Op amp Common Mode Dynamic Range(s)

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As a point of departure from the idealized circuits above, some practical basic points are now considered. Among the most evident of these is the allowable input and output dynamic ranges afforded in a real op amp. This obviously varies with not only the specific device, but also the supply voltage. While we can always optimize this performance point with device selection, more fundamental considerations come first.
Figure 1-8: Op amp input and output common mode ranges
Any real op amp will have a finite voltage range of operation, at both input and output. In modern system designs, supply voltages are dropping rapidly, and 3-5V total supply voltages are now common. This is a far cry from supply systems of the past, which were typically ±15V (30V total). Obviously, if designs are to accommodate a 3-5V supply, careful consideration must be given to maximizing dynamic range, by choosing a correct device. Choosing a device will be in terms of exact specifications, but first and foremost it should be in terms of the basic topologies used within it.
Output Dynamic Range
Figure 1-8 above is a general illustration of the limitations imposed by input and output dynamic ranges of an op amp, related to both supply rails. Any op amp will always be powered by two supply potentials, indicated by the positive rail, +VS, and the negative rail, -VS. We will define the op amp’s input and output CM range in terms of how closely it can approach these two rail voltage limits.
At the output, VOUT has two rail-imposed limits, one high or close to +VS, and one low, or close to –VS. Going high, it can range from an upper saturation limit of +VS –VSAT(HI) as a positive maximum. For example if +VS is 5V, and VSAT(HI) is 100mV, the upper VOUT limit or positive maximum is 4.9V. Similarly, going low it can range from a lower saturation limit of –VS + VSAT(LO). So, if –VS is ground (0V) and VSAT(LO) is 50mV, the lower limit of VOUT is simply 50mV.
Obviously, the internal design of a given op amp will impact this output CM dynamic range, since, when so necessary, the device itself must be designed to minimize both VSAT(HI) and VSAT(LO), so as to maximize the output dynamic range. Certain types of op amp structures are so designed, and these are generally associated with designs expressly for single-supply systems. This is covered in detail later within the chapter.
Input Dynamic Range
At the input, the CM range useful for VIN also has two rail-imposed limits, one high or close to +VS, and one low, or close to –VS. Going high, it can range from an upper CM limit of +VS - VCM(HI) as a positive maximum. For example, again using the +VS = 5V example case, if VCM(HI) is 1V, the upper VIN limit or positive CM maximum is +VS – VCM(HI), or 4V.
Figure 1-9: A graphical display of op amp input common mode range
Figure 1-9 above illustrates by way of a hypothetical op amp’s data how VCM(HI) could be specified, as shown in the upper curve. This particular op amp would operate for VCM inputs lower than the curve shown.
In practice the input CM range of real op amps is typically specified as a range of voltages, not necessarily referenced to +VS or -VS. For example, a typical ±15V operated dual supply op amp would be specified for an operating CM range of ±13V. Going low, there will also be a lower CM limit. This can be generally expressed as -VS + VCM(LO), which would appear in a graph such as Fig. 1-9 as the lower curve, for VCM(LO). If this were again a ±15V part, this could represent typical performance.
To use a single-supply example, for the –VS = 0V case, if VCM(LO) is 100mV, the lower CM limit will be 0V + 0.1V, or simply 0.1V. Although this example illustrates a lower CM range within 100mV of –VS, it is actually much more typical to see single-supply devices with lower or upper CM ranges, which include the supply rail.
In other words, VCM(LO) or VCM(HI) is 0V. There are also single-supply devices with CM ranges that include both rails. More often than not however, single-supply devices will not offer graphical data such as Fig. 1-9 for CM limits, but will simply cover performance with a tabular range of specified voltage.
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