APPLICATION INFORMATION

WIDEBAND BUFFER OPERATION

1/3

OPA3695

+5V

-5V

50

Load

W

50W

50W

50

Source

W

R

G

R

F

+

6.8 F

m

0.1 F

m

+

6.8 F

m

0.1 F

m

V

I

V

O

DIS

OPA3695

www.ti.com ............................................................................................................................................... SBOS355A – APRIL 2008 – REVISED SEPTEMBER 2008

The OPA3695 gives the exceptional ac performance

of a wideband current-feedback op amp with a highly

linear output stage. Requiring only 12.9mA/channel

supply current, the OPA3695 achieves a 900MHz

small-signal bandwidth (G = +2V/V); the high slew

rate capability of up to 4300V/

µs supports a 600MHz

2VPP large signal into a 100Ω load. The low output

headroom of 1V from either supply in a very

high-speed amplifier gives very good single +5V

operation. The OPA3695 delivers a 2VPP swing with

greater than 400MHz bandwidth operating on a single

+5V

supply.

The

primary

advantage

of

a

current-feedback video buffer (as opposed to a

slew-enhanced, low-gain, stable voltage-feedback

implementation) is a higher slew rate with lower

quiescent power and output noise.

Figure 35. DC-Coupled, Noninverting,

Figure 35 shows the dc-coupled, noninverting, dual

Bipolar-Supply, Specification and Test Circuit

power-supply circuit configuration used as the basis

for the ±5V Electrical Characteristics table and

Typical Characteristics curves. For test purposes, the

Figure

36

illustrates

the

dc-coupled,

inverting

input impedance is set to 50

Ω with a resistor to

configuration used as the basis of the Inverting

ground; the output impedance is set to 50

Ω with a

Typical Characteristic curves. Inverting operation

series output resistor. Voltage swings reported in the

offers several performance benefits. Since there is no

specifications are taken directly at the input and

common-mode signal across the input stage, the slew

output pins while load powers (dBm) are defined at a

rate for inverting operation is higher and the distortion

matched 50

Ω load. For the circuit of Figure 35, the

performance is slightly improved. An additional input

total effective amplifier loading is 100

Ω || (RF + RG) .

resistor, RM, is included in Figure 36 to set the input

For example, with a gain of +2V/V with RF and RG

impedance equal to 50

Ω. The parallel combination of

equal to 604

Ω, the equivalent amplifier loading is

RM and RG sets the input impedance. Both the

100

Ω || 1208Ω = 92.3Ω. The disable control line

noninverting and inverting applications of Figure 35

(DIS) is typically left open to ensure normal amplifier

and Figure 36 benefit from optimizing the feedback

operation. Note that while most of the information

resistor (RF) value for bandwidth (see the discussion

presented in this data sheet was characterized with

in the Gain Setting section). The typical design

100

Ω loading, performance with a standard video

sequence

is

to

select

the

RF value for best

loading

of

150

Ω

has

negligible

impact

on

bandwidth, set RG for the gain, and then set RM for

performance.

Any

changes

in

performance

are

the desired input impedance. As the gain increases

typically improved over 100

Ω loading because of

for the inverting configuration, a point is reached

lower output current demands.

where RG equals 50Ω and RM is removed; thus, the

input match is set by RG only. With RG fixed to

achieve an input match to 50

Ω, RF is simply

increased to increase gain. This approach, however,

quickly reduces the achievable bandwidth at such

high

gains.

For

gains

greater

than

10V/V,

noninverting operation is recommended to maintain

broader bandwidth.

Copyright © 2008, Texas Instruments Incorporated

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