Equation 6

:

The AD8367S can be configured either as a VGA whose gain is controlled externally through the GAIN pin or as an

AGC amplifier, using a supply voltage of 3V to 5.25V. The supply to the VPSO and VPSI pins should be decoupled

using a low inductance, 0.1uF surface-mount, ceramic capacitor as close as possible to the device. Additional supply

decoupling can be provided by small series resistors. A 10 nF capacitor from the DECL pin to the OCOM pin is

recommended to decouple the output reference voltage.

The

AD8367S is designed to operate in a 200 Ω impedance system. The output amplifier is a low output impedance

voltage buffer with a 50 Ω damping resistor to desensitize it from load reactance and parasitics. The quoted

performance includes the volt

age division between the 50 Ω resistor and the 200 Ω load. The AD8367S can be

reactively matched to an impedance other than 200 Ω by using traditional step-up and step-down matching networks

or high quality transformers.

In some applications, the printed circuit board (PCB) parasitics, in combination with the source impedance presented

by the driving stage, can present some troublesome impedance at high frequency and can potentially un-stabilize the

amplifier under certain extreme conditions, such as high gain at high temperature. To address such scenarios, it is

recommended to include a series inductor on the INPT pin as close to the device as possible. This inductor forms a

snubbing network to choke out high frequencies from entering the device. A value of 10 nH or higher is recommended

to minimize high frequency energy on the INPT pin. This inductor also minimizes the negative impact due to reflective

source conditions at high RF frequencies to ensure the amplifier operates unconditionally stable to maintain typical

device performance.

It is also recommended that stitching be used to tie ground planes together around input and output signal traces, and

under the AD8367S. This stitching forms a low impedance ground plane to ensure specified electrical performance

and to reduce thermal resistance.

The AD8367S is a general-purpose VGA suitable for use in a wide variety of applications where voltage control of

gain is needed. While having a 500 MHz bandwidth, its use is not limited to high frequency signal processing. Its

accurate, temperature- and supply-stable linear-in-dB scaling is valuable wherever it is important to have a more

dependable response to the control voltage that is usually offered by VGAs of this sort. For example, there is no

preclusion to its use in speech bandwidth systems.

frequency of the signal path and is associated with the offset control loop that eliminates the inherent variation in the

internal dc balance of the signal path as the gain is varied (offset ripple). This frequency should be chosen to be about

a decade below the lowest frequency component of the signal. If made much lower than necessary, the offset loop is

even when the output is ac-coupled because of the potential reduction of the upper and lower voltage range at this

pin.

However, in many applications these components are unnecessary because an internal network provides a default

downward with increasing capacitance. Figure 9 shows representative response curves for the indicated component

values.