LMC6584AIM datasheet(14/19 Pages) NSC

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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

Ω resis-

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.

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

B/C = 80 fA/

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 1⁄3 (1.67V) of the supply voltage to 2⁄3 (3.33V) of the

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

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

DS012041-21

FIGURE 15. Transducer Interface Application

DS012041-22

FIGURE 16. LMC6582 Used for Signal Processing

DS012041-23

FIGURE 17. Photodiode Amplifier

DS012041-26

FIGURE 18. Low Voltage Peak Detector

DS012041-27

FIGURE 19. 1 Hz Square-Wave Oscillator

LMC6584AIM Datasheet(PDF) 14 Page - National Semiconductor (TI)

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