AD8315ARM datasheet(14/20 Pages) AD

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REV. B

AD8315

–14–

Figure 9 shows the relationship between VSET and output

power (POUT) at 0.9 GHz . The overall gain control function is

linear in dB for a dynamic range of over 40 dB. Note that for VSET

voltages below 300 mV, the output power drops off steeply as

VAPC drops toward its minimum level of 250 mV.

VSET – V

–10

–20

–30

–40

0.2

0.4

0.6

0.8

1.0

1.2

1.4

–1

–2

–3

–4

1.6

+85 C

+25 C

–30 C

+85 C

+25 C

–30 C

Figure 9. POUT vs. VSET at 0.9 GHz for Dual Mode Handset

Power Amplifier Application; –30

∞C, +25∞C, and +85∞C

Enable and Power-On

The AD8315 may be disabled by pulling the ENBL pin to ground.

This reduces the supply current from its nominal level of 7.4 mA

to 4

mA. The logic threshold for turning on the device is at 1.5 V

with 2.7 V supply voltage. A plot of the enable glitch is shown in

TPC 20. Alternatively, the device can be completely disabled by

pulling the supply voltage to ground. To minimize glitch in this

mode, ENBL and VPOS should be tied together. If VPOS is

applied before the device is enabled, a narrow 750 mV glitch will

result (see TPC 27).

In both situations, the voltage on VSET should be kept below

200 mV during power-on and power-off to prevent any unwanted

transients on VAPC.

Input Coupling Options

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

the low frequency input impedance of 2.8 k

W, give a high-pass

input corner frequency of approximately 16 MHz. This sets the

minimum operating frequency. Figure 10 shows three options for

input coupling. A broadband resistive match can be implemented

by connecting a shunt resistor to ground at RFIN (Figure 10a).

This 52.3

W resistor (other values can also be used to select differ-

ent overall input impedances) combines with the input impedance

of the AD8315 to give a broadband input impedance of 50

While the input resistance and capacitance (CIN and RIN) of the

AD8315 will vary from device to device by approximately

±20%,

and over frequency (TPC 9), the dominance of the external shunt

resistor means that the variation in the overall input impedance

will be close to the tolerance of the external resistor. This method

of matching is most useful in wideband applications or in multi-

band systems where there is more than one operating frequency.

A reactive match can also be implemented as shown in Figure 10b.

This is not recommended at low frequencies as device tolerances

will dramatically vary the quality of the match because of the large

input resistance. For low frequencies, Option 10a or Option 10c

is recommended.

In Figure 10b, 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 will be 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.

RFIN

AD8315

RIN

CIN

RSHUNT

52.3

a. Broadband Resistive

RFIN

AD8315

RIN

CIN

b. Narrow Band Reactive

RFIN

AD8315

RIN

CIN

RATTN

ANTENNA

STRIPLINE

c. Series Attenuation

Figure 10. Input Coupling Options

Figure 10c 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.

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