SM73304 Datasheet, PDF (15/22 Pages) Texas Instruments – Dual and Single Precision, 17 MHz, Low Noise, CMOS Input Amplifiers with Enable

Closed Loop Output Impedance vs. Frequency
Application Notes
30159432
SM73304/SM73305
The SM73304/SM73305 are dual and single, low noise, low
offset, rail-to-rail output precision amplifiers with a wide gain
bandwidth product of 17 MHz and low supply current. The
wide bandwidth makes the SM73304/SM73305 ideal choices
for wide-band amplification in portable applications. The low
supply current along with the enable feature that is built-in on
the SM73304/SM73305 allows for even more power efficient
designs by turning the device off when not in use.
The SM73304/SM73305 are superior for sensor applications.
The very low input referred voltage noise of only 5.8 nV/
at 1 kHz and very low input referred current noise of only 10
fA/ mean more signal fidelity and higher signal-to-noise
ratio.
The SM73304/SM73305 have a supply voltage range of 1.8V
to 5.5V over a wide temperature range of 0°C to 125°C. This
is optimal for low voltage commercial applications. For appli-
cations where the ambient temperature might be less than 0°
C, the SM73304/SM73305 are fully operational at supply volt-
ages of 2.0V to 5.5V over the temperature range of −40°C to
125°C.
The outputs of the SM73304/SM73305 swing within 25 mV of
either rail providing maximum dynamic range in applications
requiring low supply voltage. The input common mode range
of the SM73304/SM73305 extends to 300 mV below ground.
This feature enables users to utilize this device in single sup-
ply applications.
The use of a very innovative feedback topology has enhanced
the current drive capability of the SM73304/SM73305, result-
ing in sourcing currents as much as 47 mA with a supply
voltage of only 1.8V.
The SM73305 is offered in the space saving TSOT23 pack-
age and the SM73304 is offered in a 10-pin MSOP. These
small packages are ideal solutions for applications requiring
minimum PC board footprint.
National Semiconductor is heavily committed to precision
amplifiers and the market segments they serves. Technical
support and extensive characterization data is available for
sensitive applications or applications with a constrained error
budget.
CAPACITIVE LOAD
The unity gain follower is the most sensitive configuration to
capacitive loading. The combination of a capacitive load
placed directly on the output of an amplifier along with the
output impedance of the amplifier creates a phase lag which
in turn reduces the phase margin of the amplifier. If phase
margin is significantly reduced, the response will be either
underdamped or the amplifier will oscillate.
The SM73304/SM73305 can directly drive capacitive loads of
up to 120 pF without oscillating. To drive heavier capacitive
loads, an isolation resistor, R_ISO in Figure 1, should be used.
This resistor and C_L form a pole and hence delay the phase
lag or increase the phase margin of the overall system. The
larger the value of R_ISO, the more stable the output voltage
will be. However, larger values of R_ISO result in reduced output
swing and reduced output current drive.
30159461
FIGURE 1. Isolating Capacitive Load
INPUT CAPACITANCE
CMOS input stages inherently have low input bias current and
higher input referred voltage noise. The SM73304/SM73305
enhance this performance by having the low input bias current
of only 50 fA, as well as, a very low input referred voltage
noise of 5.8 nV/ . In order to achieve this a larger input
stage has been used. This larger input stage increases the
input capacitance of the SM73304/SM73305. Figure 2 shows
typical input common mode input capacitance of the
SM73304/SM73305.
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