LMC66 datasheet(19/24 Pages) NSC

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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

B/C =

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

2,R3 and

4 will cause the non-inverting input of the LMC6681/2/4 to

move from 1.67V (1⁄3 of 5V) to 3.33V (2⁄3 of 5V). This voltage

behaves as the threshold voltage.

1 and C1 determine the time constant for the circuit. The

frequency of oscillation, f

OSC is

where

∆t is the time the amplifier input takes to move from

1.67V to 3.33V. The calculations are shown below.

where

τ = RC = 0.68 seconds

→ t

1 = 0.27 seconds.

and

→t

2 = 0.74 seconds

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

S = 3V, typical propagation delays would be on

the order of t

PHL = 6 µs, and tPLH = 5 µs.

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

1 and f2 are calculated as

DS012042-23

FIGURE 21. Low Voltage Peak Detector

DS012042-30

FIGURE 22. 1 Hz Square — Wave Oscillator

DS012042-31

FIGURE 23. Comparator with Hysteresis

DS012042-32

FIGURE 24. Wide-Band Band-Pass Filter

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