AD8314

–11–

REV. 0

The relationship between the input level and the setpoint voltage

Input Amplitude, see Figure 1). For example, a voltage of 1 V

on VSET is demanding a power level of 0 dBm at RFIN. The cor-

responding power level at the output of the power amplifier will be

greater than this amount due to the attenuation through the direc-

tional coupler.

When connected in a PA control loop, as shown in Figure 31,

now the effective loop response time is a much more complicated

function of the PA’s gain-control characteristics, which are very

nonlinear. A complete solution requires specific knowledge of

the power amplifier.

The transient response of this control loop is determined by the

ditionally stable (by virtue of the “dominant pole” generated

by this capacitor), but the response will be sluggish. The minimum

value ensuring stability should be used, requiring full attention

to the particulars of the power amplifier control function. Because

this is invariably nonlinear, the choice must be made for the

worst-case condition, which usually corresponds to the smallest

output from the PA, where the gain function is steepest. In practice,

an improvement in loop dynamics can often be achieved by adding

Power-On and Enable Glitch

As already mentioned, the AD8314 can be put into a low power

mode by pulling the ENBL pin to ground. This reduces the quies-

cent current from 4.5 mA to 20

µA. Alternatively, the supply can

be turned off completely to eliminate the quiescent current. Figures

13 and 23 show the behavior of the V_DN output under these

two conditions (in Figure 23, ENBL is tied to VPOS). The glitch

that results in both cases can be reduced by loading the V_DN

output.

Input Coupling Options

The internal 5 pF coupling capacitor of the AD8314, along with

the low frequency input impedance of 2 k

Ω give a high-pass input

corner frequency of approximately 16 MHz. This sets the mini-

mum operating frequency. Figure 32 shows three options for

input coupling. A broadband resistive match can be implemented

by connecting a shunt resistor to ground at RFIN (Figure 32a).

This 52.3

Ω resistor (other values can also be used to select dif-

ferent overall input impedances) resistor combines with the

input impedance of the AD8314 (3 k

Ω 2 pF) to give a broad-

band input impedance of 50

Ω. While the input resistance and

± 20% from

device to device, the dominance of the external shunt resistor

means that the variation in the overall input impedance will

be close to the tolerance of the external resistor.

At frequencies above 2 GHz, the input impedance drops below

250

Ω (see Figure 9), so it is appropriate to use a larger value of

shunt resistor. This value is calculated by plotting the input

impedance (resistance and capacitance) on a Smith Chart and

choosing the best value of shunt resistor to bring the input imped-

ance closest to the center of the chart. At 2.5 GHz, a shunt

resistor of 165

Ω is recommended.

A reactive match can also be implemented as shown in Figure

32b. This is not recommended at low frequencies as device toler-

ances will dramatically vary the quality of the match because of

the large input resistance. For low frequencies, Option a or

Option c (see below) are recommended.

In Figure 32b, the matching components are drawn as general

reactances. Depending on the frequency, the input impedance at

that frequency and the availability of standard value components,

either a capacitor or an inductor will be used. As in the previous

case, the input impedance at a particular frequency is plotted on

a Smith Chart and matching components are chosen (shunt

or series L, shunt or series C) to move the impedance to the

center of the chart. Table II gives standard component values

for some popular frequencies. Matching components for other

frequencies can be calculated using the input resistance and

reactance data over frequency which is given in Figure 9. Note

that the reactance is plotted as though it appears in parallel with

the input impedance (which it does because the reactance is prima-

rily due to input capacitance).

The impedance matching characteristics of a reactive matching

network provide voltage gain ahead of the AD8314; this increases

the device sensitivity (see Table II). The voltage gain is calculated

using the equation:

Voltage Gain

R

R

2

1

10

log

where R2 is the input impedance of the AD8314 and R1 is the

source impedance to which the AD8314 is being matched. Note

that this gain will only be achieved for a perfect match. Component

tolerances and the use of standard values will tend to reduce

the gain.

52.3

AD8314

50

50

SOURCE

RFIN

a. Broadband Resistive

50

SOURCE

AD8314

50

RFIN

X2

X1

b. Narrowband Reactive

AD8314

RFIN

STRIPLINE

c. Series Attenuation

Figure 32. Input Coupling Options

Figure 32c shows a third method for coupling the input signal

into the AD8314, applicable in applications where the input signal

is larger than the input range of the log amp. A series resistor,

connected to the RF source, combines with the input impedance