LMP7701_06 NSC [National Semiconductor], LMP7701_06 Datasheet - Page 16

• Manufacturer Part Number: LMP7701_06
• Description: Precision, CMOS Input, RRIO, Wide Supply Range Amplifiers
• Manufacturer: NSC [National Semiconductor]

www.national.com
Application Information
This equation is rearranged to find the location of the two
poles:
As shown in Equation (1), as values of R
increased, the magnitude of the poles is reduced, which in
turn decreases the bandwidth of the amplifier. Whenever
possible, it is best to choose smaller feedback resistors.
Figure 3 shows the effect of feedback resistor on the
LMP7701/LMP7702/LMP7704 bandwidth.
Equation (1) has two poles. In most cases, it is the presence
of pairs of poles that causes gain peaking. In order to elimi-
nate this effect, the poles should be placed in Butterworth
position, since poles in Butterworth position do not cause
gain peaking. To achieve a Butterworth pair, the quantity
under the square root in Equation (1) should be set to equal
−1. Using this fact and the relation between R
−A
shown in Equation (2). If R
optimum value, gain peaking will occur.
In Figure 2, C
and to increase stability. Additionally, C
V
R
FIGURE 3. Closed Loop Gain vs. Frequency
1
, the optimum value for R
F
is added to compensate for input capacitance
1
is chosen to be larger than this
1
can be found. This is
F
(Continued)
reduces or elimi-
1
1
and R
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and R
2
, R
2
are
2
(1)
(2)
=
16
nates the gain peaking that can be caused by having a larger
feedback resistor. Figure 4 shows how C
peaking.
DIODES BETWEEN THE INPUTS
The LMP7701/LMP7702/LMP7704 have a set of anti-parallel
diodes between the input pins, as shown in Figure 5. These
diodes are present to protect the input stage of the amplifier.
At the same time, they limit the amount of differential input
voltage that is allowed on the input pins. A differential signal
larger than one diode voltage drop might damage the di-
odes. The differential signal between the inputs needs to be
limited to
±
10 mA.
FIGURE 4. Closed Loop Gain vs. Frequency with
±
300 mV or the input current needs to be limited to
FIGURE 5. Input of LMP7701
Compensation
F
reduces gain
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