lm6172mdr National Semiconductor Corporation, lm6172mdr Datasheet - Page 14

• Manufacturer Part Number: lm6172mdr
• Description: Lm6172qml Dual High Speed, Low Power, Low Distortion, Voltage Feedback Amplifiers
• Manufacturer: National Semiconductor Corporation

www.national.com
Another possible source of overshoot and undershoot comes
from capacitive load at the output. Please see the section
“Driving Capacitive Loads” for more detail.
DRIVING CAPACITIVE LOADS
Amplifiers driving capacitive loads can oscillate or have ring-
ing at the output. To eliminate oscillation or reduce ringing, an
isolation resistor can be placed as shown in
combination of the isolation resistor and the load capacitor
forms a pole to increase stability by adding more phase mar-
gin to the overall system. The desired performance depends
upon the value of the isolation resistor; the bigger the isolation
resistor, the more damped (slow) the pulse response be-
comes. For LM6172, a 50Ω isolation resistor is recommended
for initial evaluation.
FIGURE 3. The LM6172 Driving a 220 pF Load
FIGURE 2. The LM6172 Driving a 510pF Load
FIGURE 1. Isolation Resistor Used
with a 30Ω Isolation Resistor
with a 50Ω Isolation Resistor
to Drive Capacitive Load
Figure
20159445
1. The
20159451
20159452
14
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
to have excessive ringing, oscillation and other degraded AC
performance in high speed circuits. As a rule, the signal traces
should be short and wide to provide low inductance and low
impedance paths. Any unused board space needs to be
grounded to reduce stray signal pickup. Critical components
should also be grounded at a common point to eliminate volt-
age drop. Sockets add capacitance to the board and can
affect frequency performance. It is better to solder the ampli-
fier directly into the PC board without using any socket.
Using Probes
Active (FET) probes are ideal for taking high frequency mea-
surements because they have wide bandwidth, high input
impedance and low input capacitance. However, the probe
ground leads provide a long ground loop that will produce er-
rors in measurement. Instead, the probes can be grounded
directly by removing the ground leads and probe jackets and
using scope probe jacks.
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 frequen-
cy. For discrete components, choose carbon composition-
type resistors and mica-type capacitors. Surface mount
components are preferred over discrete components for min-
imum inductive effect.
Large values of feedback resistors can couple with parasitic
capacitance and cause undesirable effects such as ringing or
oscillation in high speed amplifiers. For LM6172, a feedback
resistor less than 1kΩ gives optimal performance.
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. For LM6172, a feedback ca-
pacitor of 2pF is recommended.
pensation 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 individually by
FIGURE 4. Compensating for Input Capacitance
C
F
> (R
G
× C
Figure 4
IN
)/R
F
illustrates the com-
20159446