OPA646U BURR-BROWN [Burr-Brown Corporation], OPA646U Datasheet - Page 10

• Manufacturer Part Number: OPA646U
• Description: Low Power, Wide Bandwidth OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

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.
COMPENSATION
The OPA646 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
of two or greater 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.) Gain and phase response for other gains are
shown in the Typical Performance Curves.
The high-frequency response of the OPA646 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, transfer
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 capaci-
tance value is strongly dependent on circuit layout and
closed-loop gain. Using small resistor values will preserve
CAPACITIVE LOADS
The OPA646’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 buffered by connecting a small resistance, usually
5
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.
FIGURE 5. Driving Capacitive Loads.
to 25 , in series with the output as shown in Figure 5.
®
OPA646
OPA646
(R
R
R
S
S
L
typically 5 to 25 )
C
L
10
the phase margin and avoid peaking by keeping the break
frequency of this zero sufficiently high. When high closed-
loop gains are required, a three-resistor attenuator (tee
network) is recommended to avoid using large value resis-
tors with large time constants.
SETTLING TIME
Settling time is defined as the total time required, from the
input signal step, for the output to settle to within the
specified error band around the final value. This error band
is expressed as a percentage of the value of the output
transition, a 2V step. Thus, settling time to 0.01% requires
an error band of 200 V centered around the final value
of 2V.
Settling time, specified in an inverting gain of one, occurs in
only 15ns to 0.01% for a 2V step, making the OPA646 one
of the fastest settling monolithic amplifiers commercially
available. Settling time increases with closed-loop gain and
output voltage change as described in the Typical Perform-
ance Curves. Preserving settling time requires critical
attention to the details as mentioned under “Wiring Precau-
tions.” The amplifier also recovers quickly from input
overloads. Overload recovery time to linear operation from
a 50% overload is typically only 65ns.
In practice, settling time measurements on the OPA646
prove to be very difficult to perform. Accurate measurement
is next to impossible in all but the very best equipped labs.
Among other things, a fast flat-top generator and high speed
oscilloscope are needed. Unfortunately, fast flat-top genera-
tors, which settle to 0.01% in sufficient time, are scarce and
expensive. Fast oscilloscopes, however, are more commonly
available. For best results a sampling oscilloscope is recom-
mended. Sampling scopes typically have bandwidths that
are greater than 1GHz and very low capacitance inputs.
They also exhibit faster settling times in response to signals
that would tend to overload a real-time oscilloscope.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (DG) and Differential Phase (DP) are
among the more important specifications for video applica-
tions. DG is defined as the percent change in closed-loop
gain over a specified change in output voltage level. DP is
defined as the change in degrees of the closed-loop phase
over the same output voltage change. Both DG and DP are
specified at the NTSC sub-carrier frequency of 3.58MHz.
DG and DP increase with closed-loop gain and output
voltage transition. All measurements were performed using
a Tektronix model VM700 Video Measurement Set.