LMC6061 Datasheet, PDF (8/12 Pages) National Semiconductor (TI) – Precision CMOS Single Micropower Operational Amplifier

Applications Hints (Continued)
Inverting Amplifier
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Non-Inverting Amplifier
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Follower
FIGURE 5. Typical Connections of Guard Rings
The designer should be aware that when it is inappropriate
to lay out a PC board for the sake of just a few circuits, there
is another technique which is even better than a guard ring
on a PC board: Don’t insert the amplifier’s input pin into the
board at all, but bend it up in the air and use only air as an in-
sulator. Air is an excellent insulator. In this case you may
have to forego some of the advantages of PC board con-
struction, but the advantages are sometimes well worth the
effort of using point-to-point up-in-the-air wiring. See Figure
6.
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(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board).
FIGURE 6. Air Wiring
Latchup
CMOS devices tend to be susceptible to latchup due to their
internal parasitic SCR effects. The (I/O) input and output pins
look similar to the gate of the SCR. There is a minimum cur-
rent required to trigger the SCR gate lead. The LMC6061
and LMC6081 are designed to withstand 100 mA surge cur-
rent on the I/O pins. Some resistive method should be used
to isolate any capacitance from supplying excess current to
the I/O pins. In addition, like an SCR, there is a minimum
holding current for any latchup mode. Limiting current to the
supply pins will also inhibit latchup susceptibility.
Typical Single-Supply
Applications (V⁺ = 5.0 V_DC)
The extremely high input impedance, and low power con-
sumption, of the LMC6061 make it ideal for applications that
require battery-powered instrumentation amplifiers. Ex-
amples of these types of applications are hand-held pH
probes, analytic medical instruments, magnetic field detec-
tors, gas detectors, and silicon based pressure transducers.
Figure 7 shows an instrumentation amplifier that features
high differential and common mode input resistance
(>10¹⁴Ω), 0.01% gain accuracy at A_V = 100, excellent
CMRR with 1 kΩ imbalance in bridge source resistance. In-
put current is less than 100 fA and offset drift is less than
2.5 µV/˚C. R₂ provides a simple means of adjusting gain
over a wide range without degrading CMRR. R₇ is an initial
trim used to maximize CMRR without using super precision
matched resistors. For good CMRR over temperature, low
drift resistors should be used.
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