SM72501 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

Application Information
SM72501
The SM72501 is a low offset voltage, rail-to-rail input and out-
put precision amplifier with a CMOS input stage and wide
supply voltage range of 2.7V to 12V. The SM72501 has a very
low input bias current of only ±200 fA at room temperature.
The wide supply voltage range of 2.7V to 12V over the ex-
tensive temperature range of −40°C to 125°C makes the
SM72501 an excellent choice for low voltage precision appli-
cations with extensive temperature requirements.
The SM72501 has only ±37 μV of typical input referred offset
voltage and this offset is guaranteed to be less than ±500 μV
over temperature. This minimal offset voltage allows more
accurate signal detection and amplification in precision appli-
cations.
The low input bias current of only ±200 fA along with the low
input referred voltage noise of 9 nV/ gives the SM72501
superiority for use in sensor applications. Lower levels of
noise from the SM72501 means better signal fidelity and a
higher signal-to-noise ratio.
National Semiconductor is heavily committed to precision
amplifiers and the market segment they serve. Technical sup-
port and extensive characterization data is available for sen-
sitive applications or applications with a constrained error
budget.
The SM72501 is offered in the space saving 5-Pin SOT23.
This small package is an ideal solution for area constrained
PC boards and portable electronics.
CAPACITIVE LOAD
The SM72501 can be connected as a non-inverting unity gain
follower. This configuration is the most sensitive to capacitive
loading.
The combination of a capacitive load placed on the output of
an amplifier along with the amplifier's output impedance cre-
ates a phase lag which in turn reduces the phase margin of
the amplifier. If the phase margin is significantly reduced, the
response will be either underdamped or it will oscillate.
In order to drive heavier capacitive loads, an isolation resistor,
R_ISO, in Figure 1 should be used. By using this isolation re-
sistor, the capacitive load is isolated from the amplifier's
output, and hence, the pole caused by C_L is no longer in the
feedback loop. The larger the value of R_ISO, the more stable
the output voltage will be. If values of R_ISO are sufficiently
large, the feedback loop will be stable, independent of the
value of C_L. However, larger values of R_ISO result in reduced
output swing and reduced output current drive.
INPUT CAPACITANCE
CMOS input stages inherently have low input bias current and
higher input referred voltage noise. The SM72501 enhances
this performance by having the low input bias current of only
±200 fA, as well as, a very low input referred voltage noise of
9 nV/ . In order to achieve this a larger input stage has
been used. This larger input stage increases the input capac-
itance of the SM72501. The typical value of this input capac-
itance, C_IN, for the SM72501 is 25 pF. The input capacitance
will interact with other impedances such as gain and feedback
resistors, which are seen on the inputs of the amplifier, to form
a pole. This pole will have little or no effect on the output of
the amplifier at low frequencies and DC conditions, but will
play a bigger role as the frequency increases. At higher fre-
quencies, the presence of this pole will decrease phase mar-
gin and will also cause gain peaking. In order to compensate
for the input capacitance, care must be taken in choosing the
feedback resistors. In addition to being selective in picking
values for the feedback resistor, a capacitor can be added to
the feedback path to increase stability.
The DC gain of the circuit shown in Figure 2 is simply –R₂/
R₁.
30142144
FIGURE 2. Compensating for Input Capacitance
For the time being, ignore C_F. The AC gain of the circuit in
Figure 2 can be calculated as follows:
This equation is rearranged to find the location of the two
poles:
30142121
FIGURE 1. Isolating Capacitive Load
(1)
As shown in Equation 1, as values of R₁ and R₂ are increased,
the magnitude of the poles is reduced, which in turn decreas-
es the bandwidth of the amplifier. Whenever possible, it is
best to choose smaller feedback resistors. Figure 3 shows the
effect of the feedback resistor on the bandwidth of the
SM72501.
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