AD8099ACP-R2 datasheet(16/28 Pages) AD

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AD8099

Rev. B | Page 16 of 28

APPLICATIONS

USING THE AD8099

The AD8099 offers unrivaled noise and distortion performance

in low signal gain configurations. In low gain configurations

(less than15), the AD8099 requires external compensation. The

amount of gain and performance needed will determine the

compensation network.

Understanding the subtleties of the AD8099 gives the user

insight on how to exact its peak performance. Use the

component values and circuit configurations shown in the

Applications section as starting points for designs. Specific

circuit applications will dictate the final configuration and value

of your components.

CIRCUIT COMPONENTS

The circuit components are referenced in Figure 59, the

recommended noninverting circuit schematic for the AD8099.

See Table 4 for typical component values and performance data.

AD8099

0.1

µF

0.1

µF

+VS

–VS

VOUT

DISABLE

VIN

Figure 59. Wideband Noninverting Gain Configuration (SOIC)

RF and RG

—

The feedback resistor and the gain set resistor

determine the noise gain of the amplifier; typical RF values

range from 250 Ω to 499 Ω.

—Creates a zero in the loop response to compensate the pole

created by the input capacitance (including stray capacitance)

and the feedback resistor RF. CF helps reduce high frequency

peaking and ringing in the closed-loop response. Typical range

is 0.5 pF to 1.5 pF for evaluation circuits used here.

—This resistor terminates the input of the amplifier to the

source resistance of the signal source, typically 50 Ω. (This is

application specific and not always required.)

—Many high speed amplifiers in low gain configurations

require that the input stage be terminated into a nominal

impedance to maintain stability. The value of RS should be kept

to 50 Ω or lower to maintain low noise performance. At higher

gains, RS may be reduced or even eliminated. Typical range is

0 Ω to 50 Ω.

—The compensation capacitor decreases the open-loop gain

at higher frequencies where the phase is degrading. By decreas-

ing the open-loop gain here, the phase margin is increased and

the amplifier is stabilized. Typical range is 0 pF to 5 pF. The

value of CC is gain dependent.

RC—

The series lead inductance of the package and the com-

pensation capacitance (CC) forms a series resonant circuit. RC

dampens this resonance and prevents oscillations. The

recommended value of RC is 50 Ω for a closed-loop gain of 2.

This resistor introduces a zero in the open-loop response and

must be kept low so that this zero occurs at a higher frequency.

The purpose of the compensation network is to decrease the

open-loop gain. If the resistance becomes too large, the gain will

be reduced to the resistor value, and not necessarily to 0 Ω,

which is what a single capacitor would do over frequency.

Typical value range is 0 Ω to 50 Ω.

—To lower the impedance of RC , C1 is placed in parallel with

RC. C1 is not required, but greatly reduces peaking at low

closed-loop gains. The typical value range is 0 pF to 2 pF.

C2 and C3

—Bypass capacitors are connected between both

supplies for optimum distortion and PSRR performance. These

capacitors should be placed as close as possible to the supply

pins of the amplifier. For C3, C5, a 0508 case size should be

used. The 0508 case size offers reduced inductance and better

frequency response.

C4 and C2

—Electrolytic bypass capacitors.

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