OPA684IDBVT datasheet(13/25 Pages) TI

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OPA684

SBOS219A

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voltage bias. The input voltage can swing to within 1.25V of

either supply pin, giving a 2.5Vp-p input signal range cen-

tered 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 R

M and drive directly into

the blocking capacitor. The source will then see the 5k

Ω load

of the biasing network. The gain resistor (RG) is AC coupled,

giving the circuit a DC gain of +1, which puts the non-

inverting input DC bias voltage (2.5V) on the output as well.

The feedback resistor value has been adjusted from the

bipolar supply condition to re-optimize for a flat frequency

response in +5V only, gain of +2, operation. On a single +5V

supply, the output voltage can swing to within 1.0V of either

supply pin while delivering more than 70mA output current

giving 3V output swing into 100

Ω (8dBm maximum at a

matched 50

Ω load). The circuit of Figure 3 shows a blocking

capacitor driving into a 50

Ω output resistor then into a 50Ω

load. Alternatively, the blocking capacitor could be removed

if the load is tied to a supply midpoint or to ground if the DC

current required by the load is acceptable.

The circuits of Figure 3 and 4 show single-supply operation

at +5V. These same circuits may be used up to single

supplies of +12V with minimal change in the performance of

the OPA684.

LOW-POWER VIDEO LINE DRIVER APPLICATIONS

For low-power, video line driving, the OPA684 provides the

output current and linearity to support multiple load compos-

ite video signals. Figure 5 shows a typical

±5V supply video

line driver application. The improved 2nd-harmonic distortion

of the CFBplus architecture, along with the OPA684’s high

output current and voltage, gives exceptional differential gain

and phase performance for a low-power solution. As the

Typical Characteristics show, a single video load shows a

dG/dP of 0.04%/0.02

°. Multiple loads may also be driven with

< 0.1%/0.1

° dG/dP for up to 4 parallel video loads, where the

amplifier is driving an equivalent load of 37.5

Ω.

1.3k

Ω

OPA684

+5V

DIS

Ω

Ω Load

Ω Source

0.1

µF

6.8

µF

10k

Ω

10k

Ω

1.3k

Ω

0.1

µF

0.1

µF

0.1

µF

FIGURE 3. Non-inverting Single-Supply Test and Character-

ization Circuit.

Figure 4 shows the AC coupled, single +5V supply, gain of

–1V/V circuit configuration used as a basis for the +5V

Typical Characteristics. In this case, the midpoint DC bias on

the non-inverting input is also decoupled with an additional

0.1

µF decoupling capacitor. This reduces the source imped-

ance at higher frequencies for the non-inverting input bias

current noise. This 2.5V bias on the non-inverting input pin

appears on the inverting input pin and, since RG is DC

blocked by the input capacitor, will also appear at the output

pin. One advantage to inverting operation is that since there

is no signal swing across the input stage, higher slew rates

and operation to even lower supply voltages is possible. To

retain a 1Vp-p output capability, operation down to a 3V

supply is allowed. At a +3V supply, the input stage is

saturated, but for the inverting configuration of a current

feedback amplifier, wideband operation is retained even

under this condition.

FIGURE 4. Inverting Single-Supply Test and Characteriza-

tion Circuit.

1.3k

Ω

OPA684

+5V

DIS

Ω

Ω Load

Ω Source

0.1

µF

0.1

µF

6.8

µF

1.3k

Ω

10k

Ω

10k

Ω

0.1

µF

0.1

µF

52.3

Ω

OPA684

+5V

DIS

–5V

Ω

Ω Load

Supply Decoupling not shown.

Coax

VIDEO

FIGURE 5. Gain of +2 Video Cable Driver.

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