AD8315 Analog Devices, AD8315 Datasheet - Page 16

• Manufacturer Part Number: AD8315
• Description: 50 dB GSM PA Controller
• Manufacturer: Analog Devices

AD8315
A NOTE ABOUT POWER EQUIVALENCY
In using the AD8315, it must be understood that log amps do
not fundamentally respond to power. It is for this reason that
dBV (decibels above 1 V rms) are used rather than the commonly
used metric of dBm. The dBV scaling is fixed, independent of
termination impedance, while the corresponding power level is
not. For example, 224 mV rms is always −13 dBV (with one
further condition of an assumed sinusoidal waveform; see the
AD640
waveform on logarithmic intercept), and this corresponds to a
power of 0 dBm when the net impedance at the input is 50 Ω.
When this impedance is altered to 200 Ω, however, the same
voltage corresponds to a power level that is four times smaller
(P = V
the special case of a 50 Ω system and a sinusoidal signal by
simply adding 13 dB (0 dBV is then, and only then, equivalent
to 13 dBm).
Therefore, the external termination added ahead of the AD8315
determines the effective power scaling. This often takes the
form of a simple resistor (52.3 Ω provides a net 50 Ω input), but
more elaborate matching networks can be used. The choice of
impedance determines the logarithmic intercept, that is, the
input power for which the V
baseline if that relationship were continuous for all values of
V
(so many dBV) refers to the value obtained by the minimum
error straight line fit to the actual graph of V
generally, V
the calibration of the power response needs to be adjusted; the
intercept remains stable for any given arbitrary waveform.
When a true power (waveform independent) response is
needed, a mean-responding detector, such as the AD8361,
should be considered.
IN
. This is never the case for a practical log amp; the intercept
33
23
13
–7
2
3
/R) or −6 dBm. A dBV level can be converted to dBm in
data sheet for more information about the effect of
0
IN
). Where the modulation is complex, as in CDMA,
Figure 35. Typical Power-Control Curve
0.5
V
1
, P
1
1.0
SET
V
APC
vs. P
(V)
IN
1.5
function would cross the
V
2
, P
2
SET
2.0
vs. P
IN
(more
2.5
Rev. C | Page 16 of 24
The logarithmic slope, V
by which the setpoint voltage needs to be changed for each
decibel of input change (voltage or power), is, in principle,
independent of waveform or termination impedance. In
practice, it usually falls off somewhat at higher frequencies,
due to the declining gain of the amplifier stages and other
effects in the detector cells (see Figure 15).
BASIC CONNECTIONS
Figure 36 shows the basic connections for operating the
AD8315, and Figure 37 shows a block diagram of a typical
application. The AD8315 is typically used in the RF power
control loop of a mobile handset.
A supply voltage of 2.7 V to 5.5 V is required for the AD8315.
The supply to the VPOS pin should be decoupled with a low
inductance 0.1 μF surface-mount ceramic capacitor, close to the
device. The AD8315 has an internal input coupling capacitor.
This negates the need for external ac coupling. This capacitor,
along with the low frequency input impedance of the device of
approximately 2.8 kΩ, sets the minimum usable input frequency to
around 0.016 GHz. A broadband 50 Ω input match is achieved
in this example by connecting a 52.3 Ω resistor between RFIN
and ground. A plot of input impedance vs. frequency is shown
in Figure 11. Other coupling methods are also possible (see
Input Coupling Options section).
DIRECTIONAL
ATTENUATOR
RFIN
V
+V
SET
COUPLER
S
C
FLT
52.3Ω
R1
52.3Ω
1
2
3
4
Figure 37. Typical Application
NC = NO CONNECT
Figure 36. Basic Connections
RFIN
ENBL
FLTR
VSET
AD8315
SLP
RFIN
in Equation 1, which is the amount
COMM
AD8315
VPOS
VAPC
POWER
VAPC
FLTR
NC
AMP
GAIN
CONTROL
VOLTAGE
C
8
7
6
5
0.1µF
FLT
VSET
C1
+V
+V
(2.7V TO 5.5V)
S
APC
DAC
RFIN