AD8367 datasheet(14/24 Pages) AD

14 / 24 page

AD8367

Rev. A | Page 14 of 24

APPLICATIONS

The AD8367 can be configured either as a VGA whose gain

is controlled externally through the GAIN pin or as an AGC

amplifier, using a supply voltage of 2.7 V to 5.5 V. The supply

to the VPSO and VPSI pins should be decoupled using a low

inductance, 0.1 μF surface-mount, ceramic capacitor as close

as possible to the device. Additional supply decoupling can

be provided by a small series resistor. A 10 nF capacitor from

Pin DECL to Pin OCOM is recommended to decouple the

output reference voltage.

INPUT AND OUTPUT MATCHING

The AD8367 is designed to operate in a 200 Ω impedance

system. The output amplifier is a low output impedance voltage

buffer with a 50 Ω damping resistor to desensitize it from load

reactance and parasitics. The quoted performance includes the

voltage division between the 50 Ω resistor and the 200 Ω load.

The AD8367 can be reactively matched to an impedance other

than 200 Ω by using traditional step-up and step down

matching networks or high quality transformers. Table 4 lists

the 50 Ω S-parameters for the AD8367 at a VGAIN = 750 mV.

Figure 32 illustrates an example where the AD8367 is matched

to 50 Ω at 140 MHz. As shown in the Smith Chart, the input

matching network shifts the input impedance from ZIN to 50 Ω

with an insertion loss of <2 dB over a 5 MHz bandwidth. For

the output network, the 50 Ω load is made to present 200 Ω to

the AD8367 output. Table 5 provides the component values

required for 50 Ω matching at several frequencies of interest.

When added loss and noise can be tolerated, a resistive pad can

be used to provide broadband, near-matched impedances at the

device terminals and the terminations.

Minimum-loss, L-pad networks are used on the evaluation

board (see Figure 45) to allow easy interfacing to standard

50 Ω test equipment. Each pad introduces an 11.5 dB power

loss (5.5 dB voltage loss).

0.3333

–0.3333

–1

–3

0.3333

ZIN

RSOURCE

SERIES L

SHUNT C

ZIN

XSIN

100nH

ZLOAD

ZOUT

XSOUT

13pF

CAC

0.1

μF

RSOURCE

RLOAD

XPOUT

120nH

XPIN

8.2pF

AD8367

fC = 140MHz, ZIN = 197 – j34.2, RSOURCE = 50Ω

Figure 32. Reactive Matching Example for f = 140 MHz

Table 4. S-Parameters for 200 Ω System for VS = 5 V and VGAIN = 0.75 V

Frequency (MHz)

S11

S21

S12

S22

0.04

∠ −43.8°

41.1

∠ 178.8°

0.0003

∠ 76.1°

0.56

∠ −179.3°

0.09

∠ −81.5°

43.6

∠ 163.4°

0.0002

∠ 63.7°

0.55

∠ +176.1°

140

0.17

∠ −103.4°

48.0

∠ 141.4°

0.0009

∠ 130.8°

0.56

∠ +170.2°

190

0.21

∠ −111.7°

47.5

∠ 124.0°

0.0017

∠ 96.8°

0.54

∠ +166.5°

240

0.26

∠ −103.8°

48.3

∠ 107.6°

0.0018

∠ 113.5°

0.48

∠ +164.6°

Table 5. Reactive Matching Components for a 50 Ω System where RSOURCE = 50 Ω, RLOAD = 50 Ω

Frequency (MHz)

XSIN

XPIN (pF)

XSOUT (pF)

XPOUT

1.5 μH

120

180

1.8 μH

220 nH

270 nH

140

100 nH

8.2

120 nH

190

82 nH

2.7

100 nH

240

68 nH

1.5

82 nH

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