AD8315
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.
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.
40
30
4
+85 C
3
+25 C
20
2
+85 C
+25 C
10
1
–30 C
0
0
–10
–20
–1
–2
AD8315
–30 C
C
C
RFIN
R
SHUNT
52.3ꢀ
C
R
IN
–3
–4
–30
–40
IN
0
0.2
0.4
0.6
0.8
–V
1.0
1.2
1.4
1.6
V
SET
Figure 9. POUT vs. VSET at 0.9 GHz for Dual Mode Handset
a. Broadband Resistive
Power Amplifier Application; –30∞C, +25∞C, and +85∞C
AD8315
Enable and Power-On
C
C
X1
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).
RFIN
C
R
X2
IN
IN
b. Narrow Band Reactive
ANTENNA
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.
AD8315
C
C
RFIN
STRIPLINE
PA
R
ATTN
Input Coupling Options
C
R
IN
IN
The internal 5 pF coupling capacitor of the AD8315, along with
the low frequency input impedance of 2.8 kW, 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 W.
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.
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.
–14–
REV. B
ADI [ ADI ]
