Datasheet

ADA4858-3 Data Sheet
Rev. B | Page 14 of 20
APPLICATIONS INFORMATION
GAIN CONFIGURATIONS
The ADA4858-3 is a single-supply, high speed, voltage feedback
amplifier. Table 5 provides a convenient reference for quickly
determining the feedback and gain set resistor values and band-
width for common gain configurations.
Table 5. Recommended Values and Frequency Performance
1
Gain R
F
(Ω) R
G
(Ω)
Small Signal
−3 dB BW (MHz)
Large Signal 0.1 dB
Flatness (MHz)
1 402 N/A 600 88
2 301 301 350 85
5
200
40
160
35
1
Conditions: V
S
= 5 V, T
A
= 25°C, R
L
= 150 Ω.
Figure 38 and Figure 39 show the typical noninverting and
inverting configurations and the recommended bypass
capacitor values.
07714-139
+V
S
V
IN
V
OUT
ADA4858-3
+
–
10µF
0.1µF
R
G
R
F
Figure 38. Noninverting Gain Configuration
07714-140
+V
S
V
IN
V
OUT
ADA4858-3
–
+
10µF
0.1µF
R
G
R
F
Figure 39. Inverting Gain Configuration
DC-COUPLED VIDEO SIGNAL
The ADA4858-3 does not have a rail-to-rail output stage. The
output can be within 1 V of the rails. Having a charge pump on
board that can provide −3 V on a +5 V supply and −2 V on
+3.3 V supply makes this part excellent for video applications. In
dc-coupled applications, the black color has a 0 V voltage reference.
This means that the output voltage should be able to reach 0 V,
which is feasible with the presence of the charge pump. Figure 40
shows the schematic of a dc-coupled, single-supply application.
It is similar to the dual-supply application in which the input is
properly terminated with a 50 Ω resistor to ground. The amplifier
itself is set at a gain of 2 to account for the input termination loss.
The choice of R
F
and R
G
should be carefully considered for
maximum flatness vs. power dissipation trade-off. In this case, the
flatness is over 90 MHz, which is more than the high definition
video requirement.
07714-141
5V
V
IN
V
OUT
ADA4858-3
U1
+
–
C1
10µF
C2
0.1µF
R4
75Ω
R3
249Ω
R5
75Ω
R2
249Ω
R1
75Ω
–V
S
Figure 40. DC-Coupled, Single-Supply Schematic
MULTIPLE VIDEO DRIVER
In applications requiring that multiple video loads be driven
simultaneously, the ADA4858-3 can deliver 5 V supply operation.
Figure 41 shows the ADA4858-3 configured with two video
loads, and Figure 42 shows the two video load performances.
07714-142
+V
S
ADA4858-3
–
+
10µF
0.1µF
R
G
301Ω
R
F
301Ω
75Ω
CABLE
V
IN
75Ω
V
OUT
1
75Ω
75Ω
CABLE
75Ω
V
OUT
2
75Ω
75Ω
CABLE
75Ω
Figure 41. Video Driver Schematic for Two Video Loads
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
1 10 100 1000
CLOSED-LOOP GAIN (dB)
FREQUENCY (MHz)
R
L
= 150Ω
R
L
= 75Ω
V
S
= 5V
R
F
= 301Ω
G = 2
V
OUT
= 2V p-p
07714-040
Figure 42. Large Signal Frequency Response for Various Loads