AD8311

Rev. A | Page 14 of 24

Continuing with the stability analysis, the gain of the control

loop can be expressed as

kA

k

I

P

SET

OUT

+

=

∂

∂

1

(10)

where:

PA

FLT

PA

s

sC

G

k

τ

+

×

×

=

1

)

1

(

35

.

1

(dB/A)

(11)

SLP

I

A =

(A/dB)

(12)

The effect of the zero resistor, RFLT, can be easily included by

replacing (1/sCFLT) with (RFLT + 1/sCFLT). The criteria for loop

stability can be derived by setting the denominator of

Equation 10 equal to 0, giving

()

()

(

)

SLP

PA

FLT

PA

FLT

FLT

I

G

sC

s

C

sR

×

×

τ

+

+

×

+

=

1

1

35

.

1

1

0

(13)

From Equation 13, the closed-loop integration time constant is

given by

(

)

35

.

1

×

×

=

SLP

PA

FLT

ClosedLoop

I

G

C

T

(14)

The gain and phase margins of the control loop can be deduced

from the Bode plots of Equation 13.

BASIC CONNECTIONS

Figure 29 shows the basic connections for operating the

AD8311, and Figure 30 shows a block diagram of a typical

application. The AD8311 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 AD8311.

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 AD8311 has an internal input coupling capacitor,

which negates the need for external ac-coupling. This capacitor,

along with the low frequency input impedance of the device of

approximately 2.14 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 Smith chart plot of input

impedance vs. frequency is shown in Figure 14. Other coupling

methods are also possible (see the Input Coupling Options

section).

VPOS

VAPC

VSET

RFIN

COMM

FLTR

AD8311

1

2

34

5

6

+VS

(2.7V TO 5.5V)

C1

0.1

μF

RFIN

R1

52.3

Ω

VAPC

VSET

CFLT

Figure 29. Basic Connections

RFIN

VSET

AD8311

VAPC

FLTR

CFLT

DAC

POWER

AMP

RFIN

ATTENUATOR

DIRECTIONAL

COUPLER

52.3

Ω

RFLT

GAIN

CONTROL

VOLTAGE

Figure 30. Typical Application

In a power control loop, the AD8311 provides both the detector

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

output power is coupled to the RF input of the AD8311, 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

AD8311.

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 AD8311’s 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 AD8311 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 200 mV to within 100 mV of the supply

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

needs to 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 11.

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 4, Figure 5, Figure 7, and Figure 8). At 0.9 GHz, for

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

−18 dBm 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 150 mV, VAPC

unconditionally remains at its minimum level of approximately

300 mV. This feature can be used to 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 23.8 mV/dB, or approximately 42.0 dB/V.