LMC6042_17 Datasheet, PDF (13/24 Pages) Texas Instruments – CMOS Dual Micropower Operational Amplifier

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LMC6042
SNOS611E – AUGUST 1999 – REVISED MARCH 2013
(Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.)
Figure 37. Air Wiring
Typical Single-Supply Applications
(V⁺ = 5.0 V_DC)
The extremely high input impedance, and low power consumption, of the LMC6042 make it ideal for applications
that require battery-powered instrumentation amplifiers. Examples of these types of applications are hand-held
pH probes, analytic medical instruments, magnetic field detectors, gas detectors, and silicon based pressure
transducers.
The circuit in Figure 38 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
independent adjustment of the gain and common-mode rejection trim, and a total quiescent supply current of less
than 20 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 Layout for High Impedance
Work). Referring to Figure 38, the input 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,
(3)
A suggested design guideline is to minimize the difference of value between R1 through R4. This will often result
in improved resistor tempco, amplifier gain, and CMRR over temperature. If RN = R1 = R2 = R3 = R4 then the
gain equation can be simplified:
(4)
Due to the “zero-in, zero-out” performance of the LMC6042, 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
LMC6042 makes it an ideal choice for low-power instrumentation systems.
A complete instrumentation amplifier designed for a gain of 100 is shown in Figure 39. Provisions have been
made for low sensitivity trimming of CMRR and gain.
Figure 38. Two Op-Amp Instrumentation Amplifier
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