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 Performance1

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, TA = 25°C, RL = 150 Ω.

Figure 38 and Figure 39 show the typical noninverting and

inverting configurations and the recommended bypass

capacitor values.

+VS

VIN

VOUT

ADA4858-3

+

–

10µF

0.1µF

RG

RF

Figure 38. Noninverting Gain Configuration

+VS

VIN

VOUT

ADA4858-3

–

+

10µF

0.1µF

RG

RF

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 RF and RG 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.

5V

VIN

VOUT

ADA4858-3

U1

+

–

C1

10µF

C2

0.1µF

R4

75Ω

R3

249Ω

R5

75Ω

R2

249Ω

R1

75Ω

–VS

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.

+VS

ADA4858-3

–

+

10µF

0.1µF

RG

301Ω

RF

301Ω

75Ω

CABLE

VIN

75Ω

VOUT1

75Ω

75Ω

CABLE

75Ω

VOUT2

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

FREQUENCY (MHz)

RL = 150Ω

RL = 75Ω

VS = 5V

RF = 301Ω

G = 2

VOUT = 2V p-p

Figure 42. Large Signal Frequency Response for Various Loads