LMC66 datasheet(18/24 Pages) NSC

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7.0 Compensating for Input

Capacitance

It is quite common to use large values of feedback resis-

tance with amplifiers that have ultra-low input current, like

the LMC6681/2/4. Large feedback resistors can react with

small values of input capacitance due to transducers, photo-

diodes, and circuits board parasitics to reduce phase

margins.

The effect of input capacitance can be compensated for by

adding a feedback capacitor. The feedback capacitor (as in

Figure 17), C

F, is first estimated by:

IN ≤ R2CF

which typically provides significant overcompensation.

Printed circuit board stray capacitance may be larger or

smaller than that of a breadboard, so the actual optimum

value for C

F may be different. The values of CF should be

checked on the actual circuit. (Refer to the LMC660 quad

CMOS amplifier data sheet for a more detailed discussion.)

8.0 Spice Macromodel

A Spice Macromodel is available for the LMC6681/2/4. The

model includes a simulation of:

•

Input common-mode voltage range

•

Frequency and transient response

•

GBW dependence on loading conditions

•

Quiescent and dynamic supply current

•

Output swing dependence on loading conditions

and many more characteristics as listed on the macromodel

disk.

Contact the National Semiconductor Customer Response

Center at 1-800-272-9959 to obtain an operational amplifier

spice macromodel library disk.

Applications

Transducer Interface Circuits

A. PIEZOELECTRIC TRANSDUCERS

The LMC6681 can be used for processing of transducer sig-

nals as shown in the circuit below. The two 11 M

Ω resistors

provide a path for the DC currents to ground. Since the resis-

tors are bootstrapped to the output, the AC input resistance

of the LMC6681 is much higher.

An input current of 80 fA and a CMRR of 82 dB causes an in-

significant 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 offset voltage is produced at the output due to the input

current and the offset voltage of the amplifier. The LMC6682,

which can be operated off a single battery is an excellent

choice for this application with its 80 fA input current and 0.5

mV offset voltage.

DS012042-15

FIGURE 17. Canceling the Effect of Input Capacitance

DS012042-24

FIGURE 18. Transducer Interface Application

DS012042-36

FIGURE 19. LMC6681 Used for Signal Processing

DS012042-26

FIGURE 20. Photodiode Amplifier

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