OPA623AP datasheet(10/16 Pages) TI1

10 / 16 page

OPA623

FIGURE 3. Non-Inverting Current-Feedback Op Amp Configuration.

–V

+In

–In

Out

OUT

lines or loads. The buffer output is not current-limited or

protected.

As can be seen in Figure 3, the feedback in the form of a

current is applied through R

2 to the low-impedance inverting

input, and the size of R2 || R1 determines the open-loop gain

of the op amp.

The hybrid model shown in Figure 4 describes the AC

behavior of a wide-band current-feedback op amp that is not

internally compensated. The open-loop frequency response,

which is illustrated in Figure 5 for various R

2 values, is

determined by two time constants. The elements R and C

between the current source output and the output buffer form

the dominant open-loop pole TC. The signal delay time TD

modelled in the output buffer combines several small phase-

shifting time constants and delay times. They are distributed

throughout the amplifiers and are also present in the feed-

back loop. As shown in Figure 5, increasing R2 || R1 leads to

a decreasing open-loop gain. The ratio of the two time

constants TC and TD also determines the product GOL • GCL

for optimal closed-loop frequency response:

FIGURE 4. Hybrid Model OPA623.

–1

+In

Out

–In

provides a nearly constant closed-loop bandwidth, as shown

in Figure 6 for various gains with an optimal flat frequency

response. This behavior stands in contrast to op amps that

are internally compensated for stable unity-gain operation,

where the bandwidth is inversely proportional to the closed-

loop gain, sharply limiting the bandwidth and slew rate at

high output levels and gains.

In general, lower feedback resistors produce wider band-

width, more frequency response peaking, and more pulse

response overshooting. Higher feedback resistors results in

an overdamped response with little or no peaking and

overshooting.

Component pin and layout capacitances together with trace

and wire board inductances from a resonant IC circuit can

lead to oscillations of several hundreds of MHz. This very

high frequency oscillation leads to an excessive increase in

supply current which can destroy the device.

A resistor (100

Ω to 250Ω) in series and close to the high-

impedance, non-inverting input damps the LC circuit and

generates a safe operation.

THERMAL CONSIDERATIONS

The OPA623 does not require a heat sink for operation in

most environments. The use of a heat sink, however, will

The two time constants TC and TD, however, are fixed by the

op amp design. But varying R

2 || R1 externally in the

feedback loop allows for variation of the open-loop gain

OL versus the closed-loop gain GCL. This keeps the product

GOL * GCL constant, which is the theoretical condition for

optimally flat frequency response.

This variation may be beneficial when driving high capaci-

tive loads. Setting the open-loop gain externally also allows

the circuit to be optimized to a wide range of capacitive

loads, as shown in Figure 7 for a closed-loop gain of

+2V/V and a capacitive load of up to 47pF.

It should be noted here that higher open-loop gain (resulting

from lower feedback resistors) also yields lower distortion.

With external control of the open-loop characteristics of the

op amp, dynamic behavior can be tailored to individual

application requirements, and the open-loop gain selection

2TD

GOL = GCL•

OPA623AP Datasheet(PDF) 10 Page - Texas Instruments

OPA623AP Datasheet(HTML) 10 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL