LMC6681 Datasheet, PDF (18/24 Pages) National Semiconductor (TI) – Low Voltage, Rail-To-Rail Input and Output CMOS

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.
Applications
Transducer Interface Circuits
A. PIEZOELECTRIC TRANSDUCERS
DS012042-24
FIGURE 18. Transducer Interface Application
The LMC6681 can be used for processing of transducer sig-
nals as shown in the circuit below. The two 11 MΩ 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.
DS012042-15
FIGURE 17. Canceling the Effect of Input Capacitance
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:
or
R₁C_IN ≤ R₂C_F
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 C_F 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.
DS012042-36
FIGURE 19. LMC6681 Used for Signal Processing
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
DS012042-26
FIGURE 20. Photodiode Amplifier
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.
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