AD8316 datasheet(12/20 Pages) AD

12 / 20 page

REV. C

–12–

AD8316

The value of VOUT is of interest, although it is a dependent param-

eter inside the loop. It depends on the characteristics of the

power amplifier, and the value of the carrier amplitude VCW.

Using the control values derived above, that is, GO = 0.316 and

VGSC = 1 V, and assuming that the applied power is fixed at

–7 dBm (so that VCW = 100 mV rms), Equation 11 shows

max

kG V

OUT

SET

GSC

SLP

() = ()

−

()

=×

()

−

××









=−

log

139

1 044

0 0316

0 316

01 199

(15)

min

kG V

OUT

SET

GSC

SLP

() = ()

−

()

=×

()

−

××









=−=

log

031 1 044

0 0316

0 316

01 199

(16)

Both results are consistent with the assumptions made about the

amplifier control function. Note that the second term is inde-

pendent of the delivered power and is a fixed function of the

drive power.

Finally, the loop time constant for these parameters, using an

illustrative value of 2 nF for the filter capacitor CFLT, evaluates to

snF

GSC

SLP

()

()×× () =

10 44

3 072

13 95

µµ

(17)

Practical Loop

At the present time, power amplifiers, or VGAs preceding such

amplifiers, do not provide an exponential gain characteristic. It

follows that the loop dynamics (the effective time constant) will

vary with the setpoint, since the exponential function is unique

in providing constant dynamics. The procedure must therefore

be as follows. Beginning with the curve usually provided for the

power output versus APC voltage, draw a tangent at the point

on this curve where the slope is highest (see Figure 5). Using

this line, calculate the effective minimum value of the variable

VGSC, and use it in Equation 17 to determine the time constant.

(Note that the minimum in VGSC corresponds to the maximum

rate of change in the output power versus VOUT.)

For example, suppose it is found that, for a given drive power,

the amplifier generates an output power of P1 at VOUT = V1, and

P2 at VOUT = V2. Then, it is readily shown that

GSC =

() ()

–/

–

(18)

This should be used to calculate the filter capacitance. The

response time at high and low power levels (on the “shoulders”

of the curve shown in Figure 5) will be slower. Note also that it

is sometimes useful to add a zero in the closed-loop response by

placing a resistor in series with CFLT.

A Note About Power Equivalency

Users of the AD8316 must understand that log amps funda-

mentally do not respond to power. For this reason, dBV

(decibels above 1 V rms) are included in addition to the com-

monly used metric 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 wave-

form on logarithmic intercept. 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

2/R),

or –6 dBm. A dBV level may be converted to dBm in the special

case of a 50

Ω system and a sinusoidal signal simply by adding

13 dB. 0 dBV is then, and only then, equivalent to 13 dBm.

–67dBm

–47dBm

–27dBm

–7dBm

+13dBm

33dBm

23dBm

13dBm

3dBm

–7dBm

PRF

VOUT1

V2, P2

V1, P1

Figure 5. Typical Power Control Curve

Therefore, the external termination added ahead of the AD8316

determines the effective power scaling. This often takes the

form of a simple resistor (52.3

Ω will provide a net 50 Ω input),

but more elaborate matching networks may be used. The choice

of impedance determines the logarithmic intercept, that is, the

input power for which the VSET versus 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 versus PIN (more

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

the calibration of the power response needs to be adjusted; the

intercept will remain 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 decade

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, because of the declining

gain of the amplifier stages and other effects in the detector

cells (see TPC 13).

Basic Connections

Figure 6 shows the basic connections for operating the AD8316

and Figure 7 shows a block diagram of a typical application.

The AD8316 is typically used in the RF power control loop of

dual mode and trimode mobile handsets where there is more

than one RF power control line.

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