AD8370 datasheet(13/28 Pages) AD

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AD8370

Rev. 0 | Page 13 of 28

THEORY OF OPERATION

The AD8370 is a low cost, digitally controlled, fine adjustment

variable gain amplifier that provides both high IP3 and low

noise figure. The AD8370 is fabricated on an ADI proprietary

high performance 25 GHz silicon bipolar process. The –3 dB

bandwidth is approximately 750 MHz throughout the variable

gain range. The typical quiescent current of the AD8370 is

78 mA. A power-down feature reduces the current to less than

4 mA. The input impedance is approximately 200 Ω differential,

and the output impedance is approximately 100 Ω differential to

be compatible with saw filters and matching networks used in

intermediate frequency (IF) radio applications. Because there is

no feedback between the input and output and stages within the

amplifier, the input amplifier is isolated from variations in

output loading and from subsequent impedance changes, and

excellent input to output isolation is realized. Excellent distor-

tion performance and wide bandwidth make the AD8370 a

suitable gain control device for modern differential receiver

designs. The AD8370 differential input and output configuration

is ideally suited to fully differential signal chain circuit designs,

although it can be adapted to single-ended system applications,

if required.

BLOCK ARCHITECTURE

The three basic building blocks of the AD8370 are a high/low

gain selectable input preamplifier, a digitally controlled

transconductance (gm) block, and a fixed gain output stage.

INHI

INLO

OPHI

OPLO

VCCO

OCOM

ICOM

VOCM

PWUP

VCCO

OCOM

VCCI

ICOM

BIAS CELL

SHIFT REGISTER

AND LATCHES

PRE

AMP

TRANSCONDUCTANCE

OUTPUT

AMP

DATA CLCK LTCH

AD8370

Figure 35. Functional Block Diagram

PREAMPLIFIER

There are two selectable input preamplifiers. Selection is made

by the most significant bit (MSB) of the serial gain control data-

word. In the high gain mode, the overall device gain is 7.1 Volts/

Volt (17 dB) above the low gain setting. The two preamplifiers

give the AD8370 the ability to accommodate a wide range of

input amplitudes. The overlap between the two gain ranges

allows the user some flexibility based on noise and distortion

demands. See the Choosing between Gain Ranges section for

more information.

The input impedance is approximately 200 Ω differential,

regardless of which preamplifier is selected. Note that the input

impedance is formed by using active circuit elements and is not

set by passive components. See Figure 36 for a simplified

schematic of the input interface.

1mA

VCC/2

Ω

INHI/INLO

Figure 36. INHI/INLO Simplified Schematic

TRANSCONDUCTANCE STAGE

The digitally controlled gm section has 42 dB of controllable

gain and makes gain the adjustments within each gain range.

The step size resolution ranges from a fine ~ 0.07 dB up to a

coarse 6 dB per bit, depending on the gain code. As shown in

Figure 37, of the 42 dB total range, 28 dB has resolution of

better than 2 dB, and 22 dB has resolution of better than 1 dB.

The curves in Figure 37 show typical input levels that can be

applied to this amplifier at different gain settings. The maxi-

mum input was determined by finding the 1 dB compression or

expansion point of the VOUT/VSOURCE gain. Note that this is not

VOUT/VIN. In this way, the change in the input impedance of the

device is also taken into account.

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

VSOURCE [V peak] (V)

HIGH GAIN

0.1dB GAIN

–5dB GAIN

–8dB GAIN

12dB

GAIN

6dB

GAIN

<2dB

RES

<2dB

RES

<1dB

RES

<1dB

RES

<0.5dB

RESOLUTION

<0.5dB

RES

–11dB GAIN

17dB

GAIN

34dB

GAIN

–25dB GAIN

LOW GAIN

Figure 37. Gain Resolution and Nominal Input and

Output Range over the Gain Range

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