OPA650 Burr-Brown, OPA650 Datasheet - Page 9

• Manufacturer Part Number: OPA650
• Description: Wideband / Low Power Voltage Feedback OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

OUTPUT DRIVE CAPABILITY
The OPA650 has been optimized to drive 75
resistive loads. The device can drive a 2Vp-p into a 75 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.
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 (T
P
bination of the total quiescent power (P
dissipated in of the output stage (P
power. Quiescent power is simply the specified no-load
supply current times the total supply voltage across the
part. P
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
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
= 402 , V
specified maximum T
(4•(100 ||804 )) = 158mW. Maximum T
0.158W•150 C/W = 109 C.
D JA
0.01
100
0.1
1k
10
. The total internal power dissipation (P
DL
1
10k
will depend on the required output signal and load
J
S
for an OPA650N at A
= 5V, with the output at |V
SMALL-SIGNAL OUTPUT IMPEDANCE
DL
100k
= V
A
= +85 C. P
S
2
vs FREQUENCY
Frequency (Hz)
/(4•R
1M
L
) where R
D
V
J
= 10V•8.75mA + (5
= +2, R
) is given by T
DL
DQ
) to deliver load
10M
L
) and the power
G = +1
includes feed-
L
J
S
= 100 , R
D
= +85 C +
/2|, and the
) is a com-
and 100
100M
A
2
FB
)/
+
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 to 30 , 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.
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
OPA650
25
(R
R
OPA650
R
ISO
ISO
L
typically 15 to 30 )
C
L
®