Techniques to Achieve High Isolation with GaAs MMIC Switch

Chips

Rev. V4

Application Note

M515

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1

Visit www.macomtech.com for additional data sheets and product information.

M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make

changes to the product(s) or information contained herein without notice.

Mounting Techniques

1.

Mount the base of the chip directly on the

ground plane (i.e. the metal floor of the

package) and use short connections from the

ground pads of the MMIC to the ground plane.

2.

Use short ribbon bonds (0.005 to 0.010” wide)

instead of wirebonds in the connection of the

ground pads to ground.

3.

Elevate the ground plane to be at the same

level as the ground pads on the chip surface.

This can be done by:

A. Using ground plane pedestals next to the chip,

as shown.

B. Depressing the chip into a channel in the

ground plane, as shown.

Isolation performance of a GaAs MMIC switch

can be degraded by parasitic effects introduced

by the circuit in which the MMIC is used. Two

primary causes of isolation degradation are

excessive

ground

path

inductances

and

crosstalk between RF paths (external to the

MMIC).

This note will focus on techniques to

reduce ground path inductances.

The achievable isolation of a GaAs MMIC switch

is a function of how low an inductance one can

achieve between the ground pad of the MMIC

and the ground plane within a particular circuit.

The lower the inductance, of course, the higher

the isolation that can be achieved.

For

example,

the

curves

shown

for

the

MASW6010G* chip in the catalog were obtained

using coplanar RF probes directly on the chip

Because this is a coplanar probe configuration

(which also includes the ground plane), there is

virtually

no

ground

inductance

in

the

measurement. Thus, the isolation obtained in the

data sheet is for a very small ground inductance

that exists and should be considered optimum.

However,

the

isolation

curves

for

the

MASW6020G* chip in the catalog were obtained

with the chip mounted in a package having a flat

ground plane on the floor of the pads to the

ground plane. Thus, the isolation obtained in the

data sheet is when a finite inductance exists in

the ground path.

In practice when a chip is mounted into a circuit,

there will always be some finite inductance which

can degrade the isolation performance. Several

precautions can be taken to improve the isolation

within a particular circuit. (see box)

The technique used by M/A-COM in most of the

packaged MMIC switch products is that of #1

above.

Catalog performance of these products

can be used as an indicator of what can be

expected if technique #1 is used.

However,

improvements can be expected in isolation

performance if techniques 2 or 3 are used.

This note briefly described techniques to obtain

the maximum possible isolation when using a

GaAs MMIC switch chips. Several options were

outlined which involved varying degrees of

complexity. Crosstalk, not addressed here, can

also degrade isolation, and must minimized to

obtain overall performance.

* Part number MASW6010G supersedes SW-200 and

MASW6020G supersedes SW-210.

Pedestal

Ribbon Bond

Ground Plane

Chip

Ground Plane

Ribbon Bond

Chip