ADL5331

Rev. 0 | Page 9 of 16

THEORY OF OPERATION

The ADL5331 is a high performance, voltage-controlled

variable gain amplifier/attenuator for use in applications

with frequencies up to 1.2 GHz. This device is intended to

serve as an output variable gain amplifier (OVGA) for applica-

tions where a reasonably constant input level is available and

the output level adjusts over a wide range. One aspect of an

OVGA is that the output metrics, OIP3 and OP1dB, decrease

with decreasing gain.

The signal path is fully differential throughout the device 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 14 provides a simplified schematic of the ADL5331.

GAIN

CONTROL

OPHI

OPLO

INHI

INLO

TRANSIMPEDANCE

AMPLIFIER

Gm STAGE

Figure 14. 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.

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 the 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 to 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 controlled by an output

level detector, which biases the output stage for signal levels

above a threshold.

The outputs of the ADL5331 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.

The output impedance is 20 Ω differential and can drive a range

of impedances from <20 Ω to >75 Ω. Back series terminations

can be used to pad the output impedance to a desired level.

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

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 as the gain is lowered, because the gain

is reduced below the critical value. Figure 7 through Figure 9

provide details of this behavior.