OPA2680 Datasheet, PDF (17/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

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 will not normally
be a problem since most applications include a series match-
ing resistor at the output that will limit 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 packages) will, in
most cases, destroy the amplifier. Including a small series
resistor (5Ω) in the power supply line will protect against
this. Always place the 0.1µF decoupling capacitor 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 A/D converter—including
additional external capacitance which may be recommended
to improve A/D linearity. A high speed, high open-loop gain
amplifier like the OPA2680 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’s 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 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
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 Performance Curves show the recommended
R_S versus capacitive load and the resulting frequency re-
sponse at the load. Parasitic capacitive loads greater than
2pF can begin to degrade the performance of the OPA2680.
Long PC board traces, unmatched cables, and connections to
multiple devices can easily exceed this value. Always con-
sider this effect carefully, and add the recommended series
resistor as close as possible to the OPA2680 output pin (see
Board Layout Guidelines).
The criterion for setting this R_S resistor is a maximum
bandwidth, flat frequency response at the load. For the
OPA2680 operating in a gain of +2, the frequency response
at the output pin is already slightly peaked without the
capacitive load requiring relatively high values of R_S to
flatten the response at the load. Increasing the noise gain will
reduce the peaking as described previously. The circuit of
Figure 10 demonstrates this technique, allowing lower val-
ues of R_S to be used for a given capacitive load.
50Ω
175Ω
50Ω
R_NG
402Ω
+5V
Power supply decoupling
not shown.
R
1/2
OPA2680
V_O
402Ω
C_LOAD
–5V
FIGURE 10. Capacitive Load Driving with Noise Gain Tuning.
This gain of +2 circuit includes a noise gain tuning resistor
across the two inputs to increase the noise gain, increasing
the unloaded phase margin for the op amp. Although this
technique will reduce the required R_S resistor for a given
capacitive load, it does increase the noise at the output. It
also will decrease the loop gain, nominally decreasing the
distortion performance. If, however, the dominant distortion
mechanism arises from a high R_S value, significant dynamic
range improvement can be achieved using this technique.
Figure 11 shows the required R_S versus C_LOAD parametric on
noise gain using this technique. This is the circuit of Figure
10 with R_NG adjusted to increase the noise gain (increasing
the phase margin) then sweeping C_LOAD and finding the
required R_S to get a flat frequency response. This plot also
gives the required R_S versus C_LOAD for the OPA2680 oper-
ated at higher signal gains.
100
90
80
70
60
50
40
30
20
10
0
1
NG = 2
NG = 3
10
Capacitive Load (pF)
NG = 4
100
FIGURE 11. Required R_S vs Noise Gain.
DISTORTION PERFORMANCE
The OPA2680 provides good distortion performance into a
100Ω load on ±5V supplies. Relative to alternative solu-
tions, it 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 will dominate the distortion
with a negligible 3rd harmonic component. Focusing then
on the 2nd harmonic, increasing the load impedance im-
®
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OPA2680