OPA2674_17 Datasheet, PDF(22/36 Page) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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OPA2674_17 Datasheet, PDF (22/36 Pages) Texas Instruments – Dual Wideband, High Output Current Operational Amplifier with Current Limit

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OPA2674

SBOS270C â AUGUST 2003 â REVISED AUGUST 2008

33.5â¦. This impedance is added in series with RG for cal-

culating the noise gainï£§which gives NG = 3.98. This val-

ue, and the inverting input impedance of 22â¦, are inserted

into Equation 16 to get the RF that appears in Figure 12.

Note that the noninverting input in this bipolar supply in-

verting application is connected directly to ground.

It is often suggested that an additional resistor be con-

nected to ground on the noninverting input to achieve bias

current error cancellation at the output. The input bias cur-

rents for a current-feedback op amp are not generally

matched in either magnitude or polarity. Connecting a re-

sistor to ground on the noninverting input of the OPA2674

in the circuit of Figure 12 actually provides additional gain

for that input bias and noise currents, but does not de-

crease the output DC error because the input bias currents

are not matched.

OUTPUT CURRENT AND VOLTAGE

The OPA2674 provides output voltage and current capa-

bilities that are unsurpassed in a low-cost dual monolithic

op amp. Under no-load conditions at 25Â°C, the output volt-

age typically swings closer than 1V to either supply rail; the

tested (+25Â°C) swing limit is within 1.1V of either rail. Into

a 6â¦ load (the minimum tested load), it delivers more than

Â±380mA.

The specifications described previously, though familiar in

the industry, consider voltage and current limits separately.

In many applications, it is the voltage times current (or VâI

product) that is more relevant to circuit operation. Refer to

the Output Voltage and Current Limitations plot in the Typi-

cal Characteristics (see page 9). The X and Y axes of this

graph show the zero-voltage output current limit and the

zero-current output voltage limit, respectively. The four

quadrants give a more detailed view of the OPA2674 out-

put drive capabilities, noting that the graph is bounded by

a safe operating area of 1W maximum internal power dis-

sipation (in this case, for one channel only). Superimpos-

ing resistor load lines onto the plot shows that the

OPA2674 can drive Â±4V into 10â¦ or Â±4.5V into 25â¦ with-

out exceeding the output capabilities or the 1W dissipation

limit. A 100â¦ load line (the standard test circuit load)

shows the full Â±5.0V output swing capability, as stated in

the Electrical Characteristics tables. The minimum speci-

fied output voltage and current over temperature are set by

worst-case simulations at the cold temperature extreme.

Only at cold startup will the output current and voltage de-

crease to the numbers shown in the Electrical Characteris-

tics tables. As the output transistors deliver power, the

junction temperatures increase, decreasing the VBEs (in-

creasing the available output voltage swing), and increas-

ing the current gains (increasing the available output cur-

rent). In steady-state operation, the available output

voltage and current will always be greater than that shown

in the over-temperature specifications since the output

stage junction temperatures will be higher than the mini-

mum specified operating ambient.

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DRIVING CAPACITIVE LOADS

One of the most demanding and yet very common load

conditions for an op amp is capacitive loading. Often, the

capacitive load is the input of an analog-to-digital (A/D)

converterï£§including additional external capacitance that

may be recommended to improve the A/D converter linear-

ity. A high-speed, high open-loop gain amplifier like the

OPA2674 can be very susceptible to decreased stability

and closed-loop response peaking when a capacitive load

is placed directly on the output pin. When the amplifier

open-loop output resistance is considered, this capacitive

load introduces an additional pole in the signal path that

can decrease the phase margin. Several external solu-

tions to this problem have been suggested.

When the primary considerations are frequency response

flatness, pulse response fidelity, and/or distortion, the sim-

plest and most effective solution is to isolate the capacitive

load from the feedback loop by inserting a series isolation

resistor between the amplifier output and the capacitive

load. This does not eliminate the pole from the loop re-

sponse, but rather shifts it and adds a zero at a higher fre-

quency. The additional zero acts to cancel the phase lag

from the capacitive load pole, thus increasing the phase

margin and improving stability. The Typical Characteristics

show the Recommended RS vs Capacitive Load and the

resulting frequency response at the load. Parasitic capaci-

tive loads greater than 2pF can begin to degrade the per-

formance of the OPA2674. Long PC board traces, un-

matched cables, and connections to multiple devices can

easily cause this value to be exceeded. Always consider

this effect carefully, and add the recommended series re-

sistor as close as possible to the OPA2674 output pin (see

the Board Layout Guidelines section).

DISTORTION PERFORMANCE

The OPA2674 provides good distortion performance into

a 100â¦ load on Â±6V supplies. It also provides exceptional

performance into lighter loads and/or operating on a single

+5V supply. Generally, until the fundamental signal reach-

es very high frequency or power levels, the 2nd-harmonic

dominates the distortion with a negligible 3rd-harmonic

component. Focusing then on the 2nd-harmonic, increas-

ing the load impedance improves distortion directly. Re-

member that the total load includes the feedback net-

workï£§in the noninverting configuration (see Figure 1),

this is the sum of RF + RG; in the inverting configuration,

it is RF. Also, providing an additional supply decoupling ca-

pacitor (0.01ÂµF) between the supply pins (for bipolar op-

eration) improves the 2nd-order distortion slightly (3dB to

6dB).

In most op amps, increasing the output voltage swing di-

rectly increases harmonic distortion. The Typical Charac-

teristics show the 2nd-harmonic increasing at a little less

than the expected 2x rate, whereas the 3rd-harmonic in-

creases at a little less than the expected 3x rate. Where the

test power doubles, the difference between it and the

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