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BAKER’S BEST: Common-mode range can bite hard

My last column provided a glimpse inside the CMR(common mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see “Understanding CMR and instrumentation amplifiers,” EDN Europe, January 2010, pg 17, www.edn-europe.com/article.asp?articleid=3698). This story goes deeper, however, if you look at the common-mode range of the same device. Of all the performance characteristics of an INA, the most misunderstood is the common-mode-range requirement. So how do designers calculate the INA’s common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility.

Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). Pay attention to the voltage levels of the INA’s V_IN+ and V_IN- input pins, the V_OA1 and V_OA2 output levels of A₁ and A₂, and the V_OUT output-swing capability of A₃. As you work with these concepts, you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device’s normal output range.

The applied input voltages to the INA are equivalent to the commonmode input voltage plus or minus the differential input signal. The input stages of A₁ and A₂ limit the range of these two input voltages. The maximum and minimum input-voltage limits vary from device to device.

The input voltages at V_IN+ and V_IN- and the gain of A₁ and A₂ cause the internal output voltages to increase or decrease. Note that the A₁ and A₂ stages do not gain the input common-mode voltage, VCM. An input/gain-overload condition can occur if A₁’s output voltage, A₂’s output voltage, or both violate the internal output-swing restrictions. This condition is impossible to directly measure. You must be aware of the limitations of V_OA1 and V_OA2 and then calculate whether your design is at risk using the equations in Figure 1.

An example of this type of input/ gain-overload condition occurs when the in-range voltages on the INA’s inputs drive A₁, A₂, or both to their positive or negative output-swing limit. In this condition, A₃ measures an erroneous difference voltage. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A₃.

The final place to look for an input/ gain-overload condition is at the output, V_OUT, of the INA. The A₃ output restrictions are similar to those of any other amplifier. For instance, in a single- supply environment, the output swing never spans all the way to the rails.

The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A1 and A2 output stages. All of the internal amplifier’s output voltages relate to the linear common-mode input range of the complete INA. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. You can now use those graphs to your advantage.