This is the 3 op-amp instrumentation amplifier. It is a very high precision circuit and probably the most common type of instrumentation amplifier. The 3 op-amp in-amp has a preamplifier stage that applies gain followed by a subtractor stage that rejects the common-mode. Because of this, the common-mode rejection ratio increases with programmed gain, which is useful because very small signals that require a lot of gain could otherwise be overwhelmed by the common mode rejection error. A drawback to this, however, is that the output of the preamplifier stage experiences a combination of the common-mode voltage and the gained differential signal. One of the op-amps in the preamplifier stage can saturate because of this, giving rise to an extra headroom limitation. This is typically shown as a hexagon plot, showing the limits that the output can swing to for a given common-mode voltage. Some other application challenges of the three op amp in-amp are that an external gain resistor, Rg, cannot track the internal feedback resistors, Rf, over temperature, causing gain drift variation. Furthermore, because the REF pin is at the end of a resistor, any resistance in series with the REF pin unbalances the subtractor and reduces the common mode rejection ratio of the in-amp.