OPA680 datasheet(13/20 Pages) TI1

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OPA2682

APPLICATIONS INFORMATION

WIDEBAND BUFFER OPERATION

The OPA2682 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/ch

quiescent current, the OPA2682 will swing to within 1V of

either supply rail and deliver in excess of 135mA (guaran-

teed) at room temperature. This low output headroom re-

quirement, along with supply voltage independent biasing,

gives remarkable single (+5V) supply operation. The

OPA2682 will deliver greater than 200MHz bandwidth driv-

ing a 2Vp-p output into 100

Ω on a single +5V supply.

Previous current-boosted output stage amplifiers have typi-

cally suffered from very poor crossover distortion as the

output current goes through zero. The OPA2682 achieves a

comparable power gain with much better linearity. The

primary advantage of the internal current-feedback op amp

over a voltage-feedback op amp is that AC performance

(bandwidth and distortion) is relatively independent of sig-

nal 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 || 800Ω = 89Ω. The disable control line (DIS) is

typically left open (SO-14 only) to guarantee normal ampli-

fier 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 OPA2682 requires minimal input and

output voltage headroom compared to other fixed-gain buffer

amplifiers. 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 main-

tain 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. On a single +5V supply, the

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

while delivering more than 75mA output current. A demand-

ing 100

Ω load to a midpoint bias is used in this characteriza-

tion circuit. The new output stage used in the OPA2682 can

deliver large bipolar output currents into this midpoint load

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

supply, 3rd harmonic distortion plots.

HIGH SPEED INSTRUMENTATION AMPLIFIER

The front page shows an instrumentation amplifier based on

the OPA2682. The differential gain for this circuit is 2.0. The

inputs are high impedance, with only 1pF to ground at each

input. The loads on the OPA2682 outputs were made equal in

order to achieve the best harmonic distortion possible.

1/2

OPA2682

+5V

–V

–5V

Ω Load

Ω

Ω Source

400

Ω

400

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

1/2

OPA2682

+5V

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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