LM6310IM NSC [National Semiconductor], LM6310IM Datasheet - Page 12

• Manufacturer Part Number: LM6310IM
• Description: High Speed Low Power Operational Amplifier with TRI-STATE Output
• Manufacturer: NSC [National Semiconductor]

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Application Information
Using the LM6310
LIMITS AND PRECAUTIONS
SUPPLY VOLTAGE
The absolute maximum supply voltage which may be ap-
plied to the LM6310 is 12V Designers should not design for
more than 10V nominal and carefully check supply toler-
ances under all conditions so that the voltages do not ex-
ceed the maximum
DIFFERENTIAL INPUT VOLTAGE
Differential input voltage is the difference in voltage be-
tween the non-inverting (
(
voltage is
when there is no power supplied to the op amp This may
not be a problem in most conventional op amp designs
however designers should avoid using the LM6310 as com-
parator or forcing the inputs to different voltages
Very fast input pulses into high gain circuits may cause the
output to saturate leading to an overload recovery time in
the millisecond range This requires inputs which are faster
than those usually used in video systems and gain levels
which will push the output of the amplifier toward the limit of
its output swing
LAYOUT AND POWER SUPPLY BYPASSING
Since the LM6310 is a high speed (over 50 MHz) device
good high speed circuit layout practices should be followed
This should include the use of ground planes adequate
power supply bypassing removing metal from around the
input pins to reduce capacitance and careful routing of the
output signal lines to keep them away from the input pins
The power supply pins should be bypassed on both the neg-
ative and positive supply inputs with capacitors placed close
to the pins Surface mount capacitors should be used for
best performance and should be placed as close to the
pins as possible It is generally advisable to use two capaci-
tors at each supply voltage pin A small surface mount ca-
pacitor with a value of around 0 01
ceramic type with good RF performance should be placed
closest to the pin A larger capacitor usually in the range of
1 0
larger capacitor should be a device with good RF character-
istics and low ESR (equivalent series resistance) for best
results Ceramic and tantalum capacitors generally work
well as the larger capacitor
It is very important to reduce capacitance at the input and
output pins The ground plane and any other planes (power
etc ) should be ‘‘opened up’’ or removed near the pins The
opening should extend to the middle of the nearest pins as
a minimum
The LM6310 is built on a high performance bipolar process
The transistors used in this process have bandwidths much
higher that the LM6310 itself These transistors have a po-
tential to oscillate or ring at 400
layouts where the components are more than
(6 mm) away from the op amp pins These oscillations may
produce apparent shifts in voltage offset or excess current
consumption
To avoid this keep the input and output resistors as close
as possible to their respective pins Spacing within
(3 mm) or less is recommended for best results
b
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) of the op amp The absolute maximum differential input
F –4 7
g
2V across the inputs This limit also applies
F should also be placed near the pin The
a
) input and the inverting input
b
1 GHz when used in
F (10 nF) usually a
(Continued)
inch
inch
12
For best performance low inductance resistors such as
chip resistors are recommended The use of wirewound re-
sistors is strongly not recommended
DIP devices should use socket pins which are flush with the
board Conventional sockets have additional capacitance
and are not recommended Obviously the use of wire-
wrapped sockets or the ‘‘white plastic’’ push in prototype
boards is strongly not recommended
FEEDBACK RESISTOR VALUES (R
Since the LM6310 is a current feedback amplifier the value
of the feedback resistor is important to the performance of
the op amp For circuits other than voltage followers the
fastest pulse response is usually obtained with a resistor
value of 348
sistor value (R
348
cal performance curves section of this datasheet )
Current feedback amplifiers generally do not tolerate reac-
tive components in the feedback path Therefore do not
bypass the feedback resistor (R
tor This will result in instability
Overshoot and ringing of the LM6310 can be reduced by
increasing the value of the feedback resistor above 348
The value of 348
damped pulse response with about 3%– 5% overshoot
Overshoot and bandwidth peaking may be undesirable in
some designs Selecting a larger value for the feedback re-
sistor will reduce the overshoot and bandwidth peaking Too
large a value will reduce the circuit bandwidth and degrade
pulse response Do not place a capacitor across feedback
resistor
For voltage followers (A
ance A feedback resistor must be used direct connect
from the output to the inverting input will usually result in
oscillation Values in the range of 1 k –2 k
good results The pulse response photo for A
obtained with a 1 k
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 LM6310 is fast enough to pro-
duce reflected signals in many cables and long circuit
traces For best pulse performance it may be necessary to
terminate cables and long circuit traces with their character-
istic 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
(Schematics for various gains are shown in the typi-
S
To get higher gain decrease the source re-
) while leaving the feedback resistor at
will normally produce a near critically
feedback resistor
V
e a
1) there is no source resist-
F
) with a feedback capaci-
F
)
V
usually give
e a
1 was