OPA2674_17 Datasheet, PDF(21/36 Page) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

OPA2674_17 Datasheet, PDF (21/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

◁

www.ti.com

nator of Equation 15 (which is the feedback transimpe-

dance) gives an optimal target of 490â¦. As the signal gain

changes, the contribution of the NG Ã RI term in the feed-

back transimpedance changes, but the total can be held

constant by adjusting RF. Equation 16 gives an approxi-

mate equation for optimum RF over signal gain:

RF + 490 * NG RI

(16)

As the desired signal gain increases, this equation eventu-

ally suggests a negative RF. A somewhat subjective limit

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

mum value of 20â¦. Lower values load both the buffer stage

at the input and the output stage if RF gets too lowï£§actual-

ly decreasing the bandwidth. Figure 11 shows the recom-

mended RF versus NG for both Â±6V and a single +5V op-

eration. 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 correct-

ing for board parasitic not considered in the simplified anal-

ysis leading to Equation 16. The values shown in Figure 11

give a good starting point for designs where bandwidth op-

timization is desired.

600

500

+5V

400

300

Â±6V

RG = 20â¦

200

Noise Gain

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 sum-

ming junction increases the feedback impedance (the de-

nominator of Equation 15), decreasing the bandwidth. The

internal buffer output impedance for the OPA2674 is slight-

ly 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 mid-

point bias at the noninverting input through high valued re-

sistors, 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.

OPA2674

SBOS270C â AUGUST 2003 â REVISED AUGUST 2008

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 in the feed-

back element (for example, integrators, transimpedance,

and some filters) should consider a unity-gain stable, volt-

age-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 configura-

tion where the I/O impedances and signal gain from

Figure 1 are retained in an inverting circuit configuration.

+6V

50â¦

Source

97.6â¦

102â¦

Powerâsupply

decoupling not

shown.

1/2

O PA 267 4

50â¦ Load

50â¦

392â¦

â6V

Figure 12. Inverting Gain of â4 with Impedance

Matching

In the inverting configuration, two key design consider-

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

part of the signal source input impedance. If input imped-

ance 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 nor-

mally necessary to add an additional matching resistor to

ground. RG, by itself, normally is not set to the required in-

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

will determine an RF, which may be nonoptimal from a fre-

quency 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 reopti-

mize 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 50â¦ source impedance, yield-

ing an effective driving impedance of 50â¦ || 102â¦ =

▷