ADL5330

Data Sheet

Rev. B | Page 12 of 24

THEORY OF OPERATION

The ADL5330 is a high performance, voltage controlled variable

gain amplifier/attenuator for use in applications with frequencies

up to 3 GHz. This device serves as an output variable gain

amplifier (OVGA) for applications where a reasonably constant

input level is available and the output level adjusts over a wide

range. One aspect of an OVGA is the output metrics, OIP3 and

OP1dB, decrease with decreasing gain.

The signal path is fully differential throughout the device in

order to provide the usual benefits of differential signaling,

including reduced radiation, reduced parasitic feedthrough, and

reduced susceptibility to common-mode interference with other

circuits. Figure 31 provides a simplified schematic of the

ADL5330.

GAIN

CONTROL

OPHI

OPLO

INHI

INLO

TRANSIMPEDANCE

AMPLIFIER

Gm STAGE

Figure 31. Simplified Schematic

A controlled input impedance of 50 Ω is achieved through a

combination of passive and active (feedback-derived) termination

techniques in an input Gm stage. The input compression point

of the Gm stage is 1 dBm to 3 dBm, depending on the input

frequency.

Note that the inputs of the Gm stage are internally biased to a

dc level, and dc blocking capacitors are generally needed on the

inputs to avoid upsetting operation of the device.

The currents from the Gm stage are then injected into a balanced

ladder attenuator at a deliberately diffused location along the

ladder, wherein the location of the centroid of the injection

region is dependent on the applied gain control voltage. The

steering of the current injection into the ladder is accomplished

by proprietary means to achieve linear in dB gain control and

low distortion.

Linear in dB gain control is accomplished by the application of

a voltage in the range of 0 V dc to 1.4 V dc to the gain control

pin, with maximum gain occurring at the highest voltage.

The output of the ladder attenuator is passed into a fixed gain

transimpedance amplifier (TZA) to provide gain and buffer the

ladder terminating impedance from load variations. The TZA

uses feedback to improve linearity and to provide controlled

50 Ω differential output impedance. The quiescent current of

the output amplifier is adaptive; it is slaved to the gain control

voltage to conserve power at times when the gain (and output

power) are low.

The outputs of the ADL5330 require external dc bias to the

positive supply voltage. This bias is typically supplied through

external inductors. The outputs are best taken differentially to

avoid any common-mode noise that is present, but, if necessary,

can be taken single-ended from either output.

If only a single output is used, it is still necessary to provide bias

to the unused output pin, and it is advisable to arrange a reasonably

equivalent ac load on the unused output. Differential output can

be taken via a 1:1 balun into a 50 Ω environment. In virtually all

cases, it is necessary to use dc blocking in the output signal path.

At high gain settings, the noise floor is set by the input stage, in

which case the noise figure (NF) of the device is essentially

independent of the gain setting. Below a certain gain setting,

however, the input stage noise that reaches the output of the

attenuator falls below the input equivalent noise of the output

stage. In such a case, the output noise is dominated by the

output stage itself; therefore, the overall NF of the device gets

worse on a dB per dB basis, because the gain is reduced below

the critical value. Figure 9 through Figure 13 provide details of

this behavior.