ADA4859-3 Analog Devices, ADA4859-3 Datasheet - Page 13

• Manufacturer Part Number: ADA4859-3
• Description: Single-Supply, Fixed G = 2, High Speed, Video Amplifier with Charge Pump
• Manufacturer: Analog Devices

Specifications of ADA4859-3

• -3db Bandwidth: 265MHz
• Slew Rate: 740V/µs
• Vos: n/a
• Ib: 700nA
• # Opamps Per Pkg: 3
• Input Noise (nv/rthz): 17nV/rtHz
• Vcc-vee: 3V to 5.5V
• Isy Per Amplifier: n/a
• Packages: CSP

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.
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.
which simplifies to V
V
OUT
=
V
V
IN
IN
⎛ −
⎜
⎜
⎝
Figure 32. Unity Gain of Option 1
R
R
R
OUT
G
T
F
⎞
⎟
⎟
⎠
= V
+
V
GAIN OF +1
IN
IN
.
⎛
⎜
⎜
⎝
+V
R
F
S
0.1µF
R
10µF
+
G
R
G
⎞
⎟
⎟
⎠
V
OUT
Rev. 0 | Page 13 of 16
(1)
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.
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
–6
–9
3
0
1
V
V
R
V
S
OUT
L
IN
V
= 5V
= 100Ω
IN
Figure 34. Inverting Configuration (G = −1)
Figure 35. Large Signal, G = +1 and G = −1
= 2V p-p
R
Figure 33. Unity Gain of Option 2
T
R
T
10
FREQUENCY (MHz)
R
G
GAIN OF +1
GAIN OF –1
R
F
+V
OPTION 2
S
+V
G = +1,
0.1µF
10µF
S
0.01µF
10µF
100
G = –1
V
ADA4859-3
OUT
V
OUT
OPTION 1
G = +1,
1000