OPA2677IDDA Datasheet, PDF (20/38 Pages) Texas Instruments – Dual, Wideband, High Output Current Operational Amplifier

transimpedance nearly equal to the 402Ω optimum 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
on the noninverting input to achieve bias current error can-
cellation at the output. The input bias currents for a current-
feedback op amp are not generally matched in either magni-
tude or polarity. Connecting a resistor to ground on the
noninverting input of the OPA2677 in the circuit of Figure 13
actually provides additional gain for that input bias and noise
currents, but does not decrease the output DC error since the
input bias currents are not matched.
OUTPUT CURRENT AND VOLTAGE
The OPA2677 provides output voltage and current capabili-
ties that are unsurpassed in a low-cost dual monolithic op
amp. Under no-load conditions at 25°C, the output voltage
typically swings closer than 1V to either supply rail; tested at
+25°C swing limit is within 1.1V of either rail. Into a 6Ω load
(the minimum tested load), it delivers more than ±380mA
continuous and > ±1.2A peak output current.
The specifications described above, though familiar in the
industry, consider voltage and current limits separately. In many
applications, it is the voltage times current (or V-I product) that
is more relevant to circuit operation. Refer to the Output Voltage
and Current Limitations plot in the Typical Characteristics. 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 OPA2677
output drive capabilities, noting that the graph is bounded by a
safe operating area of 1W maximum internal power dissipation
(in this case for 1 channel only). Superimposing resistor load
lines onto the plot shows that the OPA2677 can drive ±4V into
10Ω or ±4.5V into 25Ω without exceeding the output capabilities
or the 1W dissipation limit. A 100Ω load line (the standard test
circuit load) shows the full ±5.0V output swing capability, as
shown in the Electrical Characteristics tables. The minimum
specified 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 decrease to the
numbers shown in the Electrical Characteristics tables. As the
output transistors deliver power, the junction temperatures
increases, decreasing the V_BEs (increasing the available output
voltage swing), and increasing the current gains (increasing the
available output current). In steady-state operation, the avail-
able 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 minimum
specified operating ambient. To maintain maximum output
stage linearity, no output short-circuit protection is provided.
This is normally not a problem because most applications
include a series-matching resistor at the output that limits the
internal power dissipation if the output side of this resistor is
shorted to ground. However, shorting the output pin directly to
the adjacent positive power-supply pin (8-pin package), will in
most cases, destroy the amplifier. If additional short-circuit
protection is required, consider using the equivalent OPA2674
that includes output current limiting. Alternatively, a small series
resistor may be included in the supply lines. Under heavy output
loads this will reduce the available output voltage swing. A 5Ω
series resistor in each power-supply lead will limit the internal
power dissipation to less than 1W for an output short circuit
while decreasing the available output voltage swing only 0.5V
for up to 100mA desired load currents. Always place the 0.1µF
power-supply decoupling capacitors after these supply current
limiting resistors directly on the supply pins.
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)
converter—including additional external capacitance that may
be recommended to improve the A/D converter linearity. A
high-speed, high open-loop gain amplifier such as the
OPA2677 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 solutions to this
problem have been suggested.
When the primary considerations are frequency response flat-
ness, pulse response fidelity, and/or distortion, the simplest 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 response, but rather shifts it and
adds a zero at a higher frequency. The additional zero acts to
cancel the phase lag from the capacitive load pole, thus
increasing the phase margin and improving stability. The Typi-
cal Characteristics show the recommended R_S vs Capacitive
Load and the resulting frequency response at the load. Parasitic
capacitive loads greater than 2pF can begin to degrade the
performance of the OPA2677. 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 resistor as close as
possible to the OPA2677 output pin (see the Board Layout
Guidelines section).
DISTORTION PERFORMANCE
The OPA2677 provides good distortion performance into a
100Ω load on ±6V supplies. Relative to alternative solutions,
it provides exceptional performance into lighter loads and/or
operation on a single +5V supply. Generally, until the funda-
mental signal reaches very high frequency or power levels,
the 2nd-harmonic dominates the distortion with a negligible
3rd-harmonic component. Focusing then on the 2nd-har-
monic, increasing the load impedance improves distortion
directly. Remember that the total load includes the feedback
network—in the noninverting configuration (see Figure 1),
this is the sum of R_F + R_G, whereas in the inverting configu-
ration it is just R_F. Also, providing an additional supply
decoupling capacitor (0.01µF) between the supply pins (for
bipolar operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
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OPA2677
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