OPA643 Burr-Brown, OPA643 Datasheet - Page 12

• Manufacturer Part Number: OPA643
• Description: Wideband Low Distortion / High Gain OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

For the values shown in Figure 6, the F
approximately 105MHz. This is less than that predicted by
simply dividing the GBP product by NG
network controls the bandwidth to a lower value while
providing full slew rate and exceptional distortion
performance due to increased loop gain at frequencies below
NG
calculated for NG
parasitics. These differ slightly from the application circuit
on the front page, since those have been adjusted for parasitics
and to account for the capacitive load (through R
ADC input.
OUTPUT VOLTAGE AND CURRENT DRIVE
The OPA643 has been optimized to drive the demanding
load of a doubly terminated transmission line. When a 50
line is driven, a series 50
and a terminating 50 load at the end of the cable are used.
Under these conditions, the cable’s impedance will appear
resistive over a wide frequency range, and the total effective
load on the OPA643 is 100 in parallel with the resistance
of the feedback network. The specifications show a
guaranteed 2.5V swing over the full temperature range into
this 100
swing at the termination resistor. The guaranteed 35mA
output current over temperature provides adequate current
drive margin for this load. Higher voltage swings (and lower
distortion) are achievable when driving higher impedance
loads.
A common IF amplifier specification which describes
available output power is the –1dB compression point. This
is usually defined at a matched 50 load to be the sinusoidal
power where the gain has compressed by –1dB vs the gain
seen at very low power levels. This compression level is
frequency dependent for an op amp, due to both bandwidth
and slew rate limitations. For frequencies well within the
bandwidth and slew rate limit of the OPA643, the –1dB
compression at a matched 50 load will be > 13dBm based
on the minimum available 1.25V swing at the load. One
common use for the –1dB compression is to predict
intermodulation intercept. This is normally 10dB greater
than the –1dB compression power for a standard RF amplifier.
This simple rule of thumb does NOT apply to the OPA643.
The high open loop gain and Class AB output stage of the
OPA643 produce a much higher intercept than the –1dB
compression would predict, as shown in the Typical
Performance Curves.
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 OPA643, 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
1
• Z
0
. The capacitor values shown in Figure 6 are
load—which will then be reduced to a 1.25V
®
OPA643
1
= 3 and NG
source resistance into the cable
2
= 7.5 with no adjustment for
1
. The compensation
–3dB
S
will be
) at the
12
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 this capacitive load from the feedback
loop by inserting a series isolation resistor between the
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, increasing the
phase margin and improving stability.
The Typical Performance Curves show the recommended
series R
response at the load. The criterion for setting this resistor is
a maximum bandwidth, flat frequency response at the load.
Since there is now a passive low pass filter from the output
pin to the load capacitor, the response at the output pin itself
is typically somewhat peaked, and becomes flat after the
rolloff 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 for a large
signal pulse response where this slight peaking causes an
overshoot in the step response at the output pin.
Parasitic capacitive loads greater than 2pF can begin to
degrade the performance of the OPA643. Long PC board
traces, unmatched cables, and connections to multiple devices
can easily exceed 2pF. Always take care to consider this, and
add the recommended series resistor as close as possible to
the OPA643 output pin (see Board Layout Guidelines).
DISTORTION PERFORMANCE
The OPA643 is capable of delivering an exceptionally low
distortion signal at high frequencies over a wide range of
gains. The distortion plots in the Typical Performance Curves
show the typical distortion under a wide variety of conditions.
Most of these plots are limited to 100dB dynamic range. The
OPA643’s distortion does not rise above –90dBc until either
the signal level exceeds 0.5V and/or the fundamental
frequency exceeds 500kHz. Distortion in the audio band is
< –120dBc.
Generally, until the fundamental signal reaches very high
frequencies or powers, the second harmonic will dominate
the distortion with negligible third harmonic component.
Focusing then on the second harmonic, increasing the load
impedance improves distortion directly. Remember that the
total load includes the feedback network—in the non-
inverting configuration this is sum of R
inverting configuration it is only R
output voltage swings lead directly to increased harmonic
distortion. A 6dB increase in output voltage swing will
generally increase the second harmonic by 12dB and the
third harmonic by 18dB. Higher signal gain settings will also
increase the second harmonic distortion. A 6dB increase in
voltage gain will raise the second and third harmonics by
S
vs Capacitive Load and the resulting frequency
F
F
(Figure 1). Larger
+ R
G
, while in the