LMC6681 National Semiconductor, LMC6681 Datasheet - Page 19

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

Low Voltage Peak Detector
The accuracy of the peak detector is dependent on the leak-
age currents of the diodes and the capacitors, and the
non-idealities of the amplifier. The parameters of the ampli-
fier which can limit the performance of this circuit are (a) Fi-
nite slew rate, (b) Input current, and (c) Maximum output cur-
rent of the amplifier.
The input current of the amplifier causes a slow discharge of
the capacitor. This phenomenon is called “drooping”. The
LMC6682 has a typical input current of 80 fA. This would
cause the capacitor to droop at a rate of dV/dt = I
80 fA/100 pF = 0.8 mV/s. Accuracy in the amplitude mea-
surement is also maintained by an offset voltage of 0.5 mV,
and an open-loop gain of 120 dB.
Oscillators
For single supply 5V operation, the output of the circuit will
swing from 0V to 5V. The voltage divider set up R
R
move from 1.67V (
behaves as the threshold voltage.
R
frequency of oscillation, f
where t is the time the amplifier input takes to move from
1.67V to 3.33V. The calculations are shown below.
where
and
4
1
t
will cause the non-inverting input of the LMC6681/2/4 to
and C
1
= 0.27 seconds.
FIGURE 22. 1 Hz Square — Wave Oscillator
= RC = 0.68 seconds
FIGURE 21. Low Voltage Peak Detector
1
determine the time constant for the circuit. The
1
⁄
3
of 5V) to 3.33V (
OSC
is
2
⁄
3
of 5V). This voltage
DS012042-30
2
, R
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B
3
/C =
and
19
Then,
LMC6681/2/4 as a Comparator
Figure 23 shows the application of the LMC6681/2/4 as a
comparator. The hysteresis is determined by the ratio of the
two resistors. Since the supply current of the LMC6681/2/4 is
less than 1 mA, it can be used as a low power comparator, in
applications where the quiescent current is an important pa-
rameter. At V
the order of t
Filters
The filter shown in Figure 24 is used to process “voice-band”
signals. The bandpass filter has a gain of 40 dB. The two
corner frequencies, f
t
2
= 0.74 seconds
FIGURE 24. Wide-Band Band-Pass Filter
FIGURE 23. Comparator with Hysteresis
S
PHL
= 3V, typical propagation delays would be on
= 6 µs, and t
1
and f
2
are calculated as
PLH
= 5 µs.
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