LM6317IM NSC [National Semiconductor], LM6317IM Datasheet - Page 6

• Manufacturer Part Number: LM6317IM
• Description: 120 MHz, Fast Settling, Low Power, Voltage Feedback Amplifier
• Manufacturer: NSC [National Semiconductor]

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Application Notes
Using the LM6317
LIMITS AND PRECAUTIONS
The absolute maximum supply voltage which may be ap-
plied to the LM6317 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 (
put of the op amp The absolute maximum differential input
for the LM6317 is 10V across the inputs This limit also ap-
plies 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 LM6317 as com-
parators or forcing the inputs to different voltages In some
designs diodes protection may be needed between the in-
puts as shown in Figure 1
Layout Consideration
PRINTED CIRCUIT BOARDS AND HIGH SPEED
OP AMPS
There are many things to consider when designing PC
boards for high speed op amps Without proper caution it is
very easy and frustrating to have excessive ringing oscilla-
tion and other degraded AC performance in high speed cir-
cuits As a rule the signal traces should be short and wide
to provide low inductance and low impedance paths Any
unused board space needs be grounded to reduce stray
signal pickup Critical components should also be grounded
at a common point to eliminate voltage drop Sockets add
capacitance to the board and can affect frequency perform-
ance It is better to solder the amplifier directly into the PC
board without using any socket
USING PROBES
Active (FET) probes are ideal for taking high frequency
measurements because they have wide bandwidth high in-
put impedance and low input capacitance However the
probe ground leads provide a long ground loop that will pro-
duce errors in measurement Instead the probes can be
grounded directly by removing the ground leads and probe
jackets and using scope probe jacks
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FIGURE 1 Input Protection for LM6317
a
) input and the inverting (
TL H 12542– 20
b
) in-
6
COMPONENTS SELECTION AND FEEDBACK RESISTOR
It is important in high speed applications to keep all compo-
nent leads short because wires are inductive at high fre-
quency For discrete components choose carbon composi-
tion-type resistors and mica-type capacitors Surface mount
components are preferred over decrete components for
minimum inductive effect
Large values of feedback resistors can couple with parasitic
capacitance and cause undersirable effects such as ringing
or oscillation in high speed amplifiers Feedback resistor
value around 1 k
COMPENSATION FOR INPUT CAPACITANCE
The combination of an amplifier’s input capacitance with the
gain setting resistors adds a pole that can cause peaking or
oscillation To solve this problem a feedback capacitor with
a value
can be used to cancel that pole The value of C
found in the DC Electrical Characteristics Table of the data-
sheet Figure 2 illustrates the compensation circuit
Power Supply Bypassing
Bypassing the power supply is necessary to maintain low
power supply impedance across frequency Both positive
and negative power supplies should be bypassed individual-
ly by placing 0 01 F creramic capacitors directly to power
supply pins and 2 2
power supply pins
FIGURE 2 Compensating for Input Capacitance
FIGURE 3 Power Supply Bypassing
C
is recommended
F l
F tantalum capacitors close to the
(R
G c
C
IN
) R
F
TL H 12542 – 22
TL H 12542 – 21
IN
can be