AD7294 datasheet(21/45 Pages) AD

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Preliminary Technical Data

AD7294

Rev. PrB | Page 21 of 45

APPLICATIONS

The AD7294 is used in a power amplifier signal chain to

achieve the optimal bias condition for the LDMOS transistor.

The main factors influencing the bias conditions are

temperature, supply voltage, gate voltage drift, and general

processing parameters. The overall performance of a power

amplifiers configuration is determined by the inherent tradeoffs

required in efficiency, gain, and linearity. Dynamically

controlling the drain bias current, in order to maintain a

constant value over temperature and time, can significantly

improve the overall performance of the power amplifier.

The AD7294 contains all the functions required for general-

purpose monitoring and control of current, voltage, and

temperature. Depending on the signal chain, a number of

amplifiers can be used in predrive and drive modes prior to the

PA final stage to increase the overall power gain of the signal,

see Figure 23 .

PRE-DRIVER

DRIVER

2-STAGE FINAL

TO ANTENNA

RF SIGNAL

AD7294

Figure 23. Typical HPA Signal Chain

This addition of power gain has an adverse effect on the overall

efficiency of the PA. To minimize the degradation of the PA’s

efficiency, the drivers must be monitored and controlled to

optimize performance.

TYPICAL RF FRONT-END APPLICATION

The circuit in Figure 24 is a typical application for the AD7294.

The device is used to monitor and control the overall

performance of a two final stage amplifiers. The gain control

and phase adjustment of the driver stage are incorporated in the

application and are carried out by the two available

uncommitted outputs of the AD7294. Both high-side current

senses measure the amount of current on the respective final

stage amplifiers. The comparator outputs, ISENSE OVER-RANGE

pins, are the controlling signals for switches on the RF inputs of

the LDMOS power FETs. If the high-side current sense reads a

value above a specified limit compared with the set-point, the

RF IN signal is switched off by the comparator.

By measuring the transmitted power (Tx) and the received

power (Rx), the device can dynamically change the drivers and

PA signal to optimize performance. This application requires a

logarithmic detector/controller, such as Analog Devices AD8317.

12-BIT

ADC

12-BIT

DAC

COMPARATORS &

REGISTERS

TEMP

SENSOR

HIGH SIDE

CURRENT

SENSE

HIGH SIDE

CURRENT

SENSE

12-BIT

DAC

12-BIT

DAC

12-BIT

DAC

VOUT A

VOUT B

VOUT C

VOUT D

FILTER

RF CHOKE

FILTER

GAIN

CONTROL

IMPEDANCE

MATCH

LDMOS

RS1(+)

RS2(+)

RS1(-)

RS2(-)

VIN 0

VIN 1

VIN 2

VIN 3

D1 (+)

D1 (-)

D0 (-)

ISENSE2

OVER-RANGE

R Sense

AD7294*

D0 (+)

SET-POINT

240mV

RS2(-)

R Sense

* ADDITIONAL PINS OMITTED FOR CLARITY

VDD

RF CHOKE

MUX

RF OUT

TX Power

Monitor

RX Power

Monitor

REF

POWER

RF OUT

RF IN

CUTOFF

ISENSE1

OVER-RANGE

Figure 24. Typical HPA Monitor and Control Application

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