AD7294 datasheet(22/45 Pages) AD

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AD7294

Preliminary Technical Data

Rev. PrB | Page 22 of 45

GAIN CONTROL OF PA

In this mode, a setpoint voltage, proportional in dB to the

desired output power, is applied to a Logarithmic Amplifier

such as the AD8317, see Figure 25. A sample of the output

power from the PA, via a directional coupler and attenuator (or

by other means), is fed to the input of the AD8317. The VOUT is

then connected to the gain control terminal of the PA. Based on

the defined relationship between VOUT and the RF input signal,

the AD8317 will adjust the voltage on VOUT (VOUT is now an

error amplifier output) until the level at the RF input

corresponds to the applied VSET. The AD7294 completes a

feedback loop, which can track the output of the AD8317 and

adjust the VSET input of the AD8317 accordingly.

INHI

VOUT

VSET

AD8317

DIRECTIONAL

COUPLER

POWER

AMPLIFIER

RF IN

ENVELOPE OF

TRANSMITTED

SIGNAL

AD7294

VOUT

VIN

INLO

CFLT

ATTENUATOR

Figure 25. Setpoint Controller Operation

VOUT of the AD8317 is applied to the gain control terminal of

the power amplifier. In order for this output power control loop

to be stable, a ground-referenced capacitor must be connected

to the CFLT pin. This capacitor integrates the error signal (which

is actually a current) that is present when the loop is not

balanced.

In a system where a Variable Gain Amplifier or

Variable Voltage Attenuator feeds the power amp only one

AD8317 is required. In such a case the gain on one of the

parts (VVA, PA) is fixed and VOUT feeds the control input of

the other.

ANALOG COMPARATOR LOOP

Utilizing the internal analog comparator loop allows the

AD7294 to control the RF switch controlling the RF IN signal to

the HPA. The ISENSEOVERRANGE 1 pin signals whether the

instrumentation amplifiers voltage from the current sense is

greater then or less then the set point 240 mV. The 240 mV is

the fixed threshold voltage of the over-range comparator and

the current sense amplifier will saturate above this. This over-

range signal can be used for the control signal on, for example,

an RF switch such as the ADG901.

HIGH SIDE

CURRENT

SENSE

RS1(+)

RS1(-)

R Sense

ISENSE1

OVERRANGE

SET-POINT

240mV

TO ADC

ADG901*

AD7294*

RF1

RF2

CTRL

RF IN

VOUT A

FILTER

RF CHOKE

RF OUT

LDMOS

RF CHOKE

VPP

Figure 26. Analog Comparator Loop

This is a useful safety feature available in the AD7294. The

analog control loop senses any high current supplied to the

power amplifier and dynamically cuts the RF signal to the gate

of the device in direct response.

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