OPA4658 Datasheet, PDF (10/14 Pages) Burr-Brown (TI) – Quad Wideband, Low Power Current Feedback OPERATIONAL AMPLIFIER

ESD PROTECTION
ESD damage has been well recognized for MOSFET de-
vices, but any semiconductor device is vulnerable to this
potentially damaging source. This is particularly true for
very high speed, fine geometry processes.
ESD damage can cause subtle changes in amplifier input
characteristics without necessarily destroying the device. In
precision operational amplifiers, this may cause a noticeable
degradation of offset voltage and drift. Therefore, ESD
handling precautions are strongly recommended when han-
dling the OPA4658.
OUTPUT DRIVE CAPABILITY
The OPA4658 has been optimized to drive 75Ω and 100Ω
resistive loads. The device can drive 2Vp-p into a 75Ω load.
This high-output drive capability makes the OPA4658 an
ideal choice for a wide range of RF, IF, and video applica-
tions. In many cases, additional buffer amplifiers are un-
needed.
Many demanding high-speed applications such as
ADC/DAC buffers require op amps with low wideband
output impedance. For example, low output impedance is
essential when driving the signal-dependent capacitances at
the inputs of flash A/D converters. As shown in Figure 3, the
OPA4658 maintains very low closed-loop output impedance
over frequency. Closed-loop output impedance increases
with frequency since loop gain is decreasing with frequency.
100
G = +2
10
max = (±V_S)² /4R_L. Note that it is the voltage across the
output transistor, and not the load, that determines the power
dissipated in the output stage.
The short-circuit condition represents the maximum amount
of internal power dissipation that can be generated. The
variation of output current with temperature is shown in the
Typical Performance Curves.
CAPACITIVE LOADS
The OPA4658’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 5Ω to 25Ω, in series with the output as shown in
Figure 4. 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
1/4
OPA4658
(R_S typically 5Ω to 25Ω)
R_S
50Ω
R_L
C_L
1
0.1
FIGURE 4. Driving Capacitive Loads.
0.01
0.001
10k
100k
1M
10M
Frequency (Hz)
100M
FIGURE 3. Closed-Loop Output Impedance vs Frequency.
THERMAL CONSIDERATIONS
The OPA4658 does not require a heat sink for operation in
most environments. At extreme temperatures and under full
load conditions a heat sink may be necessary.
The internal power dissipation is given by the equation
P_D = P_DQ + P_DL, where P_DQ is the quiescent power dissipa-
tion and P_DL is the power dissipation in the output stage due
to the load. (For ±V_S = ±5V, P_DQ = 10V x 34mA = 340mW,
max). For the case where the amplifier is driving a grounded
load (R_L) with a DC voltage (±V_OUT) the maximum value of
P_DL occurs at ±V_OUT = ±V_S/2, and is equal to P_DL,
COMPENSATION
The OPA4658 is internally compensated and is stable in
gains of two or greater, with a phase margin of approxi-
mately 66° in a gain of +2V/V. (Note that, from a stability
standpoint, an inverting gain of –1V/V is equivalent to a
noise gain of 2.) Gain and phase response for other gains are
shown in the Typical Performance Curves.
The high-frequency response of the OPA4658 in a good
layout is very flat with frequency.
DISTORTION
The OPA4658’s Harmonic Distortion characteristics into a
100Ω load are shown vs frequency and power output in the
Typical Performance Curves. Distortion can be further im-
proved by increasing the load resistance as illustrated in
Figure 5. Remember to include the contribution of the
feedback resistance when calculating the effective load re-
sistance seen by the amplifier.
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OPA4658
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