OPA683IDBVT datasheet(12/31 Pages) TI

12 / 31 page

OPA683

SBOS221E

www.ti.com

APPLICATIONS INFORMATION

VERY LOW POWER CURRENT-FEEDBACK

OPERATION

The OPA683 gives a new level of performance in very low

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

buffer architecture, the OPA683 CFBplus amplifier gives im-

proved bandwidth to higher gains than previous < 1mA

supply current amplifiers. 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 excep-

tional output power—its

±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 band-

widths, but reduced output power capability. For improved

harmonic distortion driving heavier loads, in a low power

CFBplus amplifier, consider the OPA684, while for even

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 Charac-

teristics and Typical Characteristics. For test purposes, the

input impedance is set to 50

Ω with a resistor to ground while

the output load is a 1k

Ω resistor. Voltage swings reported in

the specifications are taken directly at the input and output

pins while load powers (dBm) are interpreted as the voltage

swing at the output converted to dBm as if the load were

Ω. For the circuit of Figure 1, the total effective load will be

Ω || 2.4kΩ = 706Ω. Gain changes are most easily accom-

plished by simply resetting the RG value—holding RF con-

stant at its recommended value of 1.2k

Ω. The disable control

line (DIS) is typically left open to ensure normal amplifier

operation. It may, however, be asserted LOW to reduce the

amplifier quiescent 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 the 50

Ω. The parallel combina-

tion 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 and RM is also adjusted to hold

a 50

Ω input match. A point will be reached where R

G will

equal 50

Ω, R

M is then removed and the input match is set by

RG only. With RG fixed to achieve an input match to 50Ω, to

increase gain, RF is simply increased. This will, however,

quickly reduce the achievable bandwidth as the feedback

resistor increases from its recommended value of 1.2k

Ω. If

the source does not require an input match to 50

Ω, either

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

RG resistor alone provide the input load.

1.2k

Ω

OPA683

+5V

–5V

DIS

Ω

1.2k

Ω

G = 50Ω

0.1

µF

6.8

µF

0.1

µF

6.8

µF

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

fication and Test Circuit.

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

fication and Test Circuit.

1.2k

Ω

OPA683

+5V

–5V

DIS

52.3

Ω

1.2k

Ω

S = 50Ω

0.1

µF6.8µF

0.1

µF6.8µF

Ω

These circuits are showing

±5V operation. The same circuit

can be applied with bipolar supplies ranging from

±2.5V to

±6V. Internal supply independent biasing gives nearly the

same performance for the OPA683 over this wide range of

supplies. Generally, the optimum feedback resistor value (for

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

value as the total supply voltage across the OPA683 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 Characteristics 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 12.5k

Ω resistors)

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

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