AD8369 datasheet(13/24 Pages) AD

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REV. 0

AD8369

–13–

Gm CELLS

GAIN STEP SELECTION

DIGITAL

3dB SWITCHED

ATTENUATOR

BIAS

FIXED

GAIN

INLO

CMDC

INHI

20pF

22pF

OPLO

OPHI

VS/2

100

– 0.7

Figure 1. General Block Diagram, Control and Signal Paths Are Differential

THEORY OF OPERATION

The AD8369 is a digitally controlled fully differential VGA

based on a variation of Analog Devices’ patented X-AMP archi-

tecture (Figure 1). It provides accurate gain control over a 45 dB

span with a constant –3 dB bandwidth of 600 MHz.

The 3 dB gain steps can be controlled by a user-selectable

parallel- or serial-mode digital interface. A single pin (SENB)

selects the mode. The AD8369 is designed for optimal

operation when used in a fully differential system, although

single-ended operation is also possible. Its nominal input and

output impedances are 200

Input Attenuator and Output 3 dB Step

The AD8369 is comprised of a seven-stage R-2R ladder net-

work (eight taps) and a selected Gm stage followed by a

fixed-gain differential amplifier. The ladder provides a total

attenuation of 42 dB in 6 dB steps. The full signal is applied to

the amplifier using the first tap; at the second tap, the signal

is 6 dB lower and so on. A further 3dB interpolating gain step is

introduced at the output of the fixed gain amplifier, providing

the full 45 dB of gain span.

Fixed Gain Amplifier

The fixed gain amplifier is driven by the tap point of the R-2R

ladder network via the selected Gm cell. The output stage is a

complementary pair of current sources, loaded with internal

100

W resistors to ac ground which provides a 200 W differential

output impedance. The low frequency gain of the AD8369

can be approximated by the equation:

VIN

OUT

ËÁ

¯˜

ÁÁ

˜˜

()

200

where RL is the external load resistor in ohms and n is the gain

code; 0 is the minimum gain code and 15 is the maximum gain

code. The external load, which is in parallel combination with

the internal 200

W output resistor, affects the overall gain and

peak output swing. Note that the external load has no effect on the

gain step size.

Input and Output Interfaces

The dc working points of the differential input and output inter-

faces of the AD8369 are internally biased. The inputs INHI and

INLO are biased to a diode drop below VS/2 (~1.7 V for a 5 V

positive supply) to meet isolation and headroom constraints,

while the outputs OPHI and OPLO are centered on the supply

midpoint, VS/2, to provide the maximum output swing.

The internal VS/2 reference and the CMDC reference are buff-

ered and decoupled to ground via internal capacitors. The

input bias voltage, derived from this VS/2 reference, is brought

FREQUENCY – kHz

100

1000

10000

TPC 37. Power Supply Rejection Ratio, VS = 5 V,

RL = 1 k

W, Maximum Gain

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