OPA684IDBVT datasheet(12/25 Pages) TI

12 / 25 page

OPA684

SBOS219A

www.ti.com

APPLICATIONS INFORMATION

LOW-POWER, CURRENT-FEEDBACK OPERATION

The OPA684 gives a new level of performance in low-power,

current-feedback op amps. Using a new input stage buffer

architecture, the OPA684 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 perfor-

mance 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 operation is also

supported with similar bandwidths but with reduced output

power capability. For lower quiescent power in a CFBplus

amplifier, consider the OPA683, while for higher output

power, consider the OPA691.

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 test purposes, the input imped-

ance 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 characteristics 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 || 2000Ω = 95Ω. Gain changes are

most easily accomplished by simply resetting the RG value,

holding RF constant at its recommended value of 1kΩ. The

disable control line (DIS) is typically left open to give normal

amplifier operation. It may, however, be asserted LOW to

reduce the amplifier supply current to 100

µA typically.

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

supply circuit used as the basis of the Inverting Typical

Characteristics. Inverting operation offers several perfor-

mance benefits. Since there is no common-mode signal

across the input stage, the slew rate for inverting operation

is 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

achieve the desired gain, while RM is also adjusted to hold a

Ω input match. A point will be reached where R

G will equal

Ω, R

M is removed, and the input match is set by RG only.

With RG fixed to achieve an input match to 50Ω, increasing

RF will increase the gain. This will, however, quickly reduce

the achievable bandwidth as the feedback resistor increases

from its recommended value of 1k

Ω. If the source does not

require an input match of 50

Ω, either adjust R

M to the get the

desired load, or remove it and let the RG resistor alone

provide the input load.

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 OPA684 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 OPA684 is reduced.

Ω

OPA684

+5V

–5V

DIS

Ω

Ω Source

Ω Load

0.1

µF

6.8

µF

0.1

µF

6.8

µF

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

fications and Test Circuit.

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

fication and Test Circuit.

Ω

OPA684

+5V

–5V

DIS

Ω

52.3

Ω

Ω Load

Ω Source

0.1

µF

6.8

µF

0.1

µF

6.8

µF

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

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

Electrical and Typical Characteristics. 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 non-inverting

input. The input signal is then AC coupled into this midpoint

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