SM73302_15 Datasheet, PDF (19/31 Pages) Texas Instruments – SM73302 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier

SM73302
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SNOSB93A – AUGUST 2011 – REVISED APRIL 2013
To summarize, the following steps were taken to compensate the SM73302 for a gain of −1:
1. Values for R_c and C were calculated from the Bode plot to give an expected phase margin of 45°. The values
were based on R_IN = R_F = 2 kΩ. These calculations gave R_c = 330Ω and C = 390 pF.
2. To reduce the ringing C was increased to 2.2 nF which moved the pole of F, the feedback factor, farther
away from the A_VOL curve.
3. There was still too much ringing so 2 dB of over-compensation was added to F. This was done by
decreasing R_C to 240Ω.
The LMP7715 is the fully compensated part (without the Renewable Energy Grade), which is comparable to the
SM73302 . Using the LMP7715 in the same setup, but removing the compensation network, provided the
response shown in Figure 53 .
Figure 53. LMP7715 Response
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 SM73302 will give rise and fall times about 2.5 times faster than the LMP7715.
This is possible because the SM73302 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 47, 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 54 below shows the results.
Figure 54. R_C = 240Ω and C = 2.2 nF, Gain = +2
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