AD8336ACPZ-R7 datasheet(22/27 Pages) AD

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AD8336

Data Sheet

Rev. F | Page 22 of 27

APPLICATIONS INFORMATION

AMPLIFIER CONFIGURATION

The AD8336 amplifiers can be configured in various options. In

addition to the 60 dB gain range variable gain stage, an uncommit-

ted voltage gain amplifier is available to the user as a preamplifier.

The preamplifier connections are separate to enable noninverting

or inverting gain configurations or the use of interstage filtering.

The AD8336 can be used as a cascade connected VGA with pre-

amp input, as a standalone VGA, or as a standalone preamplifier.

This section describes some of the possible applications.

VOUT

VGAI

PRAO

GNEG

AD8336

VCOM

VPOS

GPOS

34dB

PWRA

ATTENUATOR

–60dB TO 0dB

GAIN CONTROL

INTERFACE

INPP 4

INPN

BIAS

VNEG

–

PREAMP

Figure 79. Application Block Diagram

PREAMPLIFIER

While observing just a few constraints, the uncommitted voltage

feedback preamplifier of the AD8336 can be connected in a variety

of standard high frequency operational amplifier configurations.

The amplifier is optimized for a gain of 4× (12 dB) and has a gain

bandwidth product of 600 MHz. At a gain of 4×, the bandwidth

is 150 MHz. The preamplifier gain can be adjusted to a minimum

gain of 2×; however, there will be a small peak in the response at

high frequencies. At higher preamplifier gains, the bandwidth

diminishes proportionally in conformance to the classical voltage

gain amplifier GBW relationship.

While setting the overall gain of the AD8336, the user must

consider the input-referred offset voltage of the preamplifier.

Although the offset of the attenuator and postamplifier are

almost negligible, the preamplifier offset voltage, if uncorrected,

is increased by the combined gain of the preamplifier and post-

amplifier. Therefore, for a maximum gain of 60 dB, an input offset

voltage of only 200 μV results in an error of 200 mV at the output.

Circuit Configuration for Noninverting Gain

The noninverting configuration is shown in Figure 80. The

preamplifier gain is described by the classical operational

amplifier gain equation:





Gain

The practical gain limits for this amplifier are 6 dB to 26 dB.

The gain bandwidth product is about 600 MHz, so at 150 MHz,

the maximum achievable gain is 12 dB (4×). The minimum gain

is established internally by fixed loop compensation and is 6 dB

(2×). This amplifier is not designed for unity-gain operation.

Table 5 shows the gain and bandwidth for the noninverting gain

configuration.

PRAO

34dB

AD8336

PREAMPLIFIER

–60dB TO 0dB

INPP

INPN

GAIN = 12dB

RFB1

100Ω

VGAI

VPOS

VNEG

+5V

–5V

PWRA

VCOM

RFB2

301Ω

VOUT

Figure 80. Circuit Configuration for Noninverting Gain

The preamplifier output reliably sources and sinks currents up

to 50 mA. When using ±5 V power supplies, the suggested sum

of the output resistor values is 400 Ω total for the optimal trade-

off between distortion and noise. Much of the low gain value

device characterization was performed with resistor values of

301 Ω and 100 Ω, resulting in a preamplifier gain of 12 dB (4×).

With supply voltages between ±5 V and ±12 V, the sum of the

output resistance must be increased accordingly; a total

resistance of 1 kΩ is recommended. Larger resistance values,

subject to a trade-off in higher noise performance, can be used

if circuit power and load driving is an issue. When considering

the total power dissipation, remember that the input ladder

resistance of the VGA is part of the preamplifier load.

Table 5. Gain and Bandwidth for Noninverting Preamplifier

Configuration

Preamplifier Gain

Preamplifier

BW (MHz)

Composite

Gain (dB)

Numerical

4×

150

−14 to +46

8×

−8 to +52

16×

−2 to +58

20×

0 to +60

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