Datasheet

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

= 5.0 V

)

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

plus a

differential input V

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

− 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

Product Folder Links: LMC6042