OPA2658 Datasheet, PDF (11/14 Pages) Burr-Brown (TI) – Dual Wideband, Low Power, Current Feedback OPERATIONAL AMPLIFIER

CAPACITIVE LOADS
The OPA2658’s output stage has been optimized to drive
low resistive loads. Capacitive loads, however, will decrease
the amplifier’s phase margin which may cause high fre-
quency peaking or oscillations. Capacitive loads greater than
5pF should be buffered by connecting a small resistance,
usually 10Ω to 35Ω, in series with the output as shown in
Figure 5. This is particularly important when driving high
capacitance loads such as flash A/D converters.
In general, capacitive loads should be minimized for opti-
mum high frequency performance. Coax lines can be driven
if the cable is properly terminated. The capacitance of coax
cable (29pF/foot for RG-58) will not load the amplifier
when the coaxial cable or transmission line is terminated
with its characteristic impedance.
402Ω
402Ω
1/2
OPA2658
10Ω to 35Ω
R_S
50Ω
R_L
C_L
tone, third-order spurious plot shown in Figure 7 indicates
how far below these two equal power, closely spaced, tones
the intermodulation spurious will be. The single tone power
is at a matched 50Ω load. The unique design of the OPA2658
provides much greater spurious free range than what a two-
tone third-order intermodulation intercept specification would
predict. This can be seen in Figure 7 as the spurious free
range actually increases at the higher output power levels.
5MHz HARMONIC DISTORTION vs
LOAD RESISTANCE (G = +2)
–55
–60
G = +2, V_O = 2Vp-p, f_O = 5MHz
–65
3f_O
–70
–75
–80
2f_O
–85
10
100
1k
Load Resistance (Ω)
FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance.
FIGURE 5. Driving Capacitive Loads.
COMPENSATION
The OPA2658 is internally compensated and is stable in
unity gain with a phase margin of approximately 62°, and
approximately 64° in a gain of +2V/V when used with the
recommended feedback resistor value. Frequency response
for other gains are shown in the Typical Performance Curves.
The high-frequency response of the OPA2658 in a good
layout is very flat with frequency.
TWO TONE, THIRD-ORDER SPURIOUS LEVELS
–65
20MHz
–70
–75
10MHz
–80
5MHz
–85
DISTORTION
The OPA2658’s Harmonic Distortion characteristics into a
100Ω load are shown versus frequency and power output in
the Typical Performance Curves. Distortion can be further
improved by increasing the load resistance as illustrated in
Figure 6. Remember to include the contribution of the
feedback resistance when calculating the effective load re-
sistance seen by the amplifier.
Narrowband communication channel requirements will ben-
efit from the OPA2658’s wide bandwidth and low
intermodulation distortion on low quiescent power. If output
signal power at two closely spaced frequencies is required,
third-order nonlinearities in any amplifier will cause spuri-
ous power at frequencies very near the two funda-
mental frequencies. If the two test frequencies, f₁ and f₂,
are specified in terms of average and delta frequency,
f_O = (f₁ + f₂)/2 and ∆f =  f₂ – f₁, the two, third-order,
close-in spurious tones will appear at f_O ±3 • ∆f. The two
–90
–18 –16 –14 –12 –10 –8 –6 –4 –2 0 2 4
Single Tone Power (dBm)
FIGURE 7. Third-Order Intercept Point vs Frequency.
CROSSTALK
Crosstalk is the undesired result of the signal of one channel
mixing with and reproducing itself in the output of the other
channel. Crosstalk occurs in most multichannel integrated
circuits. In dual devices, the effect of crosstalk is measured by
driving one channel and observing the output of the undriven
channel over various frequencies. The magnitude of this effect
is referenced in terms of channel- to-channel isolation and
expressed in decibels. "Input referred" points to the fact that
there is a direct correlation between gain and crosstalk, there-
fore at increased gain, crosstalk also increases by a factor
equal to that of the gain. Figure 8 illustrates the measured
effect of crosstalk in the OPA2658U.
®
11
OPA2658