ADL5336-EVALZ datasheet(17/30 Pages) AD

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Data Sheet

ADL5336

Rev. C | Page 17 of 30

THEORY OF OPERATION

CIRCUIT DESCRIPTION

The ADL5336 contains two differential VGAs, each with a

programmable, internally connected, square law detector. VGA2

includes an input select switch that allows the user to choose

between two sets of differential inputs.

The signal path of each VGA, shown in Figure 59 and Figure 60,

consists of a variable input attenuator followed by a programmable

gain amplifier (PGA).

The input attenuator is built from an 18-section resistor ladder,

providing 1.34 dB of attenuation at each successive tap point.

The resistor ladder acts as a linear input attenuator, in addition

to providing an accurate 200 Ω input impedance. The variable

transconductance (gm) stages are used to select the attenuated

signal from the appropriate tap point along the ladder and feed

this signal to the fixed gain amplifier. To realize a continuous

gain control function from discrete tap points, the gain interpolator

creates a weighted sum of signals appearing on adjacent tap points

by carefully controlling the variable gm stages.

The weighted sum of the different tap points is fed into the

programmable gain stage. The programmable gain stage achieves

its different gain settings by changing the feedback network of

the amplifier.

The input attenuator and gm stages provide analog gain control

of 24 dB, whereas the programmable gain amplifier sets the

maximum gain of each VGA.

Table 4. VGA Gain Range

Maximum Gain Word

VGA1

VGA2

VGA1 Range (dB)

VGA2 Range (dB)

−14.5 to +9.5

−10 to +14

−12 to +12.0

−7.1 to +16.9

−10 to +14.0

−5 to +19

−8.4 to +15.6

−3.1 to +20.9

ATTENUATOR LADDER

200Ω

0dB

–1.4dB

–2.8dB

–23.8dB

OPP1

INP1

GAIN1

MODE

GAIN INTERPOLATOR

PGA

INM1

–22.4dB

gm STAGES

OPM1

PGA LINEAR

VOLTAGE GAIN:

G = 3, 4, 5, 6

Figure 59. VGA1 Functional Block Diagram

PGA LINEAR

VOLTAGE GAIN:

G = 5, 7, 9, 11

IP2A

IM2A

IP2B

IM2B

ATTENUATOR LADDER

200Ω

0dB

–1.4dB

–2.8dB

–23.8dB

OPP2

GAIN2

MODE

GAIN INTERPOLATOR

PGA

–22.4dB

gm STAGES

OPM2

Figure 60. VGA2 Functional Block Diagram

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