OPA682 datasheet(13/33 Pages) TI2

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OPA692

SBOS236E

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND BUFFER OPERATION

The OPA692 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

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

external resistors. Requiring only 5.1mA quiescent current, the

OPA692 will swing to within 1V of either supply rail and deliver

in excess of 160mA at room temperature. This low output

headroom requirement, along with supply voltage indepen-

dent biasing, gives remarkable single (+5V) supply operation.

The OPA692 will deliver greater than 200MHz bandwidth

driving a 2VPP output into 100Ω on a single +5V supply.

Previous boosted output stage amplifiers have typically suf-

fered from very poor crossover distortion as the output current

goes through zero. The OPA692 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 performance (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 Electrical and

Typical Characteristics. For test purposes, the input imped-

ance 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 specifications 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 || 804Ω = 89Ω. The disable control

line (DIS) is typically left open to ensure 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 2 shows the AC-coupled, gain of +2, single-supply

circuit configuration used as the basis of the +5V Electrical

and Typical Characteristics. Though not a

rail-to-rail design,

the OPA692 requires minimal input and output voltage head-

room compared to other very wideband current-feedback op

amps. It will deliver a 3VPP output swing on a single +5V

supply with greater than 150MHz bandwidth. The key re-

quirement 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 2VPP 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 in-

cluded. 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 120mA 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 OPA692 can

deliver large bipolar output currents into this midpoint load

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

supply, 3rd-harmonic distortion typical characteristics.

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

tion and Test Circuit.

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

and Test Circuit.

OPA692

+5V

–5V

Ω Load

Ω

Ω Source

402

Ω

402

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

DIS

SINGLE-SUPPLY ADC INTERFACE

Most modern, high-performance ADCs (such as the Texas

Instruments ADS8xx and ADS9xx series) operate on a single

+5V (or lower) power supply. It has been a considerable

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

tion input signal at the ADC input for signal frequencies

OPA692

+5V

DIS

S/2

806

Ω

100

Ω

806

Ω

402

Ω

402

Ω

0.1

µF

0.1

µF

6.8

µF

0.1

µF

Ω Source

57.6

Ω

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