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Data Sheet

ADL5201

Rev. C | Page 17 of 26

CIRCUIT DESCRIPTION

BASIC STRUCTURE

The ADL5201 is a differential variable gain amplifier (VGA)

consisting of a 150 Ω digitally controlled passive attenuator

followed by a highly linear transconductance amplifier with

feedback.

ATTENUATOR

LOGIC

REF

VIN+

VIN–

VOUT+

VOUT–

DIGITAL INPUTS

PARALLEL, SPI,

FAST ATTACK

UP/DOWN

ADL5201

AMP

Figure 52. Simplified Schematic

INPUT SYSTEM

The dc voltage level at the input of the amplifier is set by an

independent internal voltage reference circuit to approximately

1.6 V. The reference is not accessible and cannot be adjusted.

The amplifier can be powered down by pulling the PWUP pin

low. In power-down mode, the total current is reduced to 7 mA

(typical). The dc level at the input remains at approximately

1.6 V, regardless of the state of the PWUP pin.

OUTPUT AMPLIFIER

Gain of the output amplifier is set to be 22 dB when driving

a 150 Ω load. The input and output resistance of this amplifier

is set to 150 Ω in matched condition. If the load or the source

resistance is not equal to 150 Ω, the following equations can be

used to determine the resulting gain and input/output resistances.

Voltage Gain = AV = 0.09 × (2000)//RL

RIN = (2000 + RL)/(1 + 0.09 × RL)

S21 (Gain) = 2 × RIN/(RIN + RS) × AV

ROUT = (2000 + RS)/(1 + 0.09 × RS)

Note that the at maximum attenuation setting, RS, as seen by

the output amplifier, is the output resistance of the attenuator,

which is 150 Ω. However, at the minimum attenuation setting,

RS is the source resistance that is connected to the input of the part.

The dc current to the outputs of each amplifier is supplied through

two external chokes. The inductance of the chokes and the

resistance of the load, in parallel with the output resistance of

the device, add a low frequency pole to the response. The para-

sitic capacitance of the chokes adds to the output capacitance of the

part. This total capacitance, in parallel with the load and output

resistance, sets the high frequency pole of the device. Generally,

the larger the inductance of the choke, the higher its parasitic

capacitance. Therefore, this trade-off must be considered when

the value and type of the choke are selected. For an operation

frequency of 15 MHz to 700 MHz driving a 150 Ω load, 1 μH

chokes with an SRF of 160 MHz or higher are recommended

(such as the 0805LS-102XJBB from Coilcraft). If higher value

chokes are used, a 4 MHz zero, due to the internal ac-coupled

feedback, causes an increase in S21 of up to 6 dB at frequencies

below 4 MHz.

The supply current of the amplifier consists of about 35 mA

through the VPOS pin and 50 mA through the two chokes

combined. The latter increases with temperature at

approximately 2.5 mA per 10°C. The total choke current increases

to 75 mA for high performance mode. The amplifier has two

output pins for each polarity, and they are oriented in an

alternating fashion. When designing the board, care should be

taken to minimize the parasitic capacitance due to the routing

that connects the corresponding outputs together. To minimize

the parasitic capacitance, a good practice is to avoid any ground

or power plane under this routing region and under the chokes.

GAIN CONTROL

The gain can be adjusted using the parallel control interface, the

serial peripheral interface, or the gain up/down interface. In

general, the gain step size is 0.5 dB, but larger sizes can be

programmed using the various interfaces, as described in the

Digital Interface Overview section. The amplifier has a maximum

gain of +20 dB (Code 0) to −11.5 dB (Code 63).

The noise figure of the amplifier is approximately 7.5 dB at the

maximum gain setting, and it increases as the gain is reduced.

The increase in noise figure is equal to the reduction in gain.

The linearity of the part, measured at the output, is first-order

independent of the gain setting. From −4 dB to +20 dB gain,

the OIP3 is approximately 50 dBm into a 150 Ω load at 200 MHz

(0 dBm per tone). At gain settings below −4 dB, the OIP3 drops

to approximately 40 dBm.

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