adl5331 Analog Devices, Inc., adl5331 Datasheet - Page 9

• Manufacturer Part Number: adl5331
• Description: 1 Mhz To 1.2 Ghz Vga With 30 Db Gain Control Range
• Manufacturer: Analog Devices, Inc.

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.
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.
INLO
INHI
Gm STAGE
Figure 14. Simplified Schematic
CONTROL
GAIN
TRANSIMPEDANCE
AMPLIFIER
OPHI
OPLO
Rev. 0 | Page 9 of 16
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.
ADL5331