OPA2674 Datasheet, PDF (22/30 Pages) Burr-Brown (TI) – Dual Wideband, High Output Current Operational Amplifier with Current Limit

OPA2674
SBOS270 − AUGUST 2003
with R_G for calculating the noise gainwhich gives
NG = 3.98. This value, and the inverting input impedance
of 22Ω, are inserted into Equation 16 to get the R_F that
appears in Figure 12. Note that the noninverting input in
this bipolar supply inverting 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 cancellation at the output. The input bias
currents for a current-feedback op amp are not generally
matched in either magnitude or polarity. Connecting a
resistor 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 decrease the output DC error because the input
bias currents are not matched.
OUTPUT CURRENT AND VOLTAGE
The OPA2674 provides output voltage and current
capabilities 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; the tested (+25°C) swing limit is within 1.1V of
either rail. Into a 6Ω load (the minimum tested load), it
delivers more than ±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 V−I
product) that is more relevant to circuit operation. Refer to
the Output Voltage and Current Limitations plot in the
Typical 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
output drive capabilities, noting that the graph is bounded
by a safe operating area of 1W maximum internal power
dissipation (in this case, for one channel only).
Superimposing resistor load lines onto the plot shows that
the OPA2674 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
stated 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 increase,
decreasing the V_BE’s (increasing the available output
voltage swing), and increasing the current gains
(increasing the available output current). In steady-state
operation, the available output voltage and current will
always be greater than that shown in the over-temperature
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specifications since the output stage junction
temperatures will be higher than the minimum specified
operating ambient.
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 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 solutions to this problem have been suggested.
When the primary considerations are frequency response
flatness, 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 Typical
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 OPA2674. Long PC board
traces, unmatched 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
OPA2674 output pin (see the Board Layout Guidelines
section).
DISTORTION PERFORMANCE
The OPA2674 provides good distortion performance into
a 100Ω load on ±6V supplies. It also provides exceptional
performance into lighter loads and/or operating on a single
+5V supply. Generally, until the fundamental 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-harmonic, 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; in the inverting
configuration, it is 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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