AD8315ARMZ datasheet(19/22 Pages) AD

19 / 22 page

Data Sheet

AD8315

Rev. D | Page 19 of 22

INPUT COUPLING OPTIONS

The internal 5 pF coupling capacitor of the AD8315, along with

the low frequency input impedance of 2.8 kΩ, give a high-pass

input corner frequency of approximately 16 MHz. This sets the

minimum operating frequency. Figure 41, Figure 42, and Figure 43

show three options for input coupling. A broadband resistive match

can be implemented by connecting a shunt resistor to ground at

RFIN (see Figure 41). This 52.3 Ω resistor (other values can also

be used to select different overall input impedances) combines with

the input impedance of the AD8315 to give a broadband input

impedance of 50 Ω. While the input resistance and capacitance

(CIN and RIN) of the AD8315 varies from device to device by

approximately ±20%, and over frequency (see Figure 12), the

dominance of the external shunt resistor means that the variation

in the overall input impedance is close to the tolerance of the

external resistor. This method of matching is most useful in

wideband applications or in multiband systems where there is

more than one operating frequency.

A reactive match can also be implemented as shown in

Figure 42. This is not recommended at low frequencies as

device tolerances dramatically vary the quality of the match

because of the large input resistance. For low frequencies,

Figure 41 or Figure 43 is recommended.

In Figure 42, the matching components are drawn as generic

reactances. Depending on the frequency, the input impedance

and the availability of standard value components, either a

capacitor or an inductor is used. As in the previous case, the

input impedance at a particular frequency is plotted on a Smith

Chart and matching components are chosen (shunt or series L,

shunt or series C) to move the impedance to the center of the chart.

AD8315

RFIN

RSHUNT

52.3V

RIN

CIN

Figure 41. Broadband Resistive Input Coupling Option

AD8315

RFIN

RIN

CIN

Figure 42. Narrow-Band Reactive Input Coupling Option

ANTENNA

STRIPLINE

AD8315

RFIN

RIN

CIN

RATTN

Figure 43. Series Attention Input Coupling Option

Figure 43 shows a third method for coupling the input

signal into the AD8315. A series resistor, connected to the RF

source, combines with the input impedance of the AD8315 to

resistively divide the input signal being applied to the input. This

has the advantage of very little power being tapped off in RF

power transmission applications.

USING THE CHIP SCALE PACKAGE

On the underside of the chip scale package, there is an exposed

paddle. This paddle is internally connected to the chip ground.

There is no thermal requirement to solder the paddle down to the

printed circuit board ground plane. However, soldering down

the paddle has been shown to increase the stability over frequency

of the AD8315 ACP response at low input power levels (that is,

at around −45 dBm) in the DCS and PCS bands.

EVALUATION BOARD

Figure 44 shows the schematic of the AD8315 MSOP evaluation

board. The layout and silkscreen of the component side are shown

in Figure 45 and Figure 46. An evaluation board is also available

for the LFCSP package (see the Ordering Guide for exact device

numbers). Apart from the slightly smaller device footprint, the

LFCSP evaluation board is identical to the MSOP board. The

board is powered by a single supply in the 2.7 V to 5.5 V range.

The power supply is decoupled by a single 0.1 μF capacitor.

Table 5 details the various configuration options of the

evaluation board.

0.1µF

TP1

VPOS

0Ω

(OPEN)

TP2

RFIN

ENBL

VSET

FLTR

VPOS

VAPC

COMM

AD8315

0Ω

VPOS

SW1

RFIN

VSET

(OPEN)

LK1

LK2

NC = NO CONNECT

VPOS

0.1µF

10kΩ

0.1µF

16.2kΩ

17.8kΩ

10kΩ

AD8031

52.3Ω

VAPC

Figure 44. Evaluation Board Schematic (MSOP)

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