LM6181IM National Semiconductor, LM6181IM Datasheet - Page 21

• Manufacturer Part Number: LM6181IM
• Description: IC AMP CURR FEEDBK 100MHZ 16SOIC
• Manufacturer: National Semiconductor
• Series: VIP™ IIr

Specifications of LM6181IM

• Amplifier Type: Current Feedback
• Number Of Circuits: 1
• Slew Rate: 2000 V/µs
• -3db Bandwidth: 100MHz
• Current - Input Bias: 5µA
• Voltage - Input Offset: 3500µV
• Current - Supply: 7.5mA
• Current - Output / Channel: 130mA
• Voltage - Supply, Single/dual (±): 7 V ~ 32 V, ±3.5 V ~ 16 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Output Type: -
• Gain Bandwidth Product: -
• Other names: *LM6181IM

Typical Applications
CURRENT FEEDBACK TOPOLOGY
For a conventional voltage feedback amplifier the resulting
small-signal bandwidth is inversely proportional to the de-
sired gain to a first order approximation based on the gain-
bandwidth concept. In contrast, the current feedback ampli-
fier topology, such as the LM6181, transcends this limitation
to offer a signal bandwidth that is relatively independent of
the closed-loop gain. Figure 1a and Figure 1b illustrate that
for closed loop gains of −1 and −5 the resulting pulse fidelity
suggests quite similar bandwidths for both configurations.
The closed-loop bandwidth of the LM6181 depends on the
feedback resistance, R
varied to adjust for the desired closed-loop gain as in Figure
2.
FIGURE 1. 1a, 1b: Variation of Closed Loop Gain
from −1 to −5 Yields Similar Responses
f
. Therefore, R
1a
1b
S
and not R
f
, must be
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21
POWER SUPPLY BYPASSING AND LAYOUT
CONSIDERATIONS
A fundamental requirement for high-speed amplifier design
is adequate bypassing of the power supply. It is critical to
maintain a wideband low-impedance to ground at the ampli-
fiers supply pins to insure the fidelity of high speed amplifier
transient signals. 10 µF tantalum and 0.1 µF ceramic bypass
capacitors are recommended for each supply pin. The by-
pass capacitors should be placed as close to the amplifier
pins as possible (0.5" or less).
FEEDBACK RESISTOR SELECTION: R
Selecting the feedback resistor, R
compensating the LM6181. For general applications the
LM6181 will maintain specified performance with an 820Ω
feedback resistor. Although this value will provide good re-
sults for most applications, it may be advantageous to adjust
this value slightly. Consider, for instance, the effect on pulse
responses with two different configurations where both the
closed-loop gains are 2 and the feedback resistors are 820Ω
and 1640Ω, respectively. Figure 3a and Figure 3b illustrate
the effect of increasing R
closed-loop gain — the amplifier bandwidth decreases. Ac-
cordingly, larger feedback resistors can be used to slow
down the LM6181 (see −3 dB bandwidth vs R
and reduce overshoot in the time domain response. Con-
versely, smaller feedback resistance values than 820Ω can
be used to compensate for the reduction of bandwidth at
high closed loop gains, due to 2nd order effects. For ex-
ample Figure 4 illustrates reducing R
the desired small signal response in an amplifier configured
for a closed loop gain of 25.
the Desired Closed Loop Gain, A
FIGURE 2. R
S
Is Adjusted to Obtain
f
while maintaining the same
f
, is a dominant factor in
f
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to 500Ω to establish
f
f
typical curves)
VCL
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