FEATURES

Complete fully calibrated measurement/control system

Accurate rms-to-dc conversion from 50 Hz to 3.8 GHz

Input dynamic range of >65 dB: −52 dBm to +8 dBm in 50 Ω FUNCTIONAL BLOCK DIAGRAM

Waveform and modulation independent, such as

GSM/CDMA/TDMA

Linear-in-decibels output, scaled 50 mV/dB

Law conformance error of 0.5 dB

All functions temperature and supply stable

Operates from 4.5 V to 5.5 V at 24 mA

Power-down capability to 1.3 mW

APPLICATIONS

Power amplifier linearization/control loops

Transmitter power controls

Transmitter signal streCM GROUPh indication (TSSI)

RF instrumentation

GENERAL DESCRIPTION

The AD8362 is a true rms-responding power detector that has

a 65 dB measurement range. It is intended for use in a variety of

high frequency communication systems and in instrumentation

requiring an accurate response to signal power. It is easy to use,

requiring only a single supply of 5 V and a few capacitors. It can

operate from arbitrarily low frequencies to over 3.8 GHz and

can accept inputs that have rms values from 1 mV to at least

1 V rms, with large crest factors, exceeding the requirements

for accurate measurement of CDMA signals.

The input signal is applied to a resistive ladder attenuator that

comprises the input stage of a variable gain amplifier (VGA).

The 12 tap points are smoothly interpolated using a proprietary

technique to provide a continuously variable attenuator, which

is controlled by a voltage applied to the VSET pin. The resulting

signal is applied to a high performance broadband amplifier. Its

output is measured by an accurate square-law detector cell. The

fluctuating output is then filtered and compared with the output

of an identical squarer, whose input is a fixed dc voltage applied

to the VTGT pin, usually the accurate reference of 1.25 V pro-

vided at the VREF pin.

The difference in the outputs of these squaring cells is integrated

in a high gain error amplifier, generating a voltage at the VOUT

pin with rail-to-rail capabilities. In a controller mode, this low

noise output can be used to vary the gain of a host system’s RF

amplifier, thus balancing the setpoint against the input power.

Optionally, the voltage at VSET can be a replica of the RF signal’s

amplitude modulation, in which case the overall effect is to

remove the modulation component prior to detection and low-

pass filtering. The corner frequency of the averaging filter can

be lowered without limit by adding an external capacitor at the

CLPF pin. The AD8362 can be used to determine the true power

of a high frequency signal having a complex low frequency

modulation envelope, or simply as a low frequency rms volt-

meter. The high-pass corner generated by its offset-nulling

loop can be lowered by a capacitor added on the CHPF pin.

Used as a power measurement device, VOUT is strapped to

VSET. The output is then proportional to the logarithm of the

rms value of the input. In other words, the reading is presented

directly in decibels and is conveniently scaled 1 V per decade,

or 50 mV/dB; other slopes are easily arranged. In controller

modes, the voltage applied to VSET determines the power level

required at the input to null the deviation from the setpoint.

The output buffer can provide high load currents.

The AD8362 has 1.3 mW power consumption when powered

down by a logic high applied to the PWDN pin. It powers up

within about 20 μs to its nominal operating current of 20 mA at

25°C. The AD8362 is supplied in a 16-lead TSSOP for operation

over the temperature range of −40°C to +85°C.