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OPA650 Fiches technique(PDF) 9 Page - Burr-Brown (TI)

[Old version datasheet] Texas Instruments acquired Burr-Brown Corporation.
No de pièce OPA650
Description  Wideband, Low Power Voltage Feedback OPERATIONAL AMPLIFIER
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Fabricant  BURR-BROWN [Burr-Brown (TI)]
Site Internet  http://www.burr-brown.com
Logo BURR-BROWN - Burr-Brown (TI)

OPA650 Fiches technique(HTML) 9 Page - Burr-Brown (TI)

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®
OPA650
9
DRIVING CAPACITIVE LOADS
The OPA650’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 frequency
peaking or oscillations. Capacitive loads greater than 10pF
should be isolated by connecting a small resistance, usually
15
Ω to 30Ω, 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. Increasing the gain from
+1 will improve the capacitive load drive due to increased
phase margin.
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 in
its characteristic impedance.
THERMAL CONSIDERATIONS
The OPA650 will not require heatsinking under most oper-
ating conditions. Maximum desired junction temperature
will limit the maximum allowed internal power dissipation
as described below. In no case should the maximum junction
temperature be allowed to exceed +175
°C.
Operating junction temperature (TJ) is given by TA +
PDθJA. The total internal power dissipation (PD) is a com-
bination of the total quiescent power (P
DQ) and the power
dissipated in of the output stage (PDL) to deliver load
power. Quiescent power is simply the specified no-load
supply current times the total supply voltage across the
part. P
DL will depend on the required output signal and load
but would, for a grounded resistive load, be at a maximum
when the output is a fixed DC voltage equal to 1/2 of either
supply voltage (assuming equal bipolar supplies). Under
this condition, P
DL = VS
2/(4•R
L) where RL includes feed-
back network loading. Note that it is the power dissipated
in the output stage and not in the load that determines
internal power dissipation. As an example, compute the
maximum T
J for an OPA650N at AV = +2, RL = 100Ω, RFB
= 402
Ω, ±V
S = ±5V, with the output at |VS/2|, and the
specified maximum T
A = +85°C. PD = 10V•8.75mA + (5
2)/
(4•(100
Ω||804Ω)) = 158mW. Maximum T
J = +85°C +
0.158W•150
°C/W = 109°C.
OUTPUT DRIVE CAPABILITY
The OPA650 has been optimized to drive 75
Ω and 100Ω
resistive loads. The device can drive a 2Vp-p into a 75
Ω load.
This high-output drive capability makes the OPA650 an ideal
choice for a wide range of RF, IF, and video applications. In
many cases, additional buffer amplifiers are unneeded.
Many demanding high-speed applications such as driving
A/D converters 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 OPA650
maintains very low-closed loop output impedance over fre-
quency. Closed-loop output impedance increases with fre-
quency since loop gain is decreasing.
FIGURE 3. Small-Signal Output Impedance vs Frequency.
SMALL-SIGNAL OUTPUT IMPEDANCE
vs FREQUENCY
Frequency (Hz)
1k
100
10
1
0.1
0.01
10k
100k
1M
100M
10M
G = +1
OPA650
C
L
R
L
R
ISO
(R
ISO typically 15Ω to 30Ω)
25
FIGURE 4. Driving Capacitive Loads.
FREQUENCY RESPONSE COMPENSATION
The OPA650 is internally compensated and is stable in unity
gain with a phase margin of approximately 60
°. However,
the unity gain buffer is the most demanding circuit configu-
ration for loop stability and oscillations are most likely to
occur in this gain. If possible, use the device in a noise gain
greater than one to improve phase margin and reduce the
susceptibility to oscillation. (Note that, from a stability
standpoint, an inverting gain of –1V/V is equivalent to a
noise gain of 2.) Frequency response for other gains are
shown in the Typical Performance Curves.
The high frequency response of the OPA650 in a good
layout is very flat with frequency. However, some circuit
configurations such as those where large feedback resis-
tances are used, can produce high-frequency gain peaking.
This peaking can be minimized by connecting a small
capacitor in parallel with the feedback resistor. This capaci-
tor compensates for the closed-loop, high-frequency, trans-
fer function zero that results from the time constant formed
by the input capacitance of the amplifier (typically 2pF after
PC board mounting), and the input and feedback resistors.
The selected compensation capacitor may be a trimmer, a
fixed capacitor, or a planned PC board capacitance. The
capacitance value is strongly dependent on circuit layout and
closed-loop gain. Using small resistor values will preserve


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