AD8362 datasheet(19/32 Pages) AD

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AD8362

Rev. D | Page 19 of 32

OPERATION IN RF MEASUREMENT MODE

BASIC CONNECTIONS

Basic connections for operating the AD8362 in measurement

mode are shown in Figure 47. While the AD8362 requires a

single supply of nominally 5 V, its performance is essentially

unaffected by variations of up to ±10%.

The supply is connected to the VPOS pin using the decoupling

network also displayed in Figure 47. The capacitors used in this

network must provide a low impedance over the full frequency

range of the input and should be placed as close as possible to

the VPOS pin. Two different capacitors are used in parallel to

reduce the overall impedance because these have different reso-

nant frequencies. The measurement accuracy is not critically

dependent on supply decoupling because the high frequency

signal path is confined to the relevant input pins. Lead lengths

from both DECL pins to ground and from INHI/INLO to the

input coupling capacitors should be as short as possible. All

COMM pins should also connect directly to the ground plane.

To place the device in measurement mode, connect VOUT to

VSET and connect VTGT directly to VREF.

DEVICE DISABLE

The AD8362 is disabled by a logic high on the PWDN pin,

which can be directly grounded for continuous operation.

When enabled, the supply current is nominally 20 mA and

essentially independent of supply voltage and input signal

strength. When powered down by a logic low on PWDN,

the supply current is reduced to 230 μA.

RECOMMENDED INPUT COUPLING

The full dynamic range of the AD8362, particularly at very

high frequencies (above 500 MHz), is realized only when the

input is presented to it in differential (balanced) form. In Figure 47,

a transmission line balun is used at the input. Having a 1:4

impedance ratio (1:2 turns ratio), the 200 Ω differential input

resistance of the AD8362 becomes 50 Ω at the input to the balun.

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

1nF

1000pF

1nF

ETC1.6-4-2-3

C10

1000pF

SIGNAL

INPUT

Z = 50Ω

100pF

0.1µF

VOUT

1nF

0.1µF

5V @ 24mA

1:4 Z-RATIO

Figure 47. Basic Connections for RF Power Measurement

The balun outputs must be ac-coupled to the input of the

AD8362. The balun used in this example (M/A-COM ETC

1.6-4-2-3) is specified for operation from 0.5 GHz to 2.5 GHz.

If a center-tapped, flux-coupled transformer is used, connect

the center tap to the DECL pins, which are biased to the same

potential as the inputs (~3.6 V).

At lower frequencies where impedance matching is not neces-

sary, the AD8362 can be driven from a low impedance differential

source, remembering the inputs must be ac-coupled.

Choosing Input Coupling Capacitors

As noted, the inputs must be ac-coupled. The input coupling

capacitors combine with the 200 Ω input impedance to create

an input high pass corner frequency equal to

fHP = 1/(200 × π × CC)

(12)

Typically, fHP should be set to at least one tenth the lowest input

frequency of interest.

Single-Ended Input Drive

As previously noted, the input stages of the AD8362 are optimally

driven from a fully balanced source, which should be provided

wherever possible. In many cases, unbalanced sources can be

applied directly to one or the other of the two input pins. The

chief disadvantage of this driving method is a 10 dB to 15 dB

reduction in dynamic range at frequencies above 500 MHz.

Figure 48 illustrates one of many ways of coupling the signal

source to the AD8362. Because the input pins are biased to

about 3.6 V (for VS = 5 V), dc-blocking capacitors are required

when driving from a grounded source. For signal frequencies

>5 MHz, a value of 1 nF is adequate. While either INHI or

INLO can be used, INHI is chosen here.

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

1nF

0.01µF

1nF

RF INPUT

100Ω

Figure 48. Input Coupling from a Single-Ended 50 Ω Source

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