APPLICATION INFORMATION

WIDEBAND MULTIPLEXER OPERATION

75W

+5V

-5V

OPA4872

75W

75W

75W

75W

To75

Load

W

523W

523W

IN0

IN1

IN2

IN3

V

IN0

V

IN1

V

IN2

V

IN3

A0

A1

SD

EN

V

OUT

0.1 F

m

Optional

0.01 F

m

6.8 F

m

+

0.1 F

m

6.8 F

m

+

OPA4872-EP

input of a current feedback amplifier. Depending on

the logic applied to channel control pins A0 and A1,

one switch is on at all times. Figure 27 represents the

The OPA4872 gives a new level of performance in

OPA4872 in this configuration. The truth table for

wideband

multiplexers.

Figure

27

shows

the

channel selection is shown in Table 1.

dc-coupled, gain of +2 V/V, dual power-supply circuit

used

as

the

basis

of

the

±5-V

Electrical

Table 1. TRUTH TABLE

Characteristics and Typical Characteristic curves. For

A0

A1

EN

SD

test purposes, the input impedance is set to 75

Ω with

0

0

0

0

IN0

a resistor to ground and the output impedance is set

to 75

Ω with a series output resistor. Voltage swings

1

0

0

0

IN1

reported in the specifications are taken directly at the

0

1

0

0

IN2

input and output pins while load powers (in dBm) are

1

1

0

0

IN3

defined at a matched 75-

Ω load. For the circuit of

X

X

1

0

Figure 27, the total effective load will be 150

Ω ||

X

X

X

1

1046

Ω = 131 Ω. Logic pins A0 and A1 control which

of the four inputs is selected while EN and SD allow

The OPA4872 is in disable mode, with a quiescent

for power reduction. One optional component is

current of 3.4mA typical, when the EN pin is set to

included in Figure 27. In addition to the usual

0V. After being placed in disable mode, the OPA4872

power-supply decoupling capacitors to ground, a

is fully enabled in 6ns. For further power savings, the

0.01-

µF capacitor is included between the two

SD pin can be used. Setting the SD pin to 5V places

power-supply pins. In practical printed circuit board

the device in shutdown mode with a standing

(PCB) layouts, this optional added capacitor typically

quiescent current of 1.1 mA. Note that in this

improves the 2nd-harmonic distortion performance by

shutdown mode, the OPA4872 requires 15ns to be

3 dB to 6 dB for bipolar supply operation.

fully powered again. The truth table for disable and

Even though the internal architecture of the OPA4872

shutdown modes can be found in Table 1.

includes current steering, it is advantageous to look

at it as four switches looking into the noninverting

Figure 27. DC-Coupled, G = +2V/V Bipolar Specification and Test Circuit (Channel 0 Selected)

Copyright © 2008, Texas Instruments Incorporated

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