OPA684 datasheet(13/36 Pages) TI2

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OPA4684

SBOS240G

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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 3VPP output swing

into 100

Ω (8dBm maximum at the matched 50Ω load). The

circuit of Figure 3 shows a blocking capacitor driving into a

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.

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 1VPP 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.

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

cations and Test Circuit.

Ω

1/4

OPA4684

+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

1.3k

Ω

1/4

OPA4684

+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

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

cations and Test Circuit.

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 OPA4684.

FIGURE 5. Noninverting Differential I/O Amplifier.

800

Ω

800

Ω

1/4

OPA4684

–V

1/4

OPA4684

DIFFERENTIAL INTERFACE APPLICATIONS

Dual and quad op amps are particularly suitable to differential

input to differential output applications. Typically, these fall into

either analog-to-digital converter (ADC) input interface or line

driver applications. Two basic approaches to differential I/O

are noninverting or inverting configurations. Since the output is

differential, the signal polarity is somewhat meaningless—the

noninverting and inverting terminology applies here to where

the input is brought into the OPA4684. Each has its advan-

tages and disadvantages. Figure 5 shows a basic starting

point for noninverting differential I/O applications.

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