OPA3680 Burr-Brown, OPA3680 Datasheet - Page 17

• Manufacturer Part Number: OPA3680
• Description: Triple / Wideband / Voltage-Feedback OPERATIONAL AMPLIFIER With Disable
• Manufacturer: Burr-Brown

FIGURE 10. Capacitive Load Driving with Noise Gain Tuning.
This gain of +2 circuit includes a noise gain tuning resistor
across the two inputs to increase the noise gain, increasing
the unloaded phase margin for the op amp. Although this
technique will reduce the required R
capacitive load, it does increase the noise at the output. It
also will decrease the loop gain, slightly decreasing the
distortion performance. If, however, the dominant distortion
mechanism arises from a high R
range improvement can be achieved using this technique.
DISTORTION PERFORMANCE
The OPA3680 provides good distortion performance into a
100 load on 5V supplies. Relative to alternative solutions,
it provides exceptional performance into lighter loads and/or
operating on a single +5V supply.
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 Performance Curves show the recommended
R
sponse at the load. Parasitic capacitive loads greater than
2pF can begin to degrade the performance of the OPA3680.
Long PC board traces, unmatched cables, and connections
to multiple devices can easily exceed this value. Always
consider this effect carefully, and add the recommended
series resistor as close as possible to the OPA3680 output
pin (see Board Layout Guidelines).
The criterion for setting this R
bandwidth, flat frequency response at the load. For the
OPA3680 operating in a gain of +2, the frequency response
at the output pin is already slightly peaked without the
capacitive load requiring relatively high values of R
flatten the response at the load. Increasing the noise gain
will reduce the peaking as described previously. The circuit
of Figure 10 demonstrates this technique, allowing lower
values of R
used to generate the Recommended R
Load plots.
S
versus capacitive load and the resulting frequency re-
50
S
50
to be used for a given capacitive load. This was
R
250
50
NG
+5V
–5V
OPA3680
1/3
S
250
value, significant dynamic
S
Power supply decoupling
resistor is a maximum
S
resistor for a given
S
not shown.
versus Capacitive
R
S
C
L
V
O
S
to
17
The distortion plots show which changes in operation will
improve distortion. Increasing the load impedance improves
distortion directly. Remember that the total load includes the
feedback network; in the non-inverting configuration
(Figure 1) this is sum of R
configuration (Figure 9), it is just R
additional supply decoupling capacitor (0.1 F) between the
supply pins (for bipolar operation) improves the 2nd-order
distortion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing in-
creases intermodulation distortion directly. The new output
stage used in the OPA3680 actually holds the difference
between fundamental power and the 3rd-order
intermodulation powers relatively constant with increasing
output power until very large output swings are required
(> 4Vp-p). The 3rd-order spurious levels are extremely low
at low output power levels. The output stage continues to
hold them low even as the fundamental power reaches very
high levels. As the Typical Performance Curves show, the
spurious intermodulation powers do not increase as pre-
dicted by a traditional intercept model. As the fundamental
power level increases, the dynamic range does not decrease
significantly. For 2 tones centered at 20MHz, with 10dBm/
tone into a matched 50
the load, which requires 8Vp-p for the overall two-tone
envelope at the output pin), the Typical Performance Curves
show 57dBc difference between the test tone powers and the
3rd-order intermodulation spurious powers. This excep-
tional performance improves further when operating at lower
frequencies.
NOISE PERFORMANCE
High slew rate, unity gain stable, voltage feedback op amps
usually achieve their slew rate at the expense of a higher
input noise voltage. The 4.8nV/ Hz input voltage noise for
the OPA3680 is, however, much lower than comparable
amplifiers. The input-referred voltage noise, and the two
input-referred current noise terms, combine to give low
output noise under a wide variety of operating conditions.
Figure 11 shows the op amp noise analysis model with all
the noise terms included. In this model, all noise terms are
taken to be noise voltage or current density terms in either
nV/ Hz or pA/ Hz.
FIGURE 11. Op Amp Noise Analysis Model.
E
RS
R
S
4kTR
4kT
R
G
S
I
BN
E
NI
R
load (i.e., 2Vp-p for each tone at
G
F
OPA3680
+ R
OPA3680
1/3
G
I
BI
, while in the inverting
R
F
F
. Also, providing an
4kT = 1.6E –20J
at 290 K
4kTR
F
E
O
®