The LMC648x have high input impedance, large common-mode range and high CMRR needed for designing instrumentation circuits. Instrumentation circuits designed with the LMC648x can reject a larger range of common-mode signals than most in-amps. This makes instrumentation circuits designed with the LMC648x an excellent choice for noisy or industrial environments. Other applications that benefit from these features include analytic medical instruments, magnetic field detectors, gas detectors, and silicon-based transducers.

A small valued potentiometer is used in series with R_G to set the differential gain of the 3-op-amp instrumentation circuit in Figure 7-2. This combination is used instead of one large valued potentiometer to increase gain trim accuracy and reduce error due to vibration. An improved design that can help increase accuracy, save cost, and reduce board space can be achieved by using the RES11A matched resistor pair series.

Figure 7-2 Low Power, Three Op Amp Instrumentation Amplifier

The Figure 7-3 shows how a high precision, high CMRR, and low drift in-amp can be achieved using two matched resistor pairs. Using a 1:4 ratio, a gain of 36V/V can be easily implemented. Other gain options are possible by using the various ratios available. One downside to the original implementation in Figure 7-2 is that very high performance, 0.01% resistors and a couple of potentiometers are needed to achieve very high common-mode rejection and gain accuracy. High accuracy resistors can be very expensive and add to board layout size and complexity. Another downside is that the temperature drift of the discrete resistors causes an increase in gain error that is not easily calibrated out.

The RES11A matched resistor pairs provide high common-mode rejection and gain-error performance due to excellent matching to less than 0.05%. The resistors are on the same substrate; therefore, the resistors drift in the same direction, minimizing temperature-related errors such as gain error drift. For a more detailed analysis of the benefits of the RES11A over discrete resistors, see the Optimizing CMRR in Differential Amplifier Circuits With Precision Matched Resistor Divider Pairs application note.

Figure 7-3 Improved Low Power, Three Op Amp Instrumentation Amplifier With RES11A

A two op amp instrumentation amplifier designed for a gain of 100V/V is shown in Figure 7-4. Low sensitivity trimming is made for offset voltage, CMRR, and gain. Low cost and low power consumption are the main advantages of this two op amp circuit. An alternative circuit with a gain of 10V/V with the RES11A is also provided for this circuit in Figure 7-5.

Higher frequency and larger common-mode range applications are best facilitated by a three op amp instrumentation amplifier.

Figure 7-4 Low Power, Two Op Amp Instrumentation Amplifier

Figure 7-5 Low Power, Two Op Amp Instrumentation Amplifier with RES11A