ADA4891-1/ADA4891-2

Rev. A | Page 13 of 20

Gain flatness is an important specification in video applications.

It represents the maximum allowable deviation in the signal

amplitude within the pass band. Tests have revealed that the

human eye is unable to distinguish brightness variations of

less than 1%, which translates into a 0.1 dB signal drop within

the pass band, or put simply, 0.1 dB gain flatness.

The PCB layout configuration and bond pads of the chip often

contribute to stray capacitance. The stray capacitance at the

inverting input forms a pole with the feedback and gain resistor.

This additional pole adds phase shift and reduces phase margin

in the closed-loop phase response, causing instability in the

amplifier and peaking in the frequency response.

Figure 43 shows the effect of using various values of Feedback

and the input stray capacitance, shifts down in frequency and

interacts significantly with the internal poles of the amplifier.

–0.4

–0.3

–0.2

–0.1

0

0.1

0.2

1

0.1

10

100

FREQUENCY (MHz)

G = +2

Figure 43. Noninverting Configuration

To get the desired 0.1 dB bandwidth, adjust the feedback

peaking.

which cancels out the pole formed by the input stray capacitance

and the gain and feedback resistor. For a first pass in determining

where the original closed-loop frequency response of the amplifier

is restored as if there is no stray input capacitance. Most often,

Figure 44 shows the effect of using various values for the feedback

input stray capacitance, together with the board parasitics, is

approximately 2 pF.

0.3

0.2

0.1

0

0.1

0.2

0.1

1

10

100

FREQUENCY (MHz)

G = 2