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What is the difference between:

  • Input swing
  • Common mode input voltage range?

I thought they were the same thing.

Yet in this datasheet for example, it says:

the MCP6231/1R/1U/2/4 family supports rail-to-rail input and output swing, with a common mode input voltage range of VDD + 300 mV to VSS – 300 mV.

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Input swing not a very specific term, so it's hard to tell what it means exactly. Common mode input range, however, is well defined. If you were to tie both inputs together, the common mode input range is the voltage range over which you can drive the two inputs with the opamp still working as intended.

Look closely at the sentence you quoted, and you will see they mean the same thing by "swing" and "common mode range". They use the loose term first, perhaps for brevity, but then the more exact term when providing specific numbers.

They are making the point that the input voltages can be anywhere within the supply range (that's what rail-to-rail means), but then get more specific and say that you can actually go up to 300 mV outside the supply range on each end.

In any case, all such blah-blah in datasheets is pretty meaningless. Such information should only be taken from the hard specs section. For example, does this sentence mean that the opamp will just tolerate the full range without damage, or that it will work correctly over that range? Are these typical values for marketing gratification only, or min/max values you can actually rely on in a design?

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In case you meant the voltage between the two inputs as "swing" take care on this. Some opamps are quite happy to have the full supply rails separating the inputs whilst other opamps would smoke under the same circumstances because they have an input circuit that will only tolerate maybe a volt or so. Usually this is due to input protection diodes.

Common mode voltage range assumes the 2 inputs are largely at the same potential and is the range that both inputs can be taken to either power rail without significantly disrupting the amplification process or causing dc offsets.

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  • \$\begingroup\$ Thanks Andy, that's good to know! Do you have any examples of OAs that assume similar potential at the inputs like that? I'd like to see how they specify such things in the datasheet compared to OAs which aren't fussy \$\endgroup\$ – CL22 Nov 10 '13 at 23:22
  • \$\begingroup\$ @Jodes off the top of my head no but it will be usually specified in absolute max ratings on the data sheet. \$\endgroup\$ – Andy aka Nov 11 '13 at 8:17
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The common mode voltage of a differential signal \$ V_+ \$ and \$ V_- \$ is defined as:

$$ V_{cm} = (V_+ + V_-)/2 $$

It is the common voltage your differential signals are superimposed upon, and as the differential signal (the input swing) is defined as:

$$ V_{dif} = V_+ - V_- $$

For example, I could define a differential signal, \$ V_{dif} \$ with 900mV common mode and 300mV differential amplitude by providing the following two voltages:

$$ V_+ = 0.9 + 0.15cos(2\pi10e3) \\ V_- = 0.9 - 0.15cos(2\pi10e3) \\ V_{cm} = 0.9 \\ V_{dif} = 0.3cos(2\pi10e3) $$

For an ideal operational amplifier, the common mode voltage doesn't matter, since it performs the amplification exclusively on the differential component. However, real opamps usually have a change in their bias conditions because of changing common mode voltages, which may appear as degrading effect such as offset voltage change.

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I don't see that it makes much sense for a device to define a common-mode input range which is larger than the allowable input swing; my guess is that the "input swing" and common mode range both extend 300mV beyond the rails, but describing the input and output swings together as being "rail-to-rail" made it clear that they both extend at least that far (the input actually swinging a little further).

In some cases, op amps may have a common mode voltage range which is smaller than the allowable input voltage swing. In such cases, for correct operation to be guaranteed, neither input may exceed the "input-swing range" [which could be specified differently for each input], and the inputs must straddle at least part of the common mode range. While an op amp will only be able to operate with any precision when both inputs are within the common mode range, having an allowable swing which goes beyond will help ensure that even if input stimuli cause an op amp to go beyond the range where it can operate precisely, it will resume proper operation when input stimuli return to acceptable levels.

To understand why this is important, imagine a hypothetical op amp, used as a voltage-follower, whose common-mode range and inverting-input swing are both limited to +/-10 volts, and which outputs -12 volts when the inverting input is driven with anything below -10V, regardless of what the non-inverting input is driven with. Such an op amp might operate perfectly as long as the input stayed within the range of +/-10 volts, but if the input were to drift even momentarily to -10.1 volt, the output could go to -12V and then be stuck there even if the input went back within range. If the op amp's common-mode range were limited to +/-10V but its voltage swing were specified to +/-12, then an input of -10.1V might cause the output to swing to -12V, but as soon as the input went back above the -10V threshold the output would start tracking the input again.

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If the CM range was the same as "input swing", then there would be no room or margin for added CM noise to be rejected and thus have zero CMRR when the input is at the supply rails. Normally CMRR is > 80 dB.

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  • \$\begingroup\$ I'm not sure I agree with that - (I assume CM noise is induced from external sources) - because the CM noise may be small, in which case the designer could utilise the full input swing, but with CMR still in effect. Perhaps you could clarify how CMRR being > 80dB validates there being a distinction between CM range and input swing? \$\endgroup\$ – CL22 Nov 10 '13 at 23:33

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