LMP7717MFE NSC [National Semiconductor], LMP7717MFE Datasheet - Page 13

• Manufacturer Part Number: LMP7717MFE
• Description: 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

Application Information
ADVANTAGES OF THE LMP7717/LMP7718
Wide Bandwidth at Low Supply Current
The LMP7717/LMP7718 are high performance op amps that
provide a GBW of 88 MHz with a gain of 10 while drawing a
low supply current of 1.15 mA. This makes them ideal for pro-
viding wideband amplification in data acquisition applications.
With the proper external compensation the LMP7717 can be
operated at gains of ±1 and still maintain much faster slew
rates than comparable unity gain stable amplifiers. The in-
crease in bandwidth and slew rate is obtained without any
additional power consumption over the LMP7715.
Low Input Referred Noise and Low Input Bias Current
The LMP7717/LMP7718 have a very low input referred volt-
age noise density (5.8 nV/
ensures a small input bias current (100 fA) and low input re-
ferred current noise (0.01 pA/
maintaining signal integrity, and makes the LMP7717/
LMP7718 ideal for audio and sensor based applications.
Low Supply Voltage
The LMP7717 and the LMP7718 have performance guaran-
teed at 2.5V and 5V supply. These parts are guaranteed to
be operational at all supply voltages between 2.0V and 5.5V,
for ambient temperatures ranging from −40°C to 125°C, thus
utilizing the entire battery lifetime. The LMP7717/LMP7718
are also guaranteed to be operational at 1.8V supply voltage,
for temperatures between 0°C and 125°C optimizing their us-
age in low-voltage applications.
RRO and Ground Sensing
Rail-to-Rail output swing provides the maximum possible dy-
namic range. This is particularly important when operating at
low supply voltages. An innovative positive feedback scheme
is used to boost the current drive capability of the output
stage. This allows the LMP7717/LMP7718 to source more
than 40 mA of current at 1.8V supply. This also limits the per-
formance of the these parts as comparators, and hence the
usage of the LMP7717 and the LMP7718 in an open-loop
configuration is not recommended. The input common-mode
range includes the negative supply rail which allows direct
sensing at ground in single supply operation.
Small Size
The small footprints of the LMP7717 packages and the
LMP7718 packages save space on printed circuit boards, and
enable the design of smaller electronic products, such as cel-
lular phones, pagers, or other portable systems. Long traces
between the signal source and the op amp make the signal
path more susceptible to noise pick up.
The physically smaller LMP7717 or LMP7718 packages allow
the op amp to be placed closer to the signal source, thus re-
ducing noise pickup and maintaining signal integrity.
USING THE DECOMPENSATED LMP7717
Advantages of Decompensated Op Amp
A unity gain stable op amp, which is fully compensated, is
designed to operate with good stability down to gains of ±1.
The large amount of compensation does provide an op amp
that is relatively easy to use; however, a decompensated op
amp is designed to maximize the bandwidth and slew rate
without any additional power consumption. This can be very
advantageous.
at 1 kHz). A CMOS input stage
). This is very helpful in
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The LMP7717/LMP7718 require a gain of ±10 to be stable.
However, with an external compensation network (a simple
RC network) these parts can be stable with gains of ±1 and
still maintain the higher slew rate. Looking at the Bode plots
for the LMP7717 and its closest equivalent unity gain stable
op amp, the LMP7715, one can clearly see the increased
bandwidth of the LMP7717. Both plots are taken with a par-
allel combination of 20 pF and 10 kΩ for the output load.
Figure 1 shows the much larger 88 MHz bandwidth of the
LMP7717 as compared to the 17 MHz bandwidth of the
LMP7715 shown in Figure 2. The decompensated LMP7717
has five times the bandwidth of the LMP7715.
What is a Decompensated Op Amp?
The differences between the unity gain stable op amp and the
decompensated op amp are shown in Figure 3. This Bode plot
assumes an ideal two pole system. The dominant pole of the
decompensated op amp is at a higher frequency, f
pared to the unity gain stable op amp which is at f
in Figure 3. This is done in order to increase the speed capa-
bility of the op amp while maintaining the same power dissi-
pation of the unity gain stable op amp. The LMP7717/
LMP7718 have a dominant pole at 1.6 kHz. The unity gain
FIGURE 1. LMP7717 A
FIGURE 2. LMP7715 A
VOL
VOL
vs. Frequency
vs. Frequency
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