OPA685 datasheet(13/31 Pages) TI2

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OPA3684

SBOS241C

www.ti.com

directly into the blocking capacitor. The source will then see

the 5k

Ω load of the biasing network as a load. The gain

resistor (RG) is AC-coupled, giving the circuit a DC gain of +1,

which puts the noninverting input DC bias voltage (2.5V) on

the output as well. The feedback resistor value has been

adjusted from the bipolar

±5V 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—easily giving a 3Vp-p output

swing into 100

Ω (8dBm maximum at the matched 50Ω 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 then 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 OPA3684.

FIGURE 3. AC-Coupled, G = +2V/V, Single-Supply Specifi-

cations and Test Circuit.

FIGURE 4. AC-Coupled, G = –1V/V, Single-Supply Specifi-

cations and Test Circuit.

FIGURE 5. Noninverting Differential I/O Amplifier.

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

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

+5V only Typical Characteristics for each channel. In this

case, the midpoint DC bias on the noninverting input is also

decoupled with an additional 0.1

µF capacitor. This reduces

the source impedance at higher frequencies for the

noninverting input bias current noise. This 2.5V bias on the

noninverting 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 opera-

tion 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 re-

tained even under this condition.

LOW-POWER, VIDEO LINE DRIVER APPLICATIONS

For low-power, video line driving, the OPA3684 provides the

output current and linearity to support 3 channels of either

single video lines, or up to 4 video lines in parallel on each

output. Figure 5 shows a typical

±5V supply video line driver

application where only one channel is shown and only a

single line is being driven. The improved 2nd-harmonic

distortion of the CFBPLUS architecture, along with the

OPA3684’s high output current and voltage, gives excep-

tional 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 be driven on each output, with minimal x-talk, while the

dG/dP is still < 0.1%/0.1

° for up to 4 parallel video loads. The

slew rate and gain of 2 bandwidth are also suitable to

moderate resolution RGB applications.

Ω

1/3

OPA3684

+5V

Ω

Ω Load

Ω Source

0.1

µF

6.8

µF

10k

Ω

10k

Ω

0.1

µF

0.1

µF

0.1

µF

DIS

1.0k

Ω

1/3

OPA3684

+5V

Ω

Ω Load

Ω Source

0.1

µF

0.1

µF

6.8

µF

1.0k

Ω

10k

Ω

10k

Ω

0.1

µF

0.1

µF

52.3

Ω

DIS

Ω

OPA3684

+5V

DIS

–5V

Ω

Ω Load

Supply decoupling not shown.

Coax

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