OPA685 datasheet(13/32 Pages) TI1

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OPA685

APPLICATIONS INFORMATION

WIDEBAND CURRENT-FEEDBACK OPERATION

The OPA685 gives a new level of performance in wideband

current-feedback op amps. Nearly constant AC performance

over a wide gain range, along with 4200V/

µs slew rate,

offers a lower power, lower cost solution for high intercept

IF amplifier requirements. While optimized at a gain of 8V/

V (12dB to a matched 50

Ω load) to give 400MHz band-

width, application from gains of 1 to 100 can be supported.

As a video line driver (gain of +2), the bandwidth extends to

900MHz, with a slew rate to support the highest pixel rates.

At gains above 20, the signal bandwidth starts to decrease

but still exceeds 50MHz up to a gain of 80V/V (32dB to a

matched 50

Ω load). Single +5V supply operation is also

supported with similar bandwidths, but reduced output power

capability. For lower speed (< 250MHz) requirements at

higher output power, consider the OPA681.

Figure 1 shows the DC-coupled, gain of +8V/V, dual power

supply circuit 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 specifications are

taken directly at the input and output pins, while load power

(dBm) is defined at a matched 50

Ω load. For the circuit of

Figure 1, the total effective load will be 100

Ω || 458Ω = 82Ω.

The disable control line (DIS) is typically left open to

guarantee normal amplifier operation. One optional compo-

nent is included in Figure 1. In addition to the usual power

supply de-coupling capacitors to ground, a 0.1

µF capacitor

is included between the two power supply pins. In practical

PC board layouts, this optional capacitor will typically

improve the 2nd harmonic distortion performance by 3dB to

6dB for bipolar supply operation.

Figure 2 shows the DC-coupled, gain of –8V/V, dual power

supply circuit used as the basis of the Inverting Typical

Performance Curves. Inverting operation offers several per-

formance benefits. Since there is no common-mode signal

across the input stage, the slew rate for inverting operation

is higher and the distortion performance is slightly im-

proved. An additional input resistor, RT, is included in

Figure 2 to set the input impedance equal to 50

Ω. The

parallel combination of RT and RG set the input impedance.

Both the non-inverting and inverting applications of Figures

1 and 2 will benefit from optimizing the feedback resistor

value for bandwidth (see the discussion in Setting Resistor

Values to Optimize Bandwidth). As the gain increases for

the inverting configuration, a point will be reached where

RG will equal 50Ω; RT is removed with the input match set

by RG only. With RG fixed to achieve an input match of

Ω, to increase gain, R

F is simply increased to get higher

gain. This will, however, quickly reduce the achievable

bandwidth as shown by the inverting gain of –16 frequency

response in the Typical Performance Curves. For gains >

12V/V (15.5dB at the matched load), non-inverting opera-

tion will give a higher bandwidth.

FIGURE 1. DC-Coupled, G = +8V/V, Bipolar Supply

Specifications and Test Circuit.

FIGURE 2. DC-Coupled, G = –8V/V, Bipolar Supply

Specifications and Test Circuit.

OPA685

+5V

DIS

–5V

Ω Load

Ω

Ω Source

56.2

Ω

402

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

0.1

µF

OPA685

+5V

–V

DIS

–5V

Ω Load

Ω

562

Ω

54.9

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

0.1

µF

Ω Source

442

Ω

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