The OPA189 datasheet https://www.ti.com/lit/ds/symlink/opa4189.pdf?ts=1696944853447&ref_url=https%253A%252F%252Fwww.google.com%252F says on p.10 the input common-mode voltage range is between (V-)-0.1V and (V+)-2.5V. It is defined for a +/-18V supply. Can this figure be extrapolated to a +/-5V supply somehow? Or is the op amp design such that I can assume the 0.1V/2.5V headroom would be constant regardless of the supply voltages? E.g. the maximum positive common-mode voltage for a +/-5V supply is still +5V - 2.5V = +2.5V?
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\$\begingroup\$ I would assume so, but measure it yourself, or check on e2e.ti.com to be sure. \$\endgroup\$– Tim WilliamsCommented Oct 11, 2023 at 9:01
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2 Answers
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These headroom figures are constant.
The internal circuit keeps the biasing currents pretty constant (as shown below) and, as a result, the common-mode range does not scale with the supply voltage range, but instead has a constant headroom to the supply rails.
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\$\begingroup\$ Interesting. Would you know how the headroom figures depend on the quiescent currents? I wondered if the relation was proportional at first but if the headroom figures are constant (confirmed by the note @Andyaka found in the datasheet) it looks more like the headroom is constant provided that changes in biasing current are only moderate (like in the graphs). \$\endgroup\$– HypCommented Oct 11, 2023 at 10:50
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\$\begingroup\$ @Hyp the limit is essentially given by the transistors close to one of the supply rails slipping into a saturated state for a given current. When an amp input stage requires 2.5 V of headroom at - say - 500 µA current, then with the same transistors that voltage will change only very slightly. A lower input stage current of - say -100 µA might bring you a few 100 mV closer to the rail but not decisively. And lower bias currents have other disadvantages. That is why op-amps with dedicated "rail-to-rail" input stages exist. \$\endgroup\$– tobaltCommented Oct 11, 2023 at 11:07
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is the op amp design such that I can assume the 0.1V/2.5V headroom would be constant regardless of the supply voltages?
We can usually assume this is true for any op-amp but, it's always a good idea to check the data sheet for contrary statements. However, I didn't see anything that might break this assumption. What I did see that somewhat supports it is this on page 21: -
This is telling me that the upper limit of the input range (+Vs - 2.5 volts) still holds for a +/- 2.5 volt power rail. I would happily assume the same is true for the lower range of input voltages.
answered Oct 11, 2023 at 10:05
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\$\begingroup\$ Oh, I didn't notice this, thanks. It does seem to confirm the upper headroom is constant down to supply voltage +/-2.5V \$\endgroup\$– HypCommented Oct 11, 2023 at 10:41