AD8336ACPZ-R7 datasheet(21/28 Pages) AD

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AD8336

Rev. 0 | Page 21 of 28

THEORY OF OPERATION

OVERVIEW

The AD8336 is the first VGA designed for operation over

exceptionally broad ranges of temperature and supply voltage.

Its performance has been characterized from temperatures

extending from −55°C to 125°C, and supply voltages from

±3 V

±12 V. It is ideal for applications requiring dc coupling, large

output voltage swings, very large gain ranges, extreme

temperature variations, or a combination thereof.

The simplified block diagram is shown in Figure 77. The

AD8336 includes a voltage feedback preamplifier, an amplifier

with a fixed gain of 34 dB, a 60 dB attenuator, and various bias

and interface circuitry. The independent voltage feedback op

amp can be used in noninverting and inverting configurations,

and functions as a preamplifier to the variable gain amplifier

(VGA). If desired, the op amp output (PRAO) and VGA input

(VGAI) pins provide for connection of an interstage filter to

eliminate noise and offset. The bandwidth of the AD8336 is dc

to 100 MHz with a gain range of 60 dB (−14 dB to +46 dB.)

For applications that require large supply voltages, a reduction

in power is advantageous. The power reduction pin (PWRA)

permits the power and bandwidth to be reduced by about half

in such applications.

VOUT

VGAI

PRAO

GNEG

VCOM

VPOS

GPOS

PWRA

–60dB TO 0dB

ATTENUATOR

AND

GAIN CONTROL

INTERFACE

BIAS

VNEG

INPP

INPN

RFB1

100Ω

4.48kΩ

34dB

12dB

–

PrA

91.43Ω

*OPTIONAL DEPEAKING CAPACITOR. SEE TEXT.

RFB2

301Ω

Figure 77. Simplified Block Diagram

To maintain low noise, the output stages of both the preamplifier

and the VGA are capable of driving relatively small load

resistances. However, at the largest supply voltages, the signal

current may exceed safe operating limits for the amplifiers and

the load current must not exceed 50 mA. With a ±12 V supply

and

±10 V output voltage at the preamplifier or VGA output,

load resistances as low as 200 Ω are acceptable.

For power supply voltages ≥ ±10 V, the maximum operating

temperature range is derated to +85°C, as the power may exceed

safe limits (see the Absolute Maximum Ratings section).

Since harmonic distortion products may increase for various

combinations of low impedance loads and high output voltage

swings, it is recommended that the user determine load and

drive conditions empirically.

PREAMPLIFIER

The gain of the uncommitted voltage feedback preamplifier is

set with external resistors. The combined preamplifier and VGA

gain is specified in two ranges, between −14 dB to +46 dB and

0 dB to 60 dB. Since the VGA gain is fixed at 34 dB (50×), the

preamp gain is adjusted for gains of 12 dB (4×) and 26 dB (200×).

With low preamplifier gains between 2× and 4×, it may be

desirable to reduce the high frequency gain with a shunt

capacitor across RFB2, to ameliorate peaking in the frequency

domain (see Figure 77). To maintain stability, the gain of the

preamplifier must be 6 dB (2×) or greater.

Typical of voltage feedback amplifier configurations, the gain-

bandwidth product of the AD8336 is fixed (at 400); thus, the

bandwidth decreases as the gain is increased beyond the nominal

gain value of 4×. For example, if the preamp gain is increased to

20×, the bandwidth reduces by a factor-of-five to about 20 MHz.

The −3 dB bandwidth of the preamplifier with a gain of 4× is

about 150 MHz, and for the 20× gain is about 30 MHz.

The preamp gain diminishes for an amplifier configured for

inverting gain, using the same value of feedback resistors as for

a noninverting amplifier, but the bandwidth remains unchanged.

For example, if the noninverting gain is 4×, the inverting gain is

−3×, but the bandwidth stays the same as in the noninverting

gain of 4×. However, because the output referred noise of the

preamplifier is the same in both cases, the input referred noise

increases as the ratio of the two gain values. For the previous

example, the input referred noise will increase by a factor of 4/3.

VGA

The architecture of the variable gain amplifier (VGA) section

of the AD8336 is based on the Analog Devices, Inc., X-AMP

(exponential amplifier), found in a wide variety of Analog

Devices variable gain amplifiers. This type of VGA combines a

ladder attenuator and interpolator, followed by a fixed-gain

amplifier.

The gain control interface is fully differential, permitting positive

or negative gain slopes. Note that the common-mode voltage of

the gain control inputs increases with increasing supply.

The gain slope is 50 dB/V and the intercept is 16.4 dB when the

nominal preamp gain is 4× (12 dB). The intercept changes with

the preamp gain; for example, when the preamp gain is set to

20× (26 dB) the intercept becomes 30.4 dB.

Pin VGAI is connected to the input of the ladder attenuator.

The ladder ratio is R/2R and the nominal resistance is 320 Ω. To

reduce preamp loading and large-signal dissipation, the input

resistance at Pin VGAI is 1.28 kΩ. Safe current density and

power dissipation levels are maintained even when large dc

signals are applied to the ladder.

The tap resistance of the resistors within the R/2R ladder is

640 Ω/3 or 213.3 Ω, the Johnson noise source of the attenuator.

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