sm72501mfx National Semiconductor Corporation, sm72501mfx Datasheet - Page 14
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sm72501mfx
Manufacturer Part Number
sm72501mfx
Description
Solarmagic Precision, Cmos Input, Rrio, Wide Supply Range Amplifier
Manufacturer
National Semiconductor Corporation
Datasheet
1.SM72501MFX.pdf
(20 pages)
www.national.com
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/
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
sistor, the capacitive load is isolated from the amplifier's
output, and hence, the pole caused by C
feedback loop. The larger the value of R
the output voltage will be. If values of R
large, the feedback loop will be stable, independent of the
value of C
output swing and reduced output current drive.
ISO
, in
Figure 1
L
FIGURE 1. Isolating Capacitive Load
. However, larger values of R
should be used. By using this isolation re-
ISO
ISO
L
gives the SM72501
ISO
is no longer in the
, the more stable
result in reduced
are sufficiently
30142121
14
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/
been used. This larger input stage increases the input capac-
itance of the SM72501. The typical value of this input capac-
itance, C
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
R
For the time being, ignore C
Figure 2
This equation is rearranged to find the location of the two
poles:
As shown in
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.
effect of the feedback resistor on the bandwidth of the
SM72501.
1
.
FIGURE 2. Compensating for Input Capacitance
IN
can be calculated as follows:
. In order to achieve this a larger input stage has
, for the SM72501 is 25 pF. The input capacitance
Equation
1, as values of R
F
. The AC gain of the circuit in
Figure 2
1
and R
Figure 3
2
30142144
are increased,
is simply –R
shows the
(1)
2
/