Datasheet

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SBOS303C − JUNE 2004 − REVISED AUGUST 2008
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19
Increasing the gain will cause the phase margin to approach
90° and the bandwidth to more closely approach the predicted
value of (GBP/NG). At a gain of +10, the 30MHz bandwidth
shown in the Electrical Characteristics agrees with that
predicted using the simple formula and the typical GBP of
280MHz.
OUTPUT DRIVE CAPABILITY
The OPA820 has been optimized to drive the demanding load
of a doubly-terminated transmission line. When a 50Ω line is
driven, a series 50Ω into the cable and a terminating 50Ω load
at the end of the cable are used. Under these conditions, the
cable impedance will appear resistive over a wide frequency
range, and the total effective load on the OPA820 is 100Ω in
parallel with the resistance of the feedback network. The
electrical characteristics show a ±3.6V swing into this
load—which will then be reduced to a ±1.8V swing at the
termination resistor. The ±75mA output drive over tempera-
ture provides adequate current drive margin for this load.
Higher voltage swings (and lower distortion) are achievable
when driving higher impedance loads.
A single video load typically appears as a 150Ω load (using
standard 75Ω cables) to the driving amplifier. The OPA820
provides adequate voltage and current drive to support up to
three parallel video loads (50Ω total load) for an NTSC signal.
With only one load, the OPA820 achieves an exceptionally low
0.01%/0.03° dG/dP error.
DRIVING CAPACITIVE LOADS
One of the most demanding, and yet very common, load
conditions for an op amp is capacitive loading. A high-speed,
high open-loop gain amplifier like the OPA820 can be very
susceptible to decreased stability and closed-loop response
peaking when a capacitive load is placed directly on the output
pin. In simple terms, the capacitive load reacts with the
open-loop output resistance of the amplifier to introduce an
additional pole into the loop and thereby decrease the phase
margin. This issue has become a popular topic of application
notes and articles, and 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. The criterion for setting the recommended resistor is
maximum bandwidth, flat frequency response at the load.
Since there is now a passive low-pass filter between the
output pin and the load capacitance, the response at the
output pin itself is typically somewhat peaked, and becomes
flat after the roll-off action of the RC network. This is not a
concern in most applications, but can cause clipping if the
desired signal swing at the load is very close to the amplifier’s
swing limit. Such clipping would be most likely to occur in pulse
response applications where the frequency peaking is
manifested as an overshoot in the step response.
Parasitic capacitive loads greater than 2pF can begin to
degrade the performance of the OPA820. 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 OPA820 output pin
(see the Board Layout section).
DISTORTION PERFORMANCE
The OPA820 is capable of delivering an exceptionally low
distortion signal at high frequencies and low gains. The
distortion plots in the Typical Characteristics show the typical
distortion under a wide variety of conditions. Most of these
plots are limited to 100dB dynamic range. The OPA820
distortion does not rise above −90dBc until either the signal
level exceeds 0.9V and/or the fundamental frequency ex-
ceeds 500kHz. Distortion in the audio band is ≤ −100dBc.
Generally, until the fundamental signal reaches very high
frequencies or powers, the 2nd-harmonic will dominate 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 this is the sum of R
F
+ R
G
, whereas in the
inverting configuration this is just R
F
(see Figure 1). Increasing
the output voltage swing increases harmonic distortion
directly. Increasing the signal gain will also increase the
2nd-harmonic distortion. Again, a 6dB increase in gain will
increase the 2nd- and 3rd-harmonic by 6dB even with a
constant output power and frequency. Finally, the distortion
increases as the fundamental frequency increases because
of the roll-off in the loop gain with frequency. Conversely, the
distortion will improve going to lower frequencies down to the
dominant open-loop pole at approximately 100kHz. Starting
from the −85dBc 2nd-harmonic for 2V
PP
into 200Ω, G = +2
distortion at 1MHz (from the Typical Characteristics), the
2nd-harmonic distortion will not show any improvement below
100kHz and will then be:
−100dB − 20log (1MHz/100kHz) = −105dBc