Data Sheet

AD8315

Rev. D | Page 15 of 22

parameter, inside the loop. It depends on the characteristics of

Equation 11

= 3.0 − 0.5 = 2.5 V

(15)

= 0.5 − 0.5 = 0

(16)

both of which results are consistent with the assumptions made

about the amplifier control function. Note that the second term

is independent of the delivered power and a fixed function of

the drive power.

RF PA

DIRECTIONAL COUPLER

RF DRIVE: UP

TO 2.5GHz

AD8315

RESPONSE-SHAPING

OF OVERALL CONTROL-

LOOP (EXTERNAL CAP)

Figure 35. Idealized Control Loop for Analysis

Finally, using the loop time constant for these parameters and

= (1/0.48)3.07 μs × 2 (nF) = 12.8 μs

(17)

PRACTICAL LOOP

At 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)

varies with the setpoint because 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 vs. the APC voltage, draw a tangent at the

point on this curve where the slope is highest (see Figure 36).

Using this line, calculate the effective minimum value of the

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

Then, it is readily shown that

(18)

This must 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 36) is slower. Note also that it is

sometimes useful to add a 0 in the closed-loop response by

this, see the Transient Response section.

33

0

23

13

3

–7

0.5

1.0

1.5

2.0

2.5

Figure 36. 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

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

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