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AD8337

Rev. A | Page 18 of 28

THEORY OF OPERATION

VOUT

GAIN

18dB

(8X)

PRAO

VNEG

VCOM

–

749Ω

107Ω

PRA

6dB

GAIN

INTERFACE

BIAS

–

VPOS

INPP

INPN

INTERPOLATOR

ATTENUATOR

–24dB TO 0dB

–

RFB1 = RFB2 = 100Ω

RFB2

RFB1

Figure 65. Block Diagram and Pinout

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 ADI

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; the preamplifier is

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

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

using larger values of RFB2, trading off bandwidth and offset

voltage. The value of RFB2 should be ≥100 Ω, because it and

an internal 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

RFB1 is selected. For values of RFB1 = 20 Ω and RFB2 = 301 Ω,

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

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

gain range increases by 18 dB, so 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:

)

ICPT(

GAIN

19.7

)

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, then

ICPT increases to 18.65 dB. Although the above 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

a VGAIN change of 0.3 V is 6 dB using a slope of 20 dB/V and

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 pin VCOM (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 Pin VCOM determines the midpoint of the gain

range. For a ground referenced design, the VGAIN range is 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 the

VGAIN range, the gain error is typically less than ±0.2 dB. The

gain control voltage can be increased or decreased to the positive

or negative rails without gain foldover.

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