LMC6041 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – CMOS Single Micropower Operational Amplifier

Applications Hints (Continued)
Typical Single-Supply Applications
(V⁺ = 5.0 V_DC)
The extremely high input impedance, and low power con-
sumption, of the LMC6041 make it ideal for applications that
require battery-powered instrumentation amplifiers. Ex-
amples of these type of applications are hand-held pH
probes, analytic medical instruments, magnetic field detec-
tors, gas detectors, and silicon based pressure transducers.
Inverting Amplifier
DS011136-8
Follower
DS011136-9
DS011136-10
Non-Inverting Amplifier
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.
DS011136-12
FIGURE 7. Two Op-Amp Instrumentation Amplifier
The circuit in Figure 7 is recommended for applications
where the common-mode input range is relatively low and
the differential gain will be in the range of 10 to 1000. This
two op-amp instrumentation amplifier features an indepen-
dent adjustment of the gain and common-mode rejection
trim, and a total quiescent supply current of less than 28 µA.
To maintain ultra-high input impedance, it is advisable to use
ground rings and consider PC board layout an important part
of the overall system design (see Printed-Circuit-Board Lay-
out for High Impedance Work). Referring to Figure 7, the in-
put voltages are represented as a common-mode input V_CM
plus a differential input V_D.
Rejection of the common-mode component of the input is
accomplished by making the ratio of R1/R2 equal to R3/R4.
So that where,
A suggested design guideline is to minimize the difference of
value between R1 through R4. This will often result in im-
proved resistor tempco, amplifier gain, and CMRR over tem-
perature. If RN = R1 = R2 = R3 = R4 then the gain equation
can be simplified:
DS011136-11
(Input pins are lifted out of PC board and soldered directly to components.
All other pins connected to PC board.)
FIGURE 6. Air Wiring
Due to the “zero-in, zero-out” performance of the LMC6041,
and output swing rail-rail, the dynamic range is only limited to
the input common-mode range of 0V to V_S–2.3V, worst case
at room temperature. This feature of the LMC6041 makes it
an ideal choice for low-power instrumentation systems.
A complete instrumentation amplifier designed for a gain of
100 is shown in Figure 8. Provisions have been made for low
sensitivity trimming of CMRR and gain.
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