LM7131BCM5X National Semiconductor, LM7131BCM5X Datasheet - Page 17

• Manufacturer Part Number: LM7131BCM5X
• Description: Tiny High Speed Single Supply Operational Amplifier
• Manufacturer: National Semiconductor

Using the LM7131
Feedback Resistor Values and Feedback
Compensation
Using large values of feedback resistances (roughly 2k) with
low gains (such gains of 2) will result in degraded pulse re-
sponse and ringing. The large resistance will form a pole
with the input capacitance of the inverting input, delaying
feedback to the amplifier. This will produce overshoot and
ringing. To avoid this, the gain setting resistors should be
scaled to lower values (below 1k) At higher gains (
values of feedback resistors can be used.
Overshoot and ringing of the LM7131 can be reduced by
adding a small compensation capacitor across the feed back
resistor. For the LM7131 values in pF to tens of pF range are
useful initial values. Too large a value will reduce the circuit
bandwidth and degrade pulse response.
Since the small stray capacitance from the circuit layout,
other components, and specific circuit bandwidth require-
ments will vary, it is often useful to select final values based
on prototypes which are similar in layout to the production
circuit boards.
Reflections
The output slew rate of the LM7131 is fast enough to pro-
duce reflected signals in many cables and long circuit traces.
For best pulse performance, it may be necessary to termi-
nate cables and long circuit traces with their characteristic
impedance to reduce reflected signals.
Reflections should not be confused with overshoot. Reflec-
tions will depend on cable length, while overshoot will de-
pend on load and feedback resistance and capacitance.
When determining the type of problem, often removing or
drastically shortening the cable will reduce or eliminate re-
flections. Overshoot can exist without a cable attached to the
op amp output.
Driving Flash A/D Converters (Video Converters)
The LM7131 has been optimized to drive flash analog to
digital converters in a +5V only system. Different flash A/D
converters have different voltage input ranges. The LM7131
has enough gain-bandwidth product to amplify standard
video level signals to voltages which match the optimum in-
put range of many types of A/D converters.
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For example, the popular 1175 type 8-bit flash A/D converter
has a preferred input range from 0.6V to 2.6V. If the input
signal has an active video range (excluding sync levels) of
approximately 700 mV, a circuit like the one in Figure 13 can
be used to amplify and drive an A/D. The 10 µF capacitor
blocks the DC components, and allows the + input of the
LM7131 to be biased through R clamp so that the minimum
output is equal to V
circuit is determined as follows:
based on the A/D input capacitance and the desired pulse
response of the system. The nominal values of 33
pF shown in the schematic may be a useful starting point,
however, signal levels, A/D converters, and system perfor-
mance requirements will require modification of these val-
ues.
The isolation resistor, R isolation should be placed close to
the output of the LM7131, which should be close to the A/D
input for best results.
R clamp is connected to a voltage level which will result in
the bottom of the video signal matching the Vrb level of the
A/D converter. This level will need to be set by clamping the
black level of the video signal. The clamp voltage will depend
on the level and polarity of the video signal. Detecting the
sync signal can be done by a circuit such as the LM1881
Video Sync Separator.
Note: This is an illustration of a conceptual use of the LM7131, not a com-
Some A/D converters have wide input ranges where the
lower reference level can be adjusted. With these convert-
ers, best distortion results are obtained if the lower end of the
output range is about 250 mV or more above the V− input of
the LM7131 more. The upper limit can be as high as 4.0V
with good results.
Output Signal Range = 2.6V (V top) = 0.6V (V bottom) =
2.0V
Gain = Output Signal Range/Input Signal = 2.857 =
2.00/0.700
Gain = (R
R isolation and C
plete design. The circuit designer will need to modify this for input pro-
tection, sync, and possibly some type of gain control for varying signal
levels.
f
/R
1
) +1 = (249 /133 ) +1
RB
f
will be determined by the designer
of the A/D converter. The gain of the
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and 5.6