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®

OPA687

APPLICATIONS INFORMATION

WIDEBAND, NON-INVERTING OPERATION

The OPA687 provides a unique combination of a very low

input voltage noise along with a very low distortion output

stage to give one of the highest dynamic range op amps

available. Its very high Gain Bandwidth Product (GBP) can

be used to either deliver high signal bandwidths at high

gains, or to deliver very low distortion signals at moderate

frequencies and lower gains. To achieve the full perfor-

mance of the OPA687, careful attention to PC board layout

and component selection is required as discussed in the

remaining sections of this data sheet.

Figure 1 shows the non-inverting gain of +20 circuit used as

the basis for most of the Typical Performance Curves. Most

of the curves were characterized using signal sources with

50

Ω driving impedance, and with measurement equipment

presenting a 50

Ω load impedance. In Figure 1, the 50Ω

ance of the test generator, while the 50

Ω series resistor at the

ment equipment load. Generally, data sheet voltage swing

output power specifications are at the matched 50

Ω load.

The total 100

Ω load at the output, combined with the 790Ω

total feedback network load, presents the OPA687 with an

effective output load of 89

Ω for the circuit of Figure 1.

Voltage feedback op amps, unlike current feedback designs,

can use a wide range of resistor values to set their gain. The

circuit of Figure 1, and the specifications at other gains, use

Using this guideline will guarantee that the noise added at the

output due to Johnson noise of the resistors will not signifi-

cantly increase the total over that due to the 0.95nV/

√Hz input

good starting point for design. Other values are certainly

acceptable if required by the design.

OPA687

+5V

–5V

–V

S

+V

S

50

Ω

V

O

V

I

50

Ω

+

0.1

µF

+

6.8

µF

6.8

µF

R

G

39.2

Ω

R

F

750

Ω

50

Ω Source

50

Ω Load

0.1

µF

FIGURE 1. Non-Inverting G = +20 Specifications and Test

Circuit.

WIDEBAND, INVERTING GAIN OPERATION

There can be significant benefits to operating the OPA687 as

an inverting amplifier. This is particularly true when a

matched input impedance is required. Figure 2 shows the

inverting gain circuit used as a starting point for the Typical

Performance Curves showing inverting mode performance.

OPA687

+5V

–5V

+V

S

–V

S

95.3

Ω

50

Ω

V

O

V

I

+

6.8

µF

0.1

µF

+

6.8

µF

0.1

µF

0.1

µF

R

F

2k

Ω

R

G

50

Ω

50

Ω Source

50

Ω Load

Driving this circuit from a 50

Ω source, and constraining the

as both the input termination resistor and the gain setting

resistor for the circuit. Although the signal gain for the

circuit of Figure 2 is double that for Figure 1, their noise

gains are equal when the 50

Ω source resistor is included.

This has the interesting effect of doubling the equivalent

GBP for the amplifier. This can be seen in comparing the

G = +12 and G = –20 small-signal frequency response

curves. Both show approximately 500MHz bandwidth with

3dB peaking, but the inverting configuration of Figure 2 is

giving 4.4dB higher signal gain. The noise gains are ap-

proximately equal in this case. If the signal source is

should be increased to be greater than the minimum value

get the desired gain. It is critical for stable operation of the

OPA687 that this driving amplifier show a very low output

impedance through frequencies exceeding the expected

closed-loop bandwidth for the OPA687.

WIDEBAND, HIGH SENSITIVITY,

TRANSIMPEDANCE DESIGN

The high Gain Bandwidth Product (GBP) and low input voltage

and current noise for the OPA687 make it an ideal wideband

transimpedance amplifier for low to moderate transimpedance

gains. Very high transimpedance gains (> 100k

Ω) will benefit

from the low input noise current of a FET-input op amp such

as the OPA655. Unity gain stability in the op amp is NOT

FIGURE 2. Inverting G = –40 Specifications and Test

Circuit.