AD8315ARMZ datasheet(16/22 Pages) AD

16 / 22 page

AD8315

Data Sheet

Rev. D | Page 16 of 22

Where the modulation is complex, as in CDMA, the calibration

of the power response must 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, must be considered.

The logarithmic slope, VSLP in Equation 1, which is the amount

by which the setpoint voltage must 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 16).

BASIC CONNECTIONS

Figure 37 shows the basic connections for operating the

AD8315, and Figure 38 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 must 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 12. Other coupling methods are also possible (see

Input Coupling Options section).

NC = NO CONNECT

RFIN

ENBL

VSET

VPOS

VAPC

COMM

FLTR

AD8315

RFIN

(2.7V TO 5.5V)

0.1µF

52.3Ω

CFLT

VSET

+VS

+VAPC

Figure 37. Basic Connections

RFIN

VSET

AD8315

VAPC

FLTR

DAC

RFIN

ATTENUATOR

52.3Ω

POWER

AMP

DIRECTIONAL

COUPLER

GAIN

CONTROL

VOLTAGE

CFLT

Figure 38. Typical Application

In a power control loop, the AD8315 provides both the detector

and controller functions. A sample of the power amplifier (PA)

output power is coupled to the RF input of the AD8315, usually

via a directional coupler. In dual-mode applications, where

there are two PAs and two directional couplers, the outputs of

the directional couplers can be passively combined (both PAs

will never be turned on simultaneously) before being applied to

the AD8315.

A setpoint voltage is applied to VSET from the controlling

source (generally, this is a DAC). Any imbalance between the

RF input level and the level corresponding to the setpoint voltage is

corrected by the AD8315 VAPC output that drives the gain control

terminal of the PA. This restores a balance between the actual

power level sensed at the input of the AD8315 and the value

determined by the setpoint. This assumes that the gain control

sense of the variable gain element is positive, that is, an increasing

voltage from VAPC tends to increase gain.

VAPC can swing from 250 mV to within 100 mV of the supply

rail and can source up to 6 mA. If the control input of the PA

must source current, a suitable load resistor can be connected

between VAPC and COMM. The output swing and current

sourcing capability of VAPC is shown in Figure 22.

RANGE ON VSET AND RFIN

The relationship between the RF input level and the setpoint

voltage follows from the nominal transfer function of the device

(see Figure 5, Figure 6, Figure 8, and Figure 9). At 0.9 GHz, for

example, a voltage of 1 V on VSET indicates a demand for −30 dBV

(−17 dBm, re 50 Ω) at RFIN. The corresponding power level at the

output of the power amplifier is greater than this amount due to

the attenuation through the directional coupler.

For setpoint voltages of less than approximately 250 mV, VAPC

remains unconditionally at the minimum level of approximately

250 mV. This feature can prevent any spurious emissions during

power-up and power-down phases.

Above 250 mV, VSET has a linear control range up to 1.4 V,

corresponding to a dynamic range of 50 dB. This results in a

slope of 23 mV/dB or approximately 43.5 dB/V.

TRANSIENT RESPONSE

The time domain response of power amplifier control loops,

using any kind of controller, is only partially determined by the

choice of filter, which, in the case of the AD8315, has a true

integrator form 1/sT, as shown in Equation 7, with a time constant

given by Equation 8. The large signal step response is also strongly

dependent on the form of the gain-control law. Nevertheless, some

simple rules can be applied. When the filter capacitor CFLT is very

large, it dominates the time domain response, but the incremental

bandwidth of this loop still varies as VAPC traverses the nonlinear

gain-control function of the PA, as shown in Figure 36.

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