OPA695IDBVT datasheet(13/32 Pages) TI

13 / 32 page

OPA695

SBOS293B

www.ti.com

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

fications and Test Circuit.

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

fications and Test Circuit.

APPLICATIONS INFORMATION

WIDEBAND CURRENT FEEDBACK OPERATION

The OPA695 gives a new level of performance in wideband

current feedback op amps. Nearly constant AC performance

over a wide gain range, along with 4300V/

µs slew rate, gives

a lower power and 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 450MHz bandwidth, applications

from gains of 1 to 40 can be supported. As a gain of +2

video line driver, the bandwidth extends to 1.4GHz with

a slew rate to support the highest pixel rates. At gains

above 20, the signal bandwidth starts to decrease, but still

exceeds 180MHz up to a gain of 40V/V (26dB to a matched

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

with similar bandwidths but reduced output power capability.

For lower speed (< 250MHz) requirements with higher output

powers, consider the OPA691.

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

supply circuit used as the basis of the

±5V Specifications and

Typical Characteristic 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 powers (dBm)

are 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 get normal

amplifier operation. The disable line must be asserted low to

shut off the OPA695. One optional component is included in

Figure 1. In addition to the usual power supply decoupling

capacitors to ground, a 0.01

µF capacitor is included between

the two power supply pins. In practical PC board layouts, this

optional added 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

Characteristic 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 improved.

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 (RF) value for bandwidth

(see the discussion in

Setting Resistor Values to Optimize

Bandwidth). The typical design sequence is to select the RF

value for best bandwidth, set RG for the gain, then set RT for

the desired input impedance. As the gain increases for the

inverting configuration, a point will be reached where RG will

equal 50

Ω, where R

T is removed and the input match is set

by RG only. With RG fixed to achieve an input match to 50Ω,

RF is simply increased, to increase gain. This will, however,

quickly reduce the achievable bandwidth, as shown by the

inverting gain of –16 frequency response in the Typical

Characteristic curves. For gains > 10V/V (14dB at the matched

load), noninverting operation is recommended to maintain

broader bandwidth.

OPA695

+5V

–V

DIS

–5V

Ω Load

Ω

562

Ω

54.9

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

Optional

0.01

µF

Ω Source

442

Ω

OPA695

+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

Optional

0.01

µF

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