AD7294BCPZ datasheet(44/48 Pages) AD

44 / 48 page

Data Sheet

AD7294

Rev. I | Page 43 of 47

APPLICATIONS INFORMATION

The AD7294 contains all the functions required for general-

purpose monitoring and control of current, voltage, and

temperature. With its 59.4 V maximum common-mode range,

the device is useful in industrial and automotive applications where

current sensing in the presence of a high common-mode voltage

is required. For example, the device is ideally suited for monitoring

and controlling a power amplifier in a cellular base station.

BASE STATION POWER AMPLIFIER MONITOR AND

CONTROL

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 tempera-

ture, supply voltage, gate voltage drift, and general processing

parameters. The overall performance of a power amplifier

configuration is determined by the inherent tradeoffs required

in efficiency, gain, and linearity. The high level of integration

offered by the AD7294 allows the use of a single chip to

dynamically control the drain bias current to maintain a constant

value over temperature and time, thus significantly improving

the overall performance of the power amplifier. The AD7294

incorporates the functionality of eight discrete components

bringing considerable board area savings over alternative

solutions.

The circuit in Figure 59 is a typical system connection diagram

for the AD7294. The device monitors and controls the overall

performance of 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, ISENSE1 OVERRANGE and ISENSE2

OVERRANGE 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 setpoint, 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 or

AD8362.

MUX

TEMP

SENSOR

HIGH SIDE

CURRENT

SENSE

HIGH SIDE

CURRENT

SENSE

VOUTA

VOUTB

VOUTC

FILTER

RF CHOKE

RF OUT

LDMOS

RS1(+)

RS2(+)

RS2(–)

RS1(–)

RS2(–)

D1+

D2+

D2–

D1–

SET-POINT

240mV

ISENSE1

OVERRANGE

ISENSE2

OVERRANGE

GAIN

CONTROL

RSENSE

VDD

LDMOS

RF OUT

12-BIT

ADC

VIN0

VIN1

VIN2

VIN3

12-BIT

DAC

12-BIT

DAC

12-BIT

DAC

12-BIT

DAC

VOUTD

GAIN

CONTROL

RF CHOKE

Tx POWER

MONITOR

Rx POWER

MONITOR

POWER

RF CUTOFF

POWER

AD7294*

RF IN

*ADDITIONAL PINS OMITTED FOR CLARITY.

REF

Figure 59. Typical HPA Monitor and Control Application

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