ADL5561

Rev. B | Page 13 of 24

CIRCUIT DESCRIPTION

BASIC STRUCTURE

The ADL5561 is a low noise, low power, fully differential amplifier/

ADC driver that uses a 3.3 V supply. It provides three gain options

(6 dB, 12 dB, and 15.5 dB) without the need for external resistors

and has wide bandwidths of 2.6 GHz for 6 dB, 2.3 GHz for 12

dB, and 2.1 GHz for 15.5 dB. Differential input impedance is

400 Ω for 6 dB, 200 Ω for 12 dB, and 133 Ω for 15.5 dB. It has a

differential output impedance of 10 Ω and an output common-

mode adjust voltage of 1.25 V to 1.85 V.

AC

VIP2

VIP1

VIN1

VIN2

Figure 30. Basic Structure

The ADL5561 is composed of a fully differential amplifier with

on-chip feedback and feed-forward resistors. The two feed-

forward resistors on each input set this pin-strappable amplifier

in three different gain configurations of 6 dB, 12 dB, and 15.5 dB.

The amplifier is designed to provide high differential open-loop

gain and an output common-mode circuit that enables the user

to change the common-mode voltage from a VCOM pin. The

amplifier is designed to provide superior low distortion at

frequencies up to and beyond 300 MHz with low noise and

low power consumption. The low distortion and noise are

realized with a 3.3 V power supply at 40 mA.

The ADL5561 is very flexible in terms of I/O coupling. It can be

ac-coupled or dc-coupled at the inputs and/or the outputs within

the specified input and output common-mode levels. The input

of the device can be configured as single-ended or differential with

similar distortion performance. Due to the internal connections

between the inputs and outputs, keep the output common-mode

voltage between 1.25 V and 1.85 V for the best distortion.

For a dc-coupled input, the input common mode should be

between 1 V and 2.3 V for the best distortion. The device has

been characterized using 2 V p-p into 200 Ω. If the inputs are

ac-coupled, the input and output common-mode voltages are

set by VCC/2 when no external circuitry is used. The ADL5561

provides an output common-mode voltage set by VCOM, which

allows driving an ADC directly without external components,

such as a transformer or ac-coupling capacitors, provided that

the VCOM of the amplifier is within the VCOM of the ADC.

For dc-coupled requirements, the input VCM must be set by the

VCOM pin in all three gain settings.