3 Practical Input Limitations

Section 2 demonstrated the common-mode limitations for different configurations by running a DC sweep. The simulation indicated that the common-mode range was violated by clamping the output signal. Furthermore, the limit always occurred at the minimum and maximum limit of the common-mode range. First, understand that the common-mode range given is the worst-case range, so when measuring actual devices, the performance is often better than the specification. Nevertheless, when designing a system, assume that the worst-case specification is a possibility due to process variation. Never base a system design on lab measurement of a few units. Always design considering the published minimum and maximum data sheet specifications.

The common-mode limitation also does not necessarily clamp the output as is generally shown in simulation models. A real-world device can introduce substantial distortion when the input range is violated rather than clamp the output. Furthermore, this effect can be dependent on other factors such as input frequency, or temperature. Figure 3-1 illustrates a case where the amplifier common-mode range limitation is dependent on frequency. The data sheet specification for this device provides the worst-case common-mode range across the entire bandwidth of the device. Lab measurements at low frequency indicate that the device has common-mode range much wider than the specification, whereas the measurements at higher frequency show substantial distortion where the signal exceeds the common-mode limit. Notice that even the higher frequency case does not clamp at the common-mode limitation, but rather introduces unacceptable distortion.

Figure 3-1 Measured Common-Mode Limitation on OPA140 versus Frequency