OPA691 datasheet(21/33 Pages) TI

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OPA2674

SBOS270A − AUGUST 2003 − REVISED MAY 2006

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supplies. Evaluating the denominator of Equation 15

(which is the feedback transimpedance) gives an optimal

target of 490

Ω. As the signal gain changes, the

contribution of the NG

× RI term in the feedback

transimpedance changes, but the total can be held

constant

by adjusting RF. Equation 16 gives an

approximate equation for optimum RF over signal gain:

+ 490 * NG

As the desired signal gain increases, this equation

eventually suggests 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 load both the buffer

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

low

actually decreasing the bandwidth. Figure 11 shows

the recommended RF versus NG for both ±6V 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 parasitic not considered in the

simplified analysis leading to Equation 16. The values

shown in Figure 11 give a good starting point for designs

where bandwidth optimization is desired.

600

500

400

300

200

Noise Gain

025

+5V

G =20Ω

±6V

Figure 11. Feedback Resistor vs Noise Gain

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 increases the feedback impedance (the

denominator of Equation 15), decreasing the bandwidth.

The internal buffer output impedance for the OPA2674 is

slightly influenced by the source impedance coming from

of the noninverting input terminal. High-source resistors

also have the effect of increasing RI, decreasing the

bandwidth. For those single-supply applications that

develop a midpoint bias at the noninverting input through

high valued resistors, the decoupling capacitor is essential

for power-supply ripple rejection, noninverting input noise

current shunting, and to minimize the high-frequency

value for RI in Figure 10.

INVERTING AMPLIFIER OPERATION

As the OPA2674 is a general-purpose, wideband

current-feedback op amp, most of the familiar op amp

application circuits are available to the designer. Those

dual op amp applications that require considerable

flexibility

the

feedback

element

(for

example,

integrators, transimpedance, and some filters) should

consider a unity-gain stable, voltage-feedback amplifier

such as the OPA2822, because the feedback resistor is

the compensation element for a current-feedback op amp.

Wideband inverting operation (and especially summing) is

particularly suited to the OPA2674. Figure 12 shows a

typical inverting configuration where the I/O impedances

and signal gain from Figure 1 are retained in an inverting

circuit configuration.

1/2

O P A 267 4

392

Ω

97.6

Ω

+6V

−6V

Ω

Ω Load

Power−supply

decoupling not

shown.

Ω

Source

102

Ω

Figure 12. Inverting Gain of −4 with Impedance

Matching

the

inverting

configuration,

two

key

design

considerations must be noted. First, the gain resistor (RG)

becomes part of the signal source input impedance. If

input impedance matching is desired (which is beneficial

whenever the signal is coupled through a cable, twisted

pair, long PCB trace, or other transmission line conductor),

it is normally necessary to add an additional matching

resistor to ground. RG, by itself, normally is not set to the

required input impedance since its value, along with the

desired gain, will determine an RF, which may be

nonoptimal from a frequency response standpoint. The

total input impedance for the source becomes the parallel

combination of RG and RM.

The second major consideration is that the signal source

impedance becomes part of the noise gain equation and

has a slight effect on the bandwidth through Equation 15.

The values shown in Figure 12 have accounted for this by

slightly decreasing RF (from the optimum values) to

reoptimize the bandwidth for the noise gain of Figure 12

(NG = 3.98). In the example of Figure 12, the RM value

combines in parallel with the external 50

Ω source

impedance, yielding an effective driving impedance of

Ω || 102Ω = 33.5Ω. This impedance is added in series

(16)

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