Using the LM6144, a 3 op amp instrumentation amplifier with rail-to-rail inputs and rail to rail output can be made. These features make these instrumentation amplifiers an excellent choice for single supply systems.

Some manufacturers use a precision voltage divider array of 5 resistors to divide the common-mode voltage to get an input range of rail-to-rail or greater. The problem with this method is that the method also divides the signal, so to even get unity gain, the amplifier must be run at high closed loop gains. This raises the noise and drift by the internal gain factor and lowers the input impedance. Any mismatch in these precision resistors reduces the CMR as well. Using the LM6144, all of these problems are eliminated.

In this example, amplifiers A and B act as buffers to the differential stage (Figure 6-1). These buffers maintain that the input impedance is over 100MΩ and the buffers eliminate the requirement for precision matched resistors in the input stage. The buffers also maintain that the difference amp is driven from a voltage source. This is necessary to maintain the CMR set by the matching of R1–R2 with R3–R4.

Figure 6-1

The gain is set by the ratio of R2/R1 and R3 equals R1 and R4 equal R2. Making R4 slightly smaller than R2 and adding a trim pot equal to twice the difference between R2 and R4 allows the CMR to be adjusted for optimum.

With both rail to rail input and output ranges, the inputs and outputs are only limited by the supply voltages. Remember that even with rail-to-rail output, the output cannot swing past the supplies so the combined common mode voltage plus the signal must not be greater than the supplies or limiting occurs.