AD8336ACPZ-R7 datasheet(20/27 Pages) AD

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AD8336

Data Sheet

Rev. F | Page 20 of 27

THEORY OF OPERATION

OVERVIEW

The AD8336 is the first VGA designed for operation over

exceptionally broad ranges of temperature and supply voltage.

The performance has been characterized from temperatures

extending from −55°C to +125°C, and supply voltages from ±3 V

to ±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

operational amplifier can be used in noninverting and inverting

configurations and functions as a preamplifier to the variable gain

amplifier (VGA). If desired, the preamplifier 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

RFB2

301Ω

4.48kΩ

34dB

12dB

91.43Ω

*OPTIONAL DEPEAKING CAPACITOR. SEE TEXT.

RFB1

100Ω

1.28kΩ

–

PREAMP

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 can

exceed safe operating limits for the amplifiers and, therefore,

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 because the power can

exceed safe limits (see the Absolute Maximum Ratings section).

Because harmonic distortion products can 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: −14 dB to +46 dB and 0 dB to 60 dB.

Since the VGA gain is fixed at 34 dB (50×), the preamplifier

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

With low preamplifier gains between 2× and 4×, it can 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 600); therefore,

the bandwidth decreases as the gain is increased beyond the

nominal gain value of 4×. For example, if the preamplifier gain

is increased to 20×, the bandwidth reduces by a factor of 5 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 preamplifier 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 increases. For the

previous example, the input-referred noise increases by a factor

of 4/3.

The output swing of the preamplifier is the same as for the VGA.

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 preamplifier gain is 4× (12 dB). The intercept changes

with the preamplifier gain; for example, when the preamplifier

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 preamplifier 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.

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