9

REV. A

AD8079

–7–

FREQUENCY – Hz

10k

1G

100k

1M

10M

100M

100

10

1

0.1

0.01

POWER = 0dBm

(223.6mV rms)

Figure 21. Output Resistance vs. Frequency

–44.0

–66.5

125

–61.5

–64.0

–35

–55

–59.0

–56.5

–54.0

–51.5

–49.0

–46.5

105

85

65

45

25

5

–15

JUNCTION TEMPERATURE –

°C

–69.0

–PSRR

+PSRR

2V SPAN

CURVES ARE FOR WORST

CASE CONDITION WHERE

ONE SUPPLY IS VARIED

WHILE THE OTHER IS

HELD CONSTANT.

Figure 22. PSRR vs. Temperature

FREQUENCY – Hz

0

–4

–84

30k

500M

100k

1M

10M

100M

–14

–24

–64

–34

–44

–54

–74

–PSRR

+PSRR

Figure 23. PSRR vs. Frequency

THEORY OF OPERATION

The AD8079, a dual current feedback amplifier, is internally

configured for a gain of either +2 (AD8079A) or +2.2

(AD8079B). The internal gain-setting resistors effectively elimi-

nate any parasitic capacitance associated with the inverting in-

put pin, accounting for the AD8079’s excellent gain flatness

response. The carefully chosen pinout greatly reduces the cross-

talk between each amplifier. Up to four back-terminated 75

Ω

video loads can be driven by each amplifier, with a typical dif-

ferential gain and phase performance of 0.01%/0.17

°, respec-

tively. The AD8079B, with a gain of +2.2, can be employed as a

single gain-trimming element in a video signal chain. Finally,

the AD8079A/B used in conjunction with our AD8116 cross-

point matrix, provides a complete turn-key solution to video

distribution.

Printed Circuit Board Layout Considerations

As to be expected for a wideband amplifier, PC board parasitics

can affect the overall closed-loop performance. If a ground

plane is to be used on the same side of the board as the signal

traces, a space (5 mm min) should be left around the signal lines

to minimize coupling. Line lengths on the order of less than

5 mm are recommended. If long runs of coaxial cable are being

driven, dispersion and loss must be considered.

Power Supply Bypassing

Adequate power supply bypassing can be critical when optimiz-

ing the performance of a high frequency circuit. Inductance in

the power supply leads can form resonant circuits that produce

peaking in the amplifier’s response. In addition, if large current

transients must be delivered to the load, then bypass capacitors

(typically greater than 1

µF) will be required to provide the best

settling time and lowest distortion. A parallel combination of

4.7

µF and 0.1 µF is recommended. Some brands of electrolytic

capacitors will require a small series damping resistor

≈ 4.7 Ω

for optimum results.

DC Errors and Noise

There are three major noise and offset terms to consider in a

current feedback amplifier. For offset errors refer to the equa-

tion below. For noise error the terms are root-sum-squared to

give a net output error. In the circuit below (Figure 24) they are

× R

sequently multiplied by the noise gain always appears at the out-

× R

2 nV/

√Hz. At low gains though the inverting input current noise

vice matching contribute to better offset and drift specifications

for the AD8079 compared to many other current feedback am-

plifiers. The typical performance curves in conjunction with the

equations below can be used to predict the performance of the

AD8079 in any application.

R

F

R

I











R

F

R

I











where:

Ω, R

Ω for AD8079B