SM73302_15 Datasheet, PDF (17/31 Pages) Texas Instruments – SM73302 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier

SM73302
www.ti.com
SNOSB93A – AUGUST 2011 – REVISED APRIL 2013
100
80
A_VOL
60
40
20
1
F
with ADDITIONAL
COMPENSATION
0
PHASE
1
F
ADDITIONAL
COMPENSATION
45° PHASE
MARGIN
2^nd POLE
f₂
GBP
-20
1k
10k 100k 1M 10M
FREQUENCY (Hz)
100M
Figure 48. SM73302 Simplified Bode Plot
To obtain stable operation with gains under 10 V/V the open loop gain margin must be reduced at high
frequencies to where there is a 45° phase margin when the gain margin of the circuit with the external
compensation is 0 dB. The pole and zero in F, the feedback factor, control the gain margin at the higher
frequencies. The distance between F and A_VOL is the gain margin; therefore, the unity gain point (0 dB) is where
F crosses the A_VOL curve.
For the example being used R_IN = R_F for a gain of −1. Therefore F = 6 dB at low frequencies. At the higher
frequencies the minimum value for F is 18 dB for 45° phase margin. From Equation 8 we have the following
relationship:
§
¨¨₁
©
+
R_F
R_IN
§
¨¨¹
©
+
R_IN || R_F
R_C
= 18 dB = 7.9
(9)
Now set R_F = R_IN = R. With these values and solving for R_C we have R_C = R/5.9. Note that the value of C does
not affect the ratio between the resistors. Once the value of the resistors is set, then the position of the pole in F
must be set. A 2 kΩ resistor is used for R_F and R_IN in this design. Therefore the value for R_C is set at 330Ω, the
closest standard value for 2 kΩ/5.9.
Rewriting Equation 5 to solve for the minimum capacitor value gives the following equation:
C = 1/(2πf_pR_C)
(10)
The feedback factor curve, F, intersects the A_VOL curve at about 12 MHz. Therefore the pole of F should not be
any larger than 1.2 MHz. Using this value and R_C = 330Ω the minimum value for C is 390 pF. Figure 49 shows
that there is too much overshoot, but the part is stable. Increasing C to 2.2 nF did not improve the ringing, as
shown in Figure 50.
Figure 49. First Try at Compensation, Gain = −1
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