications, a ground plane is strongly recom-

mended, and all GND pins must connect to it with

solid copper. While the V+ and V- power-supply pins

are common to all switches in a given package, each

input is separated with ground pins that are not inter-

nally connected to each other. This contributes to the

overall high-frequency performance by reducing chan-

nel-to-channel crosstalk. All the GND pins have ESD

diodes to V+ and V-.

In systems that have separate digital and analog (sig-

nal) grounds, connect all GND pins to analog signal

ground. Preserving a good signal ground is much more

important than preserving a digital ground. Ground cur-

rent is only a few nanoamperes.

The digital inputs have voltage thresholds determined by

and GND (V- does not influence the logic-level thresh-

old). With +5V applied to V

, the threshold is about 1.6V,

ensuring compatibility with TTL- and CMOS-logic drivers.

AC Ground and Bypassing

A ground plane is mandatory for satisfactory high-

frequency operation. Prototyping using hand wiring or

wire-wrap boards is not recommended. Connect all

GND pins to the ground plane with solid copper. (The

GND pins extend the high-frequency ground through

the package wire-frame, into the silicon itself, thus

improving isolation.) Make the ground plane solid metal

underneath the device, without interruptions. There

should be no traces under the device itself. For DIP

packages, this applies to both sides of a two-sided

board. Failure to observe this has a minimal effect on

the “on” characteristics of the switch at high frequen-

cies, but will degrade the off-isolation and crosstalk.

When using the MAX4588’s SO package on PC boards

with a buried ground plane, connect each GND pin to the

ground plane with a separate via. Do not share this via

with any other ground path. Providing a ground via on

both sides of the SMT land further enhances the off-isola-

tion by lowering the parasitic inductance. The DIP pack-

age can have the through-holes directly tied to the buried

plane, or thermally relieved as required to meet manufac-

turability requirements. Again, do not use the through-

hole pads as the current path for any other components.

Bypass all V+ and V- pins to the ground plane with sur-

face-mount 0.01µF capacitors. Locate these capacitors

as close as possible to the pins on the same side of the

board as the device. Do not use feedthroughs or vias

for bypass capacitors. If board layout dictates that the

bypass capacitors are mounted on the opposite side of

the PC board, use short feedthroughs or vias, directly

under the V+ and V- pins. Use multiple vias if possible.

If V- is 0, connect it directly to the ground plane with

solid copper. Keep all traces short.

Signal Routing

Keep all signal leads as short as possible. Separate all

signal leads from each other, and keep them away from

any other traces that could induce interference.

Separating the signal traces with generously sized

ground wires also helps minimize interference. Routing

signals via coaxial cable, terminated as close to the

MAX4588 as possible, provides the highest isolation.

Board Layout

IC sockets degrade high-frequency performance and

should not be used if signal bandwidth exceeds 5MHz.

Surface-mount parts, having shorter internal lead

frames, provide the best high-frequency performance.

Keep all bypass capacitors close to the device, and

separate all signal leads with ground planes. Such

grounds tend to be wedge-shaped as they get closer to

the device. Use vias to connect the ground planes on

each side of the board, and place the vias in the apex of

the wedge-shaped grounds that separate signal leads.

Logic-level signal lead placement is not critical.

Impedance Matching

The MAX4588 is intended for use in 75Ω systems,

where the inputs are terminated external to the IC and

the COM terminals see an impedance of 600Ω or high-

er. The MAX4588 can operate in 50Ω and 75Ω systems

with terminations through the IC. However, variations in

and R

flatness cause nonlinearities.

Crosstalk and Off-Isolation

The graphs shown in

Typical Operating Characteristics

for crosstalk and off-isolation are taken on adjacent

channels. The adjacent channel is the worst-case con-

dition. For example, NO1 has the worst off-isolation to

COM1 due to their proximity. Furthermore, NO1 has the

most crosstalk to NO2, and the least crosstalk to NO4.

Choosing channels wisely necessitates separating the

most sensitive channels from the most offensive.

Conversely, the above information also applies to the

NO5–NO8 inputs to the COM2 pin.

MAX4588

Low-Voltage, High-Isolation,

Dual 4-Channel RF/Video Multiplexer

______________________________________________________________________________________ 15

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

MAX4588CPI

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Video Switch ICs Low-Voltage, High-Isolation, Dual 4-Channel RF Video Multiplexer

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