OPA687 Burr-Brown, OPA687 Datasheet - Page 13

• Manufacturer Part Number: OPA687
• Description: Wideband / Ultra-Low Noise / Voltage Feedback OPERATIONAL AMPLIFIER With Power Down
• Manufacturer: Burr-Brown

The Typical Performance Curves show the recommended
R
at the load. Parasitic capacitive loads greater than 2pF can
begin to degrade the performance of the OPA687. 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 OPA687 output pin
(see Board Layout Guidelines).
The criterion for setting this R
bandwidth, flat frequency response at the load. For the
OPA687 operating in a gain of +20, the frequency response
at the output pin is very flat to begin with, allowing rela-
tively small values of R
As the signal gain is increased, the unloaded phase margin
will also increase. Driving capacitive loads at higher gains
will require lower R
DISTORTION PERFORMANCE
The OPA687 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 condi-
tions. Most of these plots are limited to 110dB dynamic
range. The OPA687’s distortion driving a 500
not rise above –90dBc until either the signal level exceeds
3.0V and/or the fundamental frequency exceeds 5MHz.
Generally, until the fundamental signal reaches very high
frequencies or powers, the 2nd harmonic will dominate the
distortion with negligible 3rd harmonic component. Focus-
ing then on the 2nd harmonic, increasing the load impedance
improves distortion directly. Remember that the total load
includes the feedback network, in the non-inverting configu-
ration this is sum of R
configuration this is just R
voltage swing increases harmonic distortion directly. A 6dB
increase in output swing will generally increase the 2nd
harmonic 12dB and the 3rd harmonic 18dB. 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 about 6dB even with a constant output power
and frequency. And finally, the distortion increases as the
fundamental frequency increases due to 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 200kHz.
In most applications, the 2nd harmonic will set the limit to
dynamic range. Even order non-linearities arise from slight
imbalances between the positive and negative halves of an
output sinusoid. These imbalanced non-linearities arise from
such mechanisms as voltage dependent base-collector ca-
pacitances and imbalanced source impedances looking out
of the two amplifier power pins. Once a circuit and board
layout has been determined, these imbalances can typically
be nulled out by adjusting the DC operating point for the
S
vs Capacitive Load and the resulting frequency response
S
values than shown for a gain of +20.
S
to be used for low capacitive loads.
F
F
+ R
(Figure 2). Increasing output
G
S
, while in the inverting
resistor is a maximum
load does
13
signal. An example DC tune is shown in Figure 7. This
circuit has a DC-coupled inverting signal path to the output
pin that provides gain for a small DC offsetting signal
brought into the non-inverting input pin. The output is AC-
coupled to block off this DC operating point from interact-
ing with the next stage.
FIGURE 7. DC Adjustment for 2nd Harmonic Distortion.
For a 1Vp-p output swing in the 10MHz to 20MHz region,
an output DC voltage in the 1.5V range will null the 2nd
harmonic distortion. Tests into a 200 converter input load
have shown > 20dB decrease in the 2nd harmonic using this
technique. Once the required voltage is found for a particular
board, circuit, and signal requirement, that voltage is very
repeatable from part to part and may be set permanently on
the non-inverting input. Minimal degradation from this im-
proved 2nd harmonic distortion over temperature will be
observed. An alternative means to eliminate the 2nd har-
monic distortion is to operate two OPA687s differentially as
shown on the front page of the data sheet. Both single-tone
and 2-tone even order harmonic distortions for this differen-
tial configuration are essentially unmeasureable through
30MHz for a good layout.
The OPA687 has an extremely low 3rd-order harmonic
distortion. This also gives a high 2-tone, 3rd-order
intermodulation intercept as shown in the Typical Perfor-
mance Curves. This intercept curve is defined at the 50
load when driven through a 50
direct comparisons to RF MMIC devices. This network
attenuates the voltage swing from the output pin to the load
by 6dB. If the OPA687 drives directly into the input of a
high impedance device, such as an ADC, this 6dB attenua-
tion is not taken. Under these conditions, the intercept will
increase by a minimum 6dBm. The intercept is used to
V
10k
I
5k
5k
+5V
–5V
R
G
0.1 F
20
OPA687
OPA687
matching resistor to allow
+V
–V
S
S
R
F
Supply Decoupling
Not Shown
V
O
®