AD8067

Rev. 0 | Page 16 of 24

For inverting voltage gain applications, the source impedance of the

input signal must be considered because that will set the applica-

tion’s noise gain as well as the apparent closed-loop gain. The basic

frequency equation for inverting applications is below.

SIGNAL SOURCE

–

+

–

+

–

+

S

G

F

S

G

R

R

R

R

R

(GBP)

Frequency

dB

–

Loop

Closed

+

+

+

×

=

3

S

G

F

R

R

R

–

Gain

DC

+

=

Figure 43. Input and Board Capacitances

signal source resistance.

There will be a pole in the feedback loop response formed by the

and the sum of the amplifier’s differential input capacitance,

common-mode input capacitance, and any board parasitic

capacitance. This will decrease the loop phase margin and can

cause stability problems, i.e., unacceptable peaking and ringing

in the response. To avoid this problem it is recommended that the

all wideband voltage gain applications.

S

G

S

G

F

R

R

R

R

R

Gain

Noise

ion

Configurat

Inverting

+

+

+

=

It is important that the noise gain for inverting applications be kept

above 6 for stability reasons. If the signal source driving the inverter

is another amplifier, take care that the driving amplifier shows low

output impedance through the frequency span of the expected

closed-loop bandwidth of the AD8067.

Matching the impedances at the inputs of the AD8067 is also

recommended for wideband voltage gain applications. This will

minimize nonlinear common-mode capacitive effects that can

significantly degrade settling time and distortion performance.

Resistor Selection for Wideband

Operation

Voltage feedback amplifiers can use a wide range of resistor values

to set their gain. Proper design of the application’s feedback

network requires consideration of the following issues:

The AD8067 has a low input voltage noise of 6.6 nV/√Hz. Source

increase the apparent Referred to Input (RTI) voltage noise of the

application.

• Poles formed by the amplifier’s input capacitances with the

resistances seen at the amplifier’s input terminals

• Effects of mismatched source impedances

The amplifier must supply output current to its feedback network,

as well as to the identified load. For instance, the load resistance

low distortion output current. Heavy output drive requirements

also increase the part’s power dissipation and should be taken

into account.

• Resistor value impact on the application’s output

voltage noise

• Amplifier loading effects

in Figure 43. The source impedance driving the positive input of a

noninverting buffer will form a pole primarily with the amplifier’s

common-mode input capacitance as well as any parasitic

obtainable bandwidth. For G = +10 buffers, this bandwidth limit