LMC6582AIMX NSC [National Semiconductor], LMC6582AIMX Datasheet - Page 14

• Manufacturer Part Number: LMC6582AIMX
• Description: Low Voltage, Rail-To-Rail Input and Output CMOS Operational Amplifier
• Manufacturer: NSC [National Semiconductor]

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Applications
Transducer Interface Circuits
A. PIEZ0ELECTRIC TRANSDUCERS
The LMC6582/4 can be used for processing of transducer
signals as shown in the circuit below. The two 11 M
tors provide a path for the DC currents to ground. Since the
resistors are boot-strapped to the output, the AC input resis-
tance of the LMC6582/4 is much higher.
An input current of 80 fA and a CMRR of 82 dB causes an in-
signifcant error offset voltage at the output. The rail-to-rail
performance of the amplifier also provides the maximum dy-
namic range for the transducer signals.
B. PHOTODIODE AMPLIFIERS
Photocells can be used in light measuring instruments. An
error voltage is produced at the output due to the input cur-
rent and the offset voltage of the amplifier. The LMC6582/4
which can be operated off a single battery is an excellent
choice for this application because of its 80 fA input current
and 0.5 mV offset voltage.
FIGURE 16. LMC6582 Used for Signal Processing
FIGURE 15. Transducer Interface Application
FIGURE 17. Photodiode Amplifier
DS012041-22
DS012041-23
DS012041-21
resis-
14
Low Voltage Peak Detector
The accuracy of the peak detector is dependent on the leak-
age currents of the diodes and the capacitor, and the
non-idealities of the amplifier. The parameters of the amplifer
which can limit the performance of this circuit are (a) Finite
slew rate (b) Input current, and (c) Maximum output current
of the amplifier.
The input current of the amplifier causes a slow discharge of
the capacitor. This phenomenon is called “drooping”. The
LMC6582/4 has a typical input current of 80 fA. This would
cause the capacitor to droop at a rate of dv/dt = I
100 pF = 0.8 mV/s. Accuracy in the amplitude measurement
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 0V to 5V. The voltage divider set by the resistors will
cause the input at the non-inverting terminal of the op-amp to
move
supply voltage. This voltage behaves as the threshold volt-
age, and causes the capacitor to alternately charge and dis-
charge.
R1 and C1 determine the time constant for the circuit. The
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
1
⁄
3
FIGURE 19. 1 Hz Square-Wave Oscillator
FIGURE 18. Low Voltage Peak Detector
= RC = 0.68 seconds
(1.67V) of the supply voltage to
OSC
is
2
⁄
3
DS012041-27
(3.33V) of the
B
/C = 80 fA/
DS012041-26