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参数资料
| 型号： | LM6172MDC |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 2 CHANNEL, VIDEO AMPLIFIER, UUC |
| 封装： | DIE |
| 文件页数： | 7/23页 |
| 文件大小： | 633K |
| 代理商： | LM6172MDC |
Application Notes (Continued)
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 voltage drop. Sockets add capacitance to
the board and can affect frequency performance. 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
input impedance and low input capacitance. However, the
probe ground leads provide a long ground loop that will
produce errors 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 fre-
quency.
For
discrete
components,
choose
carbon
composition-type resistors and mica-type capacitors. Sur-
face mount components are preferred over discrete compo-
nents for minimum 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 feed-
back resistor less than 1 k
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
C
F > (RG xCIN)/RF
can be used to cancel that pole. For LM6172, a feedback
capacitor of 2 pF is recommended.
Figure 4 illustrates the
compensation circuit.
01258145
FIGURE 1. Isolation Resistor Used
to Drive Capacitive Load
01258151
FIGURE 2. The LM6172 Driving a 510 pF Load
with a 30
Isolation Resistor
01258152
FIGURE 3. The LM6172 Driving a 220 pF Load
with a 50
Isolation Resistor
01258146
FIGURE 4. Compensating for Input Capacitance
LM6172
www.national.com
15
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