AD8337

Rev. B | Page 18 of 24

THEORY OF OPERATION

VOUT

GAIN

18dB

(8X)

2

1

6

PRAO

VNEG

VCOM

+

–

7

749Ω

107Ω

PRA

6dB

GAIN

INTERFACE

BIAS

+

–

8

VPOS

INPP

INPN

5

3

4

INTERPOLATOR

ATTENUATOR

–24dB TO 0dB

+

–

Figure 65. Block Diagram

OVERVIEW

The AD8337 is a low noise, single-ended, linear-in-dB, general-

purpose, variable gain amplifier (VGA) usable at frequencies

up to 100 MHz. It is fabricated using a proprietary Analog

Devices dielectrically isolated, complementary bipolar process.

The bandwidth is dc to 280 MHz and features low dc offset

voltage and an ideal nominal gain range of 0 dB to 24 dB.

Requiring about 15.5 mA, the power consumption is only

78 mW from either a single +5 V or a dual ±2.5 V supply.

Figure 65 is the circuit block diagram of the AD8337.

PREAMPLIFIER

An uncommitted, current-feedback op amp included in the

AD8337 can be used as a preamplifier to buffer the ladder

network attenuator of the X-AMP. As with any op amp, the gain

is established using external resistors, and the preamplifier is

specified with a noninverting gain of 6 dB (2×) and gain resistor

values of 100 Ω. The preamplifier gain can be increased using

compensation capacitor determines the 3 dB bandwidth, and

smaller values can compromise preamplifier stability.

Because the AD8337 is dc-coupled, larger preamp gains increase

the offset voltage. The offset voltage can be compensated by

connecting a resistor between the INPN input and the supply

voltage. If the offset is negative, the resistor value connects to the

negative supply. For ease of adjustment, a trimmer network

can be used.

For larger gains, the overall noise is reduced if a low value of

preamp gain is 16× (24.1 dB), and the input-referred noise is

approximately 1.5 nV/√Hz. For this value of gain, the overall

gain range increases by 18 dB; therefore, the gain range is

18 dB to 42 dB.

VGA

This X-AMP, with its linear-in-dB gain characteristic

architecture, yields the optimum dynamic range for receiver

applications. Referring to Figure 65, the signal path consists

of a −24 dB variable attenuator followed by a fixed gain amplifier

of 18 dB, for a total VGA gain range of −6 dB to +18 dB. With

the preamplifier configured for a gain of 6 dB, the composite

gain range is 0 dB to 24 dB.

The VGA plus preamp with 6 dB of gain implements the

following exact gain law

(dB)

V

dB

19.7

(dB)

ICPT

GAIN

V

Gain

+

⎥⎦

⎤

⎢⎣

⎡

×

=

where the nominal intercept (ICPT) is 12.65 dB.

The ICPT increases as the gain of the preamp is increased. For

example, if the gain of the preamp is increased by 6 dB, ICPT

increases to 18.65 dB. Although the previous equation shows

the exact gain law as based on statistical data, a quick estimation

of signal levels can be made using the default slope of 20 dB/V

for a particular gain setting. For example, the change in gain for

5.91 dB using the exact slope of 19.6 dB/V. This is a difference

of only 0.09 dB.

GAIN CONTROL

The gain control interface provides a high impedance input

and is referenced to VCOM pin (in a single-supply application

to midsupply at [VPOS + VNEG]/2 for optimum swing).

When dual supplies are used, VCOM is connected to ground.

The voltage on the VCOM pin determines the midpoint of the

from −0.7 V to +0.7 V with the most linear-in-dB section of the

gain control between −0.6 V and +0.6 V. In the center 80% of

gain control voltage can be increased or decreased to the positive

or negative rails without gain foldover.