Datasheet

OPA842
SBOS267D –NOVEMBER 2002–REVISED SEPTEMBER 2010 www.ti.com
than the value predicted by dividing GBP by the noise The Typical Characteristics show the recommended
gain. Increasing the gain will cause the phase margin R
S
vs Capacitive Load (see Figure 15) and the
to approach 90 degrees and the bandwidth to more resulting frequency response at the load. The
closely approach the predicted value of (GBP/NG). At criterion for setting the recommended resistor is
a gain of +10, the 21MHz bandwidth shown in the maximum bandwidth, flat frequency response at the
Electrical Characteristics agrees with that predicted load. Since there is now a passive low-pass filter
using the simple formula and the typical GBP of between the output pin and the load capacitance, the
200MHz. 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
OUTPUT DRIVE CAPABILITY
applications, but can cause clipping if the desired
The OPA842 has been optimized to drive the
signal swing at the load is very close to the amplifier’s
demanding load of a doubly-terminated transmission
swing limit. Such clipping would be most likely to
line. When a 50Ω line is driven, a series 50Ω into the
occur in pulse response applications where the
cable and a terminating 50Ω load at the end of the
frequency peaking is manifested as an overshoot in
cable are used. Under these conditions, the cable
the step response.
impedance will appear resistive over a wide
Parasitic capacitive loads greater than 2pF can begin
frequency range, and the total effective load on the
to degrade the performance of the OPA842. Long
OPA842 is 100Ω in parallel with the resistance of the
PCB traces, unmatched cables, and connections to
feedback network. The Electrical Characteristics
multiple devices can easily cause this value to be
show a +2.8V/–3.3V swing into this load—which will
exceeded. Always consider this effect carefully, and
then be reduced to a +1.4V/–1.65V swing at the
add the recommended series resistor as close as
termination resistor. The ±90mA output drive over
possible to the OPA842 output pin (see Board Layout
temperature provides adequate current drive margin
section).
for this load. Higher voltage swings (and lower
distortion) are achievable when driving higher
impedance loads. DISTORTION PERFORMANCE
A single video load typically appears as a 150Ω load The OPA842 is capable of delivering an exceptionally
(using standard 75Ω cables) to the driving amplifier. low distortion signal at high frequencies and low
The OPA842 provides adequate voltage and current gains. The distortion plots in the Typical
drive to support up to three parallel video loads (50Ω Characteristics show the typical distortion under a
total load) for an NTSC signal. With only one load, wide variety of conditions. Most of these plots are
the OPA842 achieves an exceptionally low limited to 100dB dynamic range. The OPA842
0.003%/0.008° dG/dP error. distortion does not rise above –100dBc until either
the signal level exceeds 0.5V and/or the fundamental
frequency exceeds 500kHz. Distortion in the audio
DRIVING CAPACITIVE LOADS
band is ≤ –120dBc. Generally, until the fundamental
One of the most demanding, and yet very common,
signal reaches very high frequencies or powers, the
load conditions for an op amp is capacitive loading. A
second-harmonic will dominate the distortion with a
high-speed, high open-loop gain amplifier like the
negligible third-harmonic component. Focusing then
OPA842 can be very susceptible to decreased
on the second-harmonic, increasing the load
stability and closed-loop response peaking when a
impedance improves distortion directly. Remember
capacitive load is placed directly on the output pin. In
that the total load includes the feedback network— in
simple terms, the capacitive load reacts with the
the noninverting configuration this is the sum of
open-loop output resistance of the amplifier to
R
F
+ R
G
, whereas in the inverting configuration this is
introduce an additional pole into the loop and thereby
just R
F
(see Figure 37). Increasing the output voltage
decrease the phase margin. This issue has become a
swing increases harmonic distortion directly.
popular topic of application notes and articles, and
Increasing the signal gain will also increase the
several external solutions to this problem have been
second-harmonic distortion. Again, a 6dB increase in
suggested. When the primary considerations are
gain will increase the second- and third-harmonics by
frequency response flatness, pulse response fidelity,
6dB even with a constant output power and
and/or distortion, the simplest and most effective
frequency. Finally, the distortion increases as the
solution is to isolate the capacitive load from the
fundamental frequency increases due to the roll off in
feedback loop by inserting a series isolation resistor
the loop gain with frequency. Conversely, the
between the amplifier output and the capacitive load.
distortion will improve going to lower frequencies
This does not eliminate the pole from the loop
down to the dominant open-loop pole at
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.
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