AD8337BCPZ-R2 datasheet(18/28 Pages) AD

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AD8337

Data Sheet

Rev. D | 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. Circuit 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

The uncommitted current feedback op amp included in the

AD8337 is 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. Current feedback amplifiers exhibit many

properties dissimilar from more familiar voltage feedback

amplifiers. One of the more significant differences is the

asymmetrical input impedances between inverting and non-

inverting inputs where the noninverting input impedance is

much higher. The practical effects of this difference are that

current feedback amplifiers are more commonly used in

noninverting gain applications and applications requiring

higher slew rates or bandwidths. For a description of these

current vs voltage feedback amplifiers properties, refer to

Section 1 of the Op Amp Applications Handbook, 2005.

The preamplifier gain is increased using larger values of RFB2,

trading off bandwidth and offset voltage. The value of RFB2 is to

be ≥100 Ω because the value and an internal compensation

capacitor determine 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 1.5 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)

19.7

(dB)

ICPT

GAIN

Gain















where the nominal intercept (ICPT) = 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

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 the 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 gain range. For a ground

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