OPA690 datasheet(16/23 Pages) TI

16 / 23 page

OPA691

SBOS226A

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As the desired signal gain increases, this equation will

eventually predict a negative RF. A somewhat subjective limit

to this adjustment can also be set by holding RG to a

minimum value of 20

Ω. Lower values will load both the buffer

stage at the input and the output stage if RF gets too low—

actually decreasing the bandwidth. Figure 8 shows the rec-

ommended RF versus NG for both ±5V and a single +5V

operation. The values for RF versus gain shown here are

approximately equal to the values used to generate the

Typical Characteristics. They differ in that the optimized

values used in the Typical Characteristics are also correcting

for board parasitics not considered in the simplified analysis

leading to Equation 3. The values shown in Figure 8 give a

good starting point for design where bandwidth optimization

is desired.

(e.g., integrators, transimpedance, and some filters) should

consider the unity-gain stable voltage feedback OPA680,

since the feedback resistor is the compensation element for a

current feedback op amp. Wideband inverting operation (and

especially summing) is particularly suited to the OPA691. See

Figure 9 for a typical inverting configuration where the I/O

impedances and signal gain from Figure 1 are retained in an

inverting circuit configuration.

OPA691

374

Ω

188

Ω

DIS

+5V

–5V

Ω

Ω Load

Power-supply

decoupling

not shown.

Ω

Source

68.1

Ω

FIGURE 9. Inverting Gain of –2 with Impedance Matching.

FIGURE 8. Feedback Resistor vs Noise Gain.

600

500

400

300

200

100

Noise Gain

020

+5V

±5V

The total impedance going into the inverting input may be

used to adjust the closed-loop signal bandwidth. Inserting a

series resistor between the inverting input and the summing

junction will increase the feedback impedance (denominator

of Equation 2), decreasing the bandwidth. This approach to

bandwidth control is used for the inverting summing circuit on

the front page. The internal buffer output impedance for the

OPA691 is slightly influenced by the source impedance

looking out of the noninverting input terminal. High source

resistors will have the effect of increasing RI, decreasing the

bandwidth. For those single-supply applications which de-

velop a midpoint bias at the noninverting input through high

valued resistors, the decoupling capacitor is essential for

power-supply noise rejection, noninverting input noise cur-

rent shunting, and to minimize the high frequency value for

RI in Figure 7.

INVERTING AMPLIFIER OPERATION

Since the OPA691 is a general-purpose, wideband current

feedback op amp, most of the familiar op amp application

circuits are available to the designer. Those applications that

require considerable flexibility in the feedback element

In the inverting configuration, two key design considerations

must be noted. The first is that the gain resistor (RG)

becomes part of the signal channel input impedance. If input

impedance matching is desired (which is beneficial when-

ever the signal is coupled through a cable, twisted-pair, long

PC board trace, or other transmission line conductor), it is

normally necessary to add an additional matching resistor to

ground. RG by itself is normally not set to the required input

impedance since its value, along with the desired gain, will

determine an RF which may be non-optimal from a frequency

response standpoint. The total input impedance for the

source becomes the parallel combination of RG and RM.

The second major consideration, touched on in the previous

paragraph, is that the signal source impedance becomes

part of the noise gain equation and will have slight effect on

the bandwidth through Equation 1. The values shown in

Figure 9 have accounted for this by slightly decreasing RF

(from Figure 1) to re-optimize the bandwidth for the noise

gain of Figure 9 (NG = 2.73) In the example of Figure 9, the

RM value combines in parallel with the external 50Ω source

impedance, yielding an effective driving impedance of

Ω || 68Ω = 28.8Ω. This impedance is added in series with

RG for calculating the noise gain—which gives NG = 2.73.

This value, along with the RF of Figure 9 and the inverting

input impedance of 35

Ω, are inserted into Equation 3 to get

a feedback transimpedance nearly equal to the 472

Ω opti-

mum value.

Note that the noninverting input in this bipolar supply invert-

ing application is connected directly to ground. It is often

suggested that an additional resistor be connected to ground

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