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LMC6042AIM Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMC6042AIM Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 13 page Applications Hints (Continued) 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. Typical Single-Supply Applications (V + = 5.0 V DC) The extremely high input impedance, and low power con- sumption, of the LMC6042 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, magnetic field detec- tors, gas detectors, and silicon based pressure transducers. 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 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 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: 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 sys- tems. 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. DS011137-11 (Input pins are lifted out of PC board and soldered directly to components. All other pins connected to PC board.) FIGURE 6. Air Wiring DS011137-12 FIGURE 7. Two Op-Amp Instrumentation Amplifier www.national.com 9 |
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