OPA693IDBVT datasheet(14/32 Pages) TI1

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OPA693

SBOS285A

www.ti.com

APPLICATION INFORMATION

WIDEBAND BUFFER OPERATION

The OPA693 gives the exceptional AC performance of a

wideband current-feedback op amp with a highly linear

output stage. It features internal RF and RG resistors, making

it a simple matter to select a gain of +2, +1 or –1 with no

external resistors. Requiring only 13mA supply current, the

OPA693’s output swings to within 1V of either supply with

> 700MHz small signal bandwidth and > 300MHz delivering

7VPP into a 100Ω load. This low output headroom in a very

high-speed amplifier gives remarkable single +5V operation.

The OPA693 delivers 2VPP swing with > 500MHz bandwidth

operating on a single +5V supply. The primary advantage of

a current-feedback fixed gain video buffer, as opposed to a

slew-enhanced low-gain stable voltage-feedback implemen-

tation, is a higher slew rate with lower quiescent power and

output noise.

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

supply circuit configuration used as the basis for the

±5V

Electrical Characteristics table and Typical Characteristics

curves. For test purposes, the input impedance is set to 50

Ω

with a resistor to ground and the output impedance is set to

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 || 600Ω = 85.7Ω. 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 0.01

µF capacitor can be included between the two

power-supply pins. This optional added capacitor will typi-

cally improve the 2nd harmonic distortion performance by

3dB to 6dB.

Figure 2 shows the DC-coupled, gain of +1V/V buffer con-

figuration used as a starting point for the gain of +1V/V

Typical Characteristic curves. In this case, the inverting input

resistor, RG, is left open giving a very broadband gain of +1

performance. While the test circuit shows a 50

Ω input resis-

tor, a buffer application is typically transforming from a

source that cannot drive a heavy load to a 100

Ω load, such

as shown in Figure 2. The noninverting input impedance of

the OPA693 is typically 100k

Ω || 2pF. Driving directly into

the noninverting input will provide this very light load to the

source. However, the source must still provide the noninverting

input bias current required by the input stage to operate. An

alternative approach to a gain of +1 buffer is described in the

Wideband Unity Gain Buffers section of this data sheet.

OPA693

+5V

−5V

Ω Load

Ω

Ω Source

300

Ω

300

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

DIS

Figure 1. DC-Coupled, G = +2, Bipolar-Supply, Specification

and Test Circuit.

Figure 3 shows the DC-coupled, gain of –1V/V buffer con-

figuration used as a starting point for the gain of –1V/V

Typical Characteristic curves. The input impedance is set to

Ω using the parallel combination of an external 60.4Ω

resistor and the internal 300

Ω R

G resistor. The noninverting

input is tied directly to ground. Since the internal design for

the OPA693 is current-feedback, trying to get improved DC

accuracy by including a resistor on the noninverting input to

ground is ineffective. Using a direct short to ground on the

noninverting input reduces both the contribution of the DC

bias current and noise current to the output error. While the

external 60.4

Ω is used here to match to the 50Ω source from

the test equipment, the maximum input impedance in this

configuration is limited to the 300

Ω R

G resistor even with the

RM resistor removed. Unlike the noninverting unity gain

buffer application, removing RM does not strongly impact the

DC operating point because the short on the noninverting

input of Figure 3 provides the DC operating voltage. This

application of the OPA693 provides a very broadband, high-

output, signal inverter.

Figure 2. DC-Coupled, G = +1V/V, Bipolar-Supply, Specifica-

tion and Test Circuit.

OPA693

+5V

−5V

Open

Ω Load

Ω

Ω Source

300

Ω

300

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

DIS

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