ADL5206ACPZ-R7 datasheet(19/25 Pages) AD

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Data Sheet

ADL5206

Rev. 0 | Page 19 of 25

THEORY OF OPERATION

BASIC STRUCTURE

The ADL5206 is a differential, digitally controlled VGA, which is

also known as a DGA. The DGA consists of a 100 Ω differential

input, digitally controlled passive attenuator, followed by a

digitally controlled gain amplifier. On-chip logic circuitry maps

the gain codes such that all gain changes, from the maximum

gain to minimum gain, are accomplished by only using the

digitally controlled resistors in the feedback of the amplifier.

This technique does not require a digital step attenuator (DSA)

on the input of the amplifier, thus providing SFDR increases

as gain reduces. This topology also allows all 30 dB of gain

reduction in the feedback with a total noise figure degradation

of 7 dB only over the total 30 dB gain range at 700 MHz. The

differential output impedance of the amplifier is 10 Ω.

CONTROL AND LOGIC CIRCUITRY

The ADL5206 features three different gain control interfaces:

serial, parallel, or up and down control, which is determined by

the combination of the MODE1 and MODE0 pins. For details

on controlling the gain in each of these modes, see the Digital

Interface Overview section. Typically, the gain step size is 1 dB.

Larger step sizes can be programmed, as described in the Digital

Interface Overview section. The amplifier has a maximum gain

of 32 dB (Gain Code 00000) to a minimum gain of 2 dB

(Gain Code 11110 to Gain Code 11111).

COMMON-MODE VOLTAGE

The ADL5206 is flexible in terms of input and output coupling.

The ADL5206 can be ac-coupled or dc-coupled at the inputs

and/or outputs within the specified output common-mode

voltage reference range of 1.2 V to 1.8 V for the 3.3 V supply

and 1.4 V to 2.7 V for the 5 V supply, depending on the supply

voltage. If no external output common-mode voltage is applied,

the input and output common-mode voltages are set internally

to half of the supply voltage.

The output common-mode voltages of the ADL5206 are

controlled by the voltages on the VCM pin. The VCM pin is

connected internally through 5 kΩ resistors to the VPOS pin

as well as to the exposed pad. As a result, the common-mode

output voltage is preset internally to half of the supply voltage at

VPOS. Alternatively, the VCM pin can be connected to the

common-mode voltage reference output from an ADC, and

thus the common-mode levels between the amplifier and the

ADC can be matched without requiring any external components.

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