LMP7717MFE NSC [National Semiconductor], LMP7717MFE Datasheet - Page 17

• Manufacturer Part Number: LMP7717MFE
• Description: 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

Non-Inverting Compensation
For the non-inverting amp the same theory applies for estab-
lishing the needed compensation. When setting the inverting
configuration for a gain of −1, F has a value of 2. For the non-
inverting configuration both F and the actual gain are the
same, making the non-inverting configuration more difficult to
compensate. Using the same circuit as shown in Figure 4, but
setting up the circuit for non-inverting operation (gain of +2)
results in similar performance as the inverting configuration
with the inputs set to half the amplitude to compensate for the
additional gain. Figure 11 below shows the results.
The response shown in Figure 11 is close to the response
shown in Figure 9. The part is actually slightly faster in the
non-inverting configuration. Decreasing the value of R
around 200Ω can decrease the negative overshoot but will
have slightly longer rise and fall times. The other option is to
add a small resistor in series with the input signal. Figure 12
shows the performance of the LMP7715 with no compensa-
tion. Again the decompensated parts are almost 2.5 times
faster than the fully compensated op amp.
The most difficult op amp configuration to stabilize is the gain
of +1. With proper compensation the LMP7717/LMP7718 can
be used in this configuration and still maintain higher speeds
FIGURE 11. R
FIGURE 12. LMP7715 Response Gain = +2
C
= 240Ω and C = 2.2 nF, Gain = +2
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30010883
C
to
17
than the fully compensated parts. Figure 13 shows the gain =
1, or the buffer configuration, for these parts.
Figure 13 is the result of using Equation 5 and additional ex-
perimentation in the lab. R
necessary to introduce another pole at the input stage for
good performance at gain = +1. Equation 5 is shown below
with R
Using 2 kΩ for R
290Ω. The closest standard value for R
fine tuning in the lab R
as the optimum values. R
at the non-inverting pin inserts another pole into the compen-
sation for the LMP7717. Adding this pole and slightly reducing
the compensation for 1/F (using a slightly higher resistor value
for R
14 is the response of the circuit shown in Figure 13. Figure
15 shows the response of the LMP7715 in the buffer config-
uration with no compensation and R
FIGURE 13. LMP7717 with Lead-Lag Compensation for
FIGURE 14. R
C
) gives the optimum response for a gain of +1. Figure
IN
=
∞
.
Non-Inverting Configuration
F
and solving for R
C
= 330Ω and C = 10 nF, Gain = +1
C
= 330Ω and R
P
together with the input capacitance
P
is not part of Equation 5, but it is
C
P
P
gives R
= R
C
= 1.5 kΩ were chosen
is 300Ω. After some
F
= 0.
C
30010884
= 2000/6.9 =
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