OPA685 datasheet(12/31 Pages) TI2

12 / 31 page

OPA3684

SBOS241C

www.ti.com

APPLICATIONS INFORMATION

LOW-POWER, CURRENT-FEEDBACK OPERATION

The triple-channel OPA3684 gives a new level of perfor-

mance in low-power, current-feedback op amps. Using a

new input stage buffer architecture, the OPA3684 CFBPLUS

amplifier holds nearly constant AC performance over a wide

gain range. This closed-loop internal buffer gives a very low

and linearized impedance at the inverting node, isolating the

amplifier’s AC performance from gain element variations.

This allows both the bandwidth and distortion to remain

nearly constant over gain, moving closer to the ideal current-

feedback performance of gain bandwidth independence.

This low-power amplifier also delivers exceptional output

power—it’s

±4V swing on ±5V supplies with > 100mA output

drive gives excellent performance into standard video loads

or doubly-terminated 50

Ω cables. Single +5V supply opera-

tion is also supported with similar bandwidths but with re-

duced output power capability. For lower quiescent power in

a CFBPLUS amplifier, consider the OPA683 family; while for

higher output power, consider the OPA691 family.

Figure 1 shows the DC-coupled, gain of +2, dual power-

supply circuit used as the basis of the

±5V Electrical and

Typical Characteristics for each channel. For test purposes,

the input impedance is set to 50

Ω with a resistor to ground

and the output impedance is set to 50

Ω with a series output

resistor. Voltage swings reported in the Electrical Character-

istics are taken directly at the input and output pins while load

powers (dBm) are defined at a matched 50

Ω load. For

the circuit of Figure 1, the total effective load will be

100

Ω || 1600Ω = 94Ω. Gain changes are most easily accom-

plished by simply resetting the RG value, holding RF constant

at its recommended value of 800

Ω.

mode signal across the input stage, the slew rate for inverting

operation is typically higher and the distortion performance is

slightly improved. An additional input resistor, RM, is included

in Figure 2 to set the input impedance equal to 50

Ω. The

parallel combination of RM and RG set the input impedance.

As the desired gain increases for the inverting configuration,

RG is adjusted to achieved the desired gain, while RM is also

adjusted to hold a 50

Ω input match. A point will be reached

where RG will equal 50Ω, RM is removed, and the input match

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

Ω, increasing R

F will increase the gain. This will, however,

quickly reduce the achievable bandwidth as the feedback

resistor increases from its recommended value of 800

Ω. If

the source does not require an input match to 50

Ω, either

adjust RM to get the desired load, or remove it and let the RG

resistor alone provide the input load.

800

Ω

1/3

OPA3684

+5V

–5V

Ω

800

Ω

Ω Source

Ω Load

0.1

µF

6.8

µF

0.1

µF

6.8

µF

DIS

FIGURE 1. DC-Coupled, G = +2V/V, Bipolar Supply Speci-

fications and Test Circuit.

FIGURE 2. DC-Coupled, G = –1V/V, Bipolar Supply Specifi-

cations and Test Circuit.

Figure 2 shows the DC-coupled, gain of –1V/V, dual power-

supply circuit used as the basis of the Inverting Typical

Characteristics for each channel. Inverting operation offers

several performance benefits. Since there is no common-

These circuits show

±5V operation. The same circuits can be

applied with bipolar supplies from

±2.5V to ±6V. Internal

supply independent biasing gives nearly the same perfor-

mance for the OPA3684 over this wide range of supplies.

Generally, the optimum feedback resistor value (for nomi-

nally flat frequency response at G = +2) will increase in value

as the total supply voltage across the OPA3684 is reduced.

See Figure 3 for the AC-coupled, single +5V supply, gain of

+2V/V circuit configuration used as a basis for the +5V only

Electrical and Typical Characteristics for each channel. The

key requirement of broadband single-supply operation is to

maintain input and output signal swings within the usable

voltage ranges at both the input and the output. The circuit

of Figure 3 establishes an input midpoint bias using a simple

resistive divider from the +5V supply (two 10k

Ω resistors) to

the noninverting input. The input signal is then AC-coupled

into this midpoint voltage bias. The input voltage can swing

to within 1.25V of either supply pin, giving a 2.5Vp-p input

signal range centered between the supply pins. The input

impedance of Figure 3 is set to give a 50

Ω input match. If the

source does not require a 50

Ω match, remove this and drive

800

Ω

1/3

OPA3684

+5V

–5V

Ω

53.6

Ω

800

Ω

Ω Load

Ω Source

0.1

µF

6.8

µF

0.1

µF

6.8

µF

DIS

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