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Recently I've been looking at single-ended to differential converters. An engineer designed an SE/DE converter with +7 and -4V supply rails. It got me thinking as to why he chose such odd values for the rails.

The first stage of the converter is a standard inverting op-amp, with the non-inverting input set to 1.67V. The single-ended input voltage range is +/-5V - I thought this would be a problem for the inverting stage, since it would have an input voltage lower than its supply voltage. I then realised (after an embarrassingly long time) that due to the feedback of the inverting amp, the op-amp's input doesn't actually go below the supply. I then wanted to know how low the supply voltage could be, before damage would occur. The circuit its as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

This is what I came up with :

equation

So this, I believe, gives you the minimum power rails required so that the output doesn't saturate; in simulation this works. Does it also tell me the minimum power rails needed, to ensure the inputs don't go above/below the power rails?

I'm sorry I keep asking questions recently, it's just great to have such a vast knowledge base to ask questions to!

Thanks in advance, and sorry for the rambling, I hope it makes sense!

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  • \$\begingroup\$ Though the equations give some insight into what the circuit may look like, it would be easier to answer the question with a schematic. \$\endgroup\$
    – user4574
    Dec 9, 2020 at 23:23
  • \$\begingroup\$ Sorry, I was initially writing on my phone, I have updated the question. \$\endgroup\$ Dec 10, 2020 at 10:37

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So this, I believe, gives you the minimum power rails required so that the output doesn't saturate

Those formulas are correct under the assumption that the op-amp can output all the way to the power rails.

Typically you want to give yourself some margin though. How much depends on exactly which part number you are using, but 0.25V to 0.5V would be a good starting point for may rail to rail output type op-amps.

  • Many op-amps can't output all the way to the rail.
  • For those op-amps that have rail to rail output the drive strength usually becomes very weak as you come close to the rails. For example an op-amp datasheet might say something like 1mA output at 100mV from the rail. This means that if your output goes close to the rails you won't be able to drive much load and you may not be able to operate at high frequencies.

Does it also tell me the minimum power rails needed, to ensure the inputs don't go above/below the power rails?

In this configuration both the + and - inputs of the op-amp are held at Vref (1.67V) regardless of the input. Note that this only remains true if the output doesn't get too close to the power rails.

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  • \$\begingroup\$ Hey thanks for the answer! I wasn't aware of the low drive strength at the rails - that's very interesting. Regarding your last point, does that mean it's protected for any input voltage? Or, only an input voltage that, when the outputs are saturated, it still doesn't go above/below the rails? In short, does it give you more margin ? \$\endgroup\$ Dec 12, 2020 at 8:41
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    \$\begingroup\$ The op-amp holding the minus input at Vref is contingent on the output not saturating. Once it saturates you must redo the analysis with R2 shorted to one of the rails on the output side of the op-amp. \$\endgroup\$
    – user4574
    Dec 13, 2020 at 3:11
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    \$\begingroup\$ Also note that while the input terminals of an op-amp are normally high impedance. If you try to drive them beyond the rails they will usually sink current. The maximum rating section of the datasheet will tell you how much you can put in without damaging the part. For example the TL081 can take 10mA into the input terminals without damage. With R1=2K and R2=1K you could put +/-35V into your circuit before there would be 10mA into the minus pin. If you changed R1=20K and R2=10K then you would be able to put in 215V before exceeding the input current rating. \$\endgroup\$
    – user4574
    Dec 13, 2020 at 3:19

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