OPA682 datasheet(13/21 Pages) TI1

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OPA682

APPLICATIONS INFORMATION

WIDEBAND BUFFER OPERATION

The OPA682 gives the exceptional AC performance of a

wideband current feedback op amp with a highly linear, high

power output stage. It features internal RF and RG resistors

which make it easy to select a gain of +2, +1 or –1 without

any external resistors. Requiring only 6mA quiescent cur-

rent, the OPA682 will swing to within 1V of either supply

rail and deliver in excess of 135mA guaranteed at room

temperature. This low output headroom requirement, along

with supply voltage independent biasing, gives remarkable

single (+5V) supply operation. The OPA682 will deliver

greater than 200MHz bandwidth driving a 2Vp-p output into

100

Ω on a single +5V supply. Previous boosted output stage

amplifiers have typically suffered from very poor crossover

distortion as the output current goes through zero. The

OPA682 achieves a comparable power gain with much

better linearity. The primary advantage of a current feedback

op amp over a voltage feedback op amp is that AC perfor-

mance (bandwidth and distortion) is relatively independent

of signal gain.

Figure 1 shows the DC coupled, gain of +2, dual power

supply circuit configuration used as the basis of the

±5V

Specifications and Typical Performance Curves. For test

purposes, the input impedance 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 speci-

fications 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

|| 800

Ω = 89Ω. The disable control line (DIS) is typically left

open to guarantee normal amplifier operation. In addition to

the usual power supply decoupling capacitors to ground, a

0.1

µF capacitor can be included between the two power

supply pins. This optional added capacitor will typically

improve the 2nd harmonic distortion performance by 3dB to

6dB.

FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifi-

cation and Test Circuit.

FIGURE 2. AC-Coupled, G = +2, Single Supply Specifica-

tion and Test Circuit.

Figure 2 shows the AC coupled, gain of +2, single-supply

circuit configuration used as the basis of the +5V Specifica-

tions and Typical Performance Curves. Though not a “rail-

to-rail” design, the OPA682 requires minimal input and

output voltage headroom compared to other very wideband

current feedback op amps. It will deliver a 3Vp-p output

swing on a single +5V supply with greater than 150MHz

bandwidth. 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 2 establishes an input midpoint

bias using a simple resistive divider from the +5V supply

(two 806

Ω resistors). The input signal is then AC-coupled

into this midpoint voltage bias. The input voltage can swing

to within 1.5V of either supply pin, giving a 2Vp-p input

signal range centered between the supply pins. The input

impedance matching resistor (57.6

Ω) used for testing is

adjusted to give a 50

Ω input match when the parallel

combination of the biasing divider network is included. The

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

gain of +1—which puts the input DC bias voltage (2.5V) on

the output as well. Again, on a single +5V supply, the output

voltage can swing to within 1V of either supply pin while

delivering more than 80mA output current. A demanding

100

Ω load to a midpoint bias is used in this characterization

circuit. The new output stage used in the OPA682 can

deliver large bipolar output currents into this midpoint load

with minimal crossover distortion, as shown by the +5V

supply, 3rd harmonic distortion plots.

SINGLE-SUPPLY A/D CONVERTER INTERFACE

Most modern, high performance A/D converters (such as the

Burr-Brown ADS8xx and ADS9xx series) operate on a

single +5V (or lower) power supply. It has been a consider-

able challenge for single-supply op amps to deliver a low

distortion input signal at the ADC input for signal frequen-

OPA682

+5V

–5V

Ω Load

Ω

Ω Source

400

Ω

400

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

DIS

OPA682

+5V

DIS

S/2

806

Ω

100

Ω

806

Ω

400

Ω

400

Ω

0.1

µF

0.1

µF

6.8

µF

0.1

µF

Ω Source

57.6

Ω

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