LMC6681 National Semiconductor, LMC6681 Datasheet - Page 20

• Manufacturer Part Number: LMC6681
• Description: Low Voltage / Rail-To-Rail Input and Output CMOS
• Manufacturer: National Semiconductor

www.national.com
Filters
The LMC6681/2/4, with its rail-to-rail input common-mode
voltage range and high gain (120 dB typical, R
extremely well suited for such filter applications. The
rail-to-rail input range allows for large input signals to be pro-
cessed without distortion. The high gain means that the cir-
cuit can provide filtering and gain in one stage, instead of the
typical two stage filter. This implies a reduction in cost, and
savings of space and power.
This is an illustration of the conceptual use of the LMC6681/
2/4. The selectivity of the filter can be improved by increas-
ing the order (number of poles) of the design.
Sample-and-Hold Circuits
When the “Switch” is closed during the Sample Interval,
C
“Switch” is open, C
the high input impedance of the LMC6681.
Errors in the “hold” voltage are caused by the input current of
the amplifier, the leakage current of the CD4066, and the
leakage current of the capacitor. While an input current of 80
fA minimizes the accumulation rate for error in this circuit, the
LMC6681’s CMRR of 82 dB allows excellent accuracy
throughout the amplifier’s rail-to-rail dynamic capture range.
HOLD
charges up to the value of the input signal when the
FIGURE 25. Sample-and-Hold Application
(Continued)
HOLD
retains this value as it is buffered by
L
= 10 k ) is
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Battery Monitoring Circuit
The LMC6681/2/4 has been optimized for performance at
3V, and also has guaranteed specs at 1.8V and 2.2V. In por-
table applications, the R
notebook, or any other computer which the battery is power-
ing. A desired output voltage can be achieved by
manipulating the ratios of the feedback resistors. During the
charging cycle, the current flows out of the battery as shown.
While during discharge, the current is in the reverse direc-
tion. Since the current can range from a few milliamperes to
amperes, the amplifier will have to sense a signal below
ground during the discharge cycle. At 3V, the LMC6681/2/4
can accept a signal up to 300 mV below ground. The
common-mode voltage range of the LMC6681/2/4, which ex-
tends beyond both rails, is thus a very useful feature in this
application.
A typical offset voltage of 0.5 mV, and CMRR of 82 dB main-
tain accuracy in the circuit output, while the rail-to-rail output
performance allows for a maximum signal range.
FIGURE 27. Circuit Used to Sense Discharging
FIGURE 26. Circuit Used to Sense Charging
LOAD
represents the laptop/
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