Datasheet

Data Sheet ADA4859-3
Rev. A | Page 13 of 16
APPLICATIONS INFORMATION
USING THE ADA4859-3 IN GAINS EQUAL TO +1, −1
The ADA4859-3 was designed to offer outstanding video
performance, simplify applications, and minimize board area.
The ADA4859-3 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.
Although the ADA4859-3 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 32. In this configuration, the −IN input
pin is tied to the output. (Feedback is provided through the two
internal 550 Ω resistors in parallel), and the input is applied to
the noninverting input. The noise gain for this configuration is 1.
0.1µF
10µF
V
IN
R
T
V
OUT
+V
S
GAIN OF +1
07715-130
Figure 32. Unity Gain of Option 1
Option 2
Another option exists for running the ADA4859-3 as a unity-
gain amplifier. In this configuration, the noise gain is +2, see
Figure 33. 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 33.








+
+








−
=
G
G
F
IN
G
F
IN
OUT
R
R
R
V
R
R
VV
(1)
which simplifies to V
OUT
= V
IN
.
0.1µF
V
IN
R
T
V
OUT
+V
S
GAIN OF +1
07715-131
10µF
R
F
R
G
Figure 33. 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 34.
0.01µF
V
IN
R
T
V
OUT
+V
S
GAIN OF –1
07715-132
10µF
Figure 34. Inverting Configuration (G = −1)
Figure 35 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 the G = +1 Option 2 has better flatness and no
peaking compared to Option 1.
3
–9
–6
–3
0
1 10 100 1000
CLOSED-LOOP GAIN (dB)
FREQUENCY (MHz)
G = +1,
OPTION 1
G = +1,
OPTION 2
G = –1
07715-031
V
S
= 5V
V
OUT
= 2V p-p
R
L
= 100Ω
Figure 35. Large Signal, G = +1 and G = −1