LMP7717, LMP7718

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SNOSAY7H – MARCH 2007 – REVISED MARCH 2013

For large signal response the rise and fall times are dominated by the slew rate of the op amps. Even though

both parts are quite similar the LMP7717 will give rise and fall times about 2.5 times faster than the LMP7715.

This is possible because the LMP7717 is a decompensated op amp and even though it is being used at a gain of

−1, the speed is preserved by using a good technique for external compensation.

Non-Inverting Compensation

For the non-inverting amp the same theory applies for establishing 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 49, 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 56 below shows the results.

Figure 56. RC = 240Ω and C = 2.2 nF, Gain = +2

Figure 57. LMP7715 Response Gain = +2

The response shown in Figure 56 is close to the response shown in Figure 54. The part is actually slightly faster

in the non-inverting configuration. Decreasing the value of RC to 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 57 shows the performance of the LMP7715 with no compensation. 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 than the fully compensated

parts. Figure 58 shows the gain = 1, or the buffer configuration, for these parts.

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