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AD8337

Rev. A | Page 19 of 28

The gain scaling factor (gain slope) is designed for 20 dB/V; this

relatively low slope ensures that noise on the GAIN input is not

unduly amplified. Since a VGA functions as a multiplier, it is

important to make sure that the GAIN input does not inadver-

tently modulate the output signal with unwanted noise on the

gain control pin. Because of its high input impedance, a simple

low-pass filter can be added to the GAIN input to filter

unwanted noise.

OUTPUT STAGE

The output stage is a Class AB, voltage-feedback, complemen-

tary, emitter-follower with a fixed gain of 18 dB, similar to the

preamplifier in speed and bandwidth. Because of the ac-beta

roll-off of the output devices and the inherent reduction in

feedback beyond the −3 dB bandwidth, the impedance looking

into the output pin of the preamp and output stages appears to be

inductive (increasing impedance with increasing frequency).

The high speed output amplifier used in the AD8337 can drive

large currents, but its stability is susceptible to capacitive

loading. A small series resistor mitigates the effects of

capacitive loading (see the Applications section).

ATTENUATOR

The input resistance of the VGA attenuator is nominally 265 Ω.

Assuming the default preamplifier feedback network RFB1 + RFB2

is 200 Ω, the effective preamplifier load is about 114 Ω. The

attenuator is composed of eight 3.01 dB sections for a total

attenuation range of −24.08 dB. Following the attenuator is a

fixed gain amplifier with 8× (18.06 dB) gain. Because of this

relatively low gain, the output offset is kept well below 20 mV

over temperature; the offset is largest at maximum gain when

the preamplifier offset is amplified. The VCOM pin defines the

common-mode reference for the output as seen in Figure 65.

SINGLE-SUPPLY OPERATION AND AC COUPLING

If the AD8337 is to be operated from a single 5 V supply,

the bias supply for VCOM must be a very low impedance

2.5 V reference, especially if dc coupling is used. If the device

is dc-coupled, the VCOM source must be able to handle the

preamplifier and VGA dynamic load currents in addition to

the bias currents.

When ac coupling the preamplifier input, a bias network and

bypass capacitor must be connected to the opposite polarity

input pin. The bias generator for Pin VCOM must provide the

dynamic current to the preamplifier feedback network and the

VGA attenuator. For many single 5 V applications, a reference

such as the ADR43 and a good op amp provide an adequate

VCOM source if a 2.5 V supply is unavailable.

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