SM72501 Datasheet, PDF (15/20 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

DIODES BETWEEN THE INPUTS
The SM72501 has 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 volt-
age drop might damage the diodes. The differential signal
between the inputs needs to be limited to ±300 mV or the input
current needs to be limited to ±10 mA.
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FIGURE 3. Closed Loop Gain vs. Frequency
Equation 1 has two poles. In most cases, it is the presence of
pairs of poles that causes gain peaking. In order to eliminate
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 R₁ and R₂, R₂ = −A_V R₁, the optimum
value for R₁ can be found. This is shown in Equation 2. If R₁
is chosen to be larger than this optimum value, gain peaking
will occur.
(2)
In Figure 2, C_F is added to compensate for input capacitance
and to increase stability. Additionally, C_F reduces or elimi-
nates the gain peaking that can be caused by having a larger
feedback resistor. Figure 4 shows how C_F reduces gain peak-
ing.
FIGURE 5. Input of SM72501
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FIGURE 4. Closed Loop Gain vs. Frequency with
Compensation
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