FREQUENCY RESPONSE COMPENSATION
The OPA655 is internally compensated to be stable at unity
gain into a 100Ω load with a nominal phase margin of 58°.
This unity gain phase margin shows a slight peaking in the
frequency response and requires a very low inductance
shorting connection from the output pin to the inverting
input pin for minimal peaking. This stable broadband perfor-
mance at unity gain lends itself well to integrator and buffer
applications.
Phase margin and flatness will improve at higher gains.
Since phase margin is slightly load dependent, flatness in a
gain of +2 can be modified by changing the loading. Very
flat performance is shown in the Typical Performance Curves
using a 100Ω feedback and 100Ω load. This may be peaked
up by increasing the load or feedback resistors or rolled off
by decreasing them. Recall that an inverting gain of –1 is
equivalent to a gain of +2 for bandwidth purposes, i.e. noise
gain equal to 2. The external compensation techniques
developed for voltage feedback op amps can be applied to
this device. For example, in the non-inverting configuration,
placing a capacitor across the feedback resistor will reduce
the gain to +1 starting at f = 1/(2π R
F
C
F
) Hz. Alternatively,
in the inverting configuration, the bandwidth may be limited
without modifying the low frequency inverting gain by
placing a series RC network to ground on the inverting node.
This has the effect of increasing the noise gain at higher
frequencies, thereby limiting the bandwidth for the inverting
input signal through the gain-bandwidth product.
At higher gains, the gain-bandwidth product (240MHz) of
this voltage feedback topology will limit the achievable
signal bandwidth. If FET input is not required and higher
bandwidths at higher gains are needed, consider the broad
bandwidth available from a current feedback op amp such as
the OPA658.
DRIVING CAPACITIVE LOADS
The high open loop gain and Class AB output stage of the
OPA655 are optimized for driving the low impedance of
doubly terminated cables. Capacitive loads directly on the
output pin can decrease phase margin leading to frequency
response peaking and possibly sustained oscillations. This
effect is particularly pronounced at unity gain and becomes
less significant at higher gains. Frequency response flatness
can be maintained into a capacitive load by isolating it with
a resistor as shown in Figure 11. The Typical Performance
Curves show a plot of the minimum value for R
ISO
to hold
a flat frequency response as C
L
is increased. The 1kΩ shunt
load across C
L
shown in Figure 11 was the probe load for
this measurement and should be considered optional.
PULSE AND OVERDRIVE PERFORMANCE
High speed amplifiers like the OPA655 can provide an
extremely fast settling time for a pulse input. Excellent
frequency response flatness and phase linearity are required
R
ISO
OPA655
V
IN
1kΩ
R
IN
C
L
FIGURE 11. Driving a Capacitive Load.
to get the best settling times. As shown in the specifications
table, settling time for a 1V step at a gain of +1 for the
OPA655 is an extremely fast 8ns to 0.1%. This specification
is defined as the time required, after the input transition, for
the output to settle within a specified error band around its
final value. For a 1V step, 0.1% settling corresponds to an
error band of
±1mV.
For the best settling times, little or no
peaking in the frequency response can be allowed. Using the
recommended R
ISO
for capacitive loads will limit this peak-
ing and reduce the settling times. Fast, extremely fine scale
settling (0.01%) requires close attention to the ground return
currents in the supply de-coupling capacitors. De-coupling
the output stage power supply connections (+V
S2
+ (–V
S2
))
separately from the main supply inputs will improve both
settling and harmonic distortion performance.
As can be observed in the typical performance curves, the
OPA655 recovers very quickly from an input overdrive. For
non-inverting operation, recovery is immediate for negative
overdrives and < 10ns for a positive going overdrive signal.
For inverting mode operation, such as transimpedance am-
plifiers, recovery is immediate to input overdrives that do
not build up input voltages that exceeds the common mode
input range. Unlike older FET input amplifiers, overdriving
the inputs does not cause the output to invert phase and/or
latch. Inputs that exceed the positive supply voltage will,
however, cause the output to reverse and swing negative—
but no latching will occur.
HARMONIC DISTORTION
The Typical Performance Curves show the very low har-
monic distortion that OPA655 can deliver into a 100Ω load
over a wide range of operating conditions. Generally, distor-
tion improves at lower gains, lower signal swings, lower
frequencies, and higher loads. Figure 12 shows significant
improvement in second harmonic distortion as the load is
increased, and relative insensitivity of the third harmonic to
load conditions. For measurement purposes, these distortion
levels were increased from those listed in the specification
table by increasing the gain to +5. Narrowband communica-
tions systems will benefit from the very low third order
distortion vs load which will provide very low intermodulation
spurs.
®
OPA655
12
BURR-BROWN [ BURR-BROWN CORPORATION ]
