LMC6001AIH NSC [National Semiconductor], LMC6001AIH Datasheet - Page 9

• Manufacturer Part Number: LMC6001AIH
• Description: Ultra Ultra-Low Input Current Amplifier
• Manufacturer: NSC [National Semiconductor]

Applications Hints
ure 4 . To have a significant effect, guard rings should be
placed on both the top and bottom of the PC board. This PC
foil must then be connected to a voltage which is at the same
voltage as the amplifier inputs, since no leakage current can
flow between two points at the same potential. For example,
a PC board trace-to-pad resistance of 10
mally considered a very large resistance, could leak 5 pA if
the trace were a 5V bus adjacent to the pad of the input.
This would cause a 500 times degradation from the
LMC6001’s actual performance. If a guard ring is used and
held within 1 mV of the inputs, then the same resistance of
10
small amount of leakage will degrade the extremely low input
current performance of the LMC6001. See Figure 5 for typi-
cal connections of guard rings for standard op-amp
configurations.
12
FIGURE 5. Typical Connections of Guard Rings
will only cause 10 fA of leakage current. Even this
FIGURE 4. Examples of Guard
Ring in PC Board Layout
Non-Inverting Amplifier
Inverting Amplifier
Follower
(Continued)
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, which is nor-
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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 .
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board).
Another potential source of leakage that might be over-
looked is the device package. When the LMC6001 is manu-
factured, the device is always handled with conductive finger
cots. This is to assure that salts and skin oils do not cause
leakage paths on the surface of the package. We recom-
mend that these same precautions be adhered to, during all
phases of inspection, test and assembly.
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 LMC6001 is
designed to withstand 100 mA surge current on the I/O pins.
Some resistive method should be used to isolate any capaci-
tance from supplying excess current to the I/O pins. In addi-
tion, like an SCR, there is a minimum holding current for any
latchup mode. Limiting current to the supply pins will also in-
hibit latchup susceptibility.
Typical Applications
The extremely high input resistance, and low power con-
sumption, of the LMC6001 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, electrostatic field de-
tectors and gas chromotographs.
Two Opamp, Temperature
Compensated pH Probe Amplifier
The signal from a pH probe has a typical resistance between
10 M
important that the amplifier input currents be as small as
possible. The LMC6001 with less than 25 fA input current is
an ideal choice for this application.
The theoretical output of the standard Ag/AgCl pH probe is
59.16 mV/pH at 25˚C with 0V out at a pH of 7.00. This output
is proportional to absolute temperature. To compensate for
this, a temperature compensating resistor, R1, is placed in
and 1000 M . Because of this high value, it is very
FIGURE 6. Air Wiring
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