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What is the smallest voltage a comparator can detect? Is it possible to set the reference voltage to a few millivolts, just above the noise margin for using a comparator circuit?

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    \$\begingroup\$ You can easily do that, with the right comparator. Did you have one you're asking specifically about? \$\endgroup\$ – Samuel Jan 10 '14 at 7:43
  • \$\begingroup\$ I dont have a specific one in mind, maybe just using LM311. \$\endgroup\$ – Sherby Jan 10 '14 at 7:47
  • \$\begingroup\$ You can always use a low offset opamp as a comparator. \$\endgroup\$ – alexan_e Jan 10 '14 at 9:28
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    \$\begingroup\$ @alexan_e Modern comparators and modern op-amps are comparable in low offset ratings, and comparators being optimized for a bi-state output, generally work a bit better, because they don't need to have internal filter stages for linearization across the band of interest. \$\endgroup\$ – Anindo Ghosh Jan 10 '14 at 12:11
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    \$\begingroup\$ @alexan_e Input stages of op-amps and comparators based on similar dies would be near-identical, other than the limiting diodes in an op-amp. Output stages are the issue - phase compensation, and op-amp output transistors designed to not operate at saturation. So for LT1013, there will be a corresponding comparator part, of course, but they aren't interchangeable. These two documents (1, 2) make interesting reading on the subject. \$\endgroup\$ – Anindo Ghosh Jan 10 '14 at 14:18
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The smallest voltage difference that a comparator can detect is a moving target: New comparators with increasingly low offset and hysteresis voltages are released by manufacturers from time to time. For instance:

  • Texas Instruments LMP7300 has a worst-case offset voltage of 0.75 mV, and an adjustable hysteresis down to 1 mV/mV.
  • Analog Devices AD790 has a 0.25 mV worst-case offset and hysteresis of 0.5 mV

The question appears to have an ambiguity on desired specifications:

Setting of reference voltage is essentially independent of the noise margin budget, and dependent on the maximum guaranteed input offset voltage. The reference can go down to the ground rail, or single-digit millivolts above it, in many comparators.

The acceptable noise margin on the other hand is addressed by the comparator's hysteresis, and as indicated above, this can be fractional milliVolts.

Thus, if one were to surmise from the wording of the question that the application requires comparator functions with a reference at the ground rail (0 Volts) and a noise margin of single-digit millivolts, comparators from many major manufacturers easily meet these criteria, such as the ones listed above.

For more stringent requirement specifications, a low noise preconditioning stage is often used, which may raise the signal bias point and / or provide buffering or amplification of the source signal. The problem then becomes one of minimizing noise in the preconditioning stage.

Nanovolt signals such as in medical equipment are commonly handled using such preconditioning, largely freeing the design from comparator constraints.

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Since your initial question was concerning noise threshold, maybe we should re-phrase the question to imply "the smallest DELTA voltage" which could be maybe 50% or 100 % over the noise voltage (this ratio very dependent upon your application which isn't mentioned). I have used comparators with an offset of 500uV to sense sample-to-sample deltas in the micro-volt region where the actual DC level was not important, just the delta between two signals.

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