LMC6042AIJ datasheet(13/24 Pages) TI

13 / 24 page

LMC6042

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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 VDC)

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 VCM plus a

differential input VD.

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 VS − 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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