AD8315ARMZ datasheet(16/24 Pages) AD

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AD8315

Rev. C | Page 16 of 24

–7

V2, P2

V1, P1

VAPC (V)

0.5

1.0

1.5

2.0

2.5

Figure 35. Typical Power-Control Curve

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 data sheet for more information about the effect of

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 = V2/R) or −6 dBm. A dBV level can be converted to dBm in

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 VSET vs. PIN function would cross the

baseline if that relationship were continuous for all values of

VIN. This is never the case for a practical log amp; the intercept

(so many dBV) refers to the value obtained by the minimum

error straight line fit to the actual graph of VSET vs. PIN (more

generally, VIN). Where the modulation is complex, as in CDMA,

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.

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

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).

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 36. Basic Connections

RFIN

VSET

AD8315

VAPC

FLTR

DAC

RFIN

ATTENUATOR

52.3Ω

POWER

AMP

DIRECTIONAL

COUPLER

GAIN

CONTROL

VOLTAGE

CFLT

Figure 37. Typical Application

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