ADA4856-3

Rev. 0 | Page 13 of 20

APPLICATIONS INFORMATION

USING THE ADA4856-3 IN GAINS EQUAL TO +1, −1

The ADA4856-3 was designed to offer outstanding video

performance, simplify applications, and minimize board area.

The ADA4856-3 is a triple amplifier with on-chip feedback

and gain set resistors. The gain is fixed internally at G = +2. The

inclusion of the on-chip resistors not only simplifies the design of

the application but also eliminates six surface-mount resistors,

saving valuable board space and lowering assembly costs.

Whereas the ADA4856-3 has a fixed gain of G = +2, it can be

used in other gain configurations, such as G = −1 and G = +1.

Unity-Gain Operation

Option 1

There are two options for obtaining unity gain (G = +1). The

first is shown in Figure 33. In this configuration, the –IN input

pin is tied to the output (feedback is now provided with the two

internal 402 Ω resistors in parallel), and the input is applied to

the noninverting input. The noise gain for this configuration is 1.

0.1µF

0.1µF

GAIN OF +1

10µF

10µF

Figure 33. Unity Gain of Option 1

Option 2

Another option exists for running the ADA4856-3 as a unity-

gain amplifier. In this configuration, the noise gain is +2, see

Figure 34. The frequency response and transient response for

this configuration closely match the gain of +2 plots because the

noise gains are equal. This method does have twice the noise gain

of Option 1; however, in applications that do not require low noise,

Option 2 offers less peaking and ringing. By tying the inputs

together, the net gain of the amplifier becomes 1. Equation 1 shows

the transfer characteristic for the schematic shown in Figure 34.

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛ −

=

G

G

F

IN

G

F

IN

OUT

R

R

R

V

R

R

V

V

(1)

0.1µF

0.1µF

GAIN OF +1

10µF

10µF

Figure 34. Unity Gain of Option 2

Inverting Unity-Gain Operation

In this configuration, the noninverting input is tied to ground

and the input signal is applied to the inverting input. The noise

gain for this configuration is +2, see Figure 35.

0.1µF

0.1µF

GAIN OF –1

10µF

10µF

Figure 35. Inverting Configuration (G = −1)

Figure 36 shows the small signal frequency response for both

gain of +1 (Option 1 and Option 2) and gain of −1 configurations.

It is clear that G = +1, Option 2 has better flatness and no peaking

compared to Option 1.

6

3

0

–3

–6

–9

–12

1

10

100

1000

FREQUENCY (MHz)

OPTION 1

G = +1

OPTION 2

G = +1

G = –1

Figure 36. G = +1 and G = −1