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I have used a voltage adaptor as power supply. The output is 5.17 V but I need exactly 5v for my circuit. I can't use voltage regulator (7805) because The minimum input voltage is 7 V. And my circuit needs 1 A current. The only reason I insist on it is that if the voltage has less telorance, The ADC would be more accurate. What's your suggestion? Thanks

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    \$\begingroup\$ Use SEPIC converter. Or a flyback. Or just an isolated 5V to 5V converter. Or boost then buck. \$\endgroup\$ – Gregory Kornblum Jul 28 '17 at 6:48
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    \$\begingroup\$ A 7805 is only spec'd to +-5% so it may be worse than 5.17V \$\endgroup\$ – sstobbe Jul 28 '17 at 6:51
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    \$\begingroup\$ Most devices have a tolerance on voltage input. You might not have a problem. Edit your post to include the part number and link to datasheet. \$\endgroup\$ – Transistor Jul 28 '17 at 7:10
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    \$\begingroup\$ Use a dedicated precision voltage reference for the ADC so it's accuracy is less dependent on the supply voltage. \$\endgroup\$ – pericynthion Jul 28 '17 at 7:32
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You can get LDOs (Low DropOut regulators) that have 100mV or even less required headroom. But, you'd have to be exquisitely careful not to drop another few 10s of mV in connectors or wiring and blow that headroom budget.

A buck-boost converter will convert up or down to give you the voltage you want.

It would probably be best to take a step back and realise that 'exactly 5v' is not possible, even in theory. There is always a tolerance. Does your 5v load really require a tighter tolerance than 5.2v? Most loads will specify +/- 10% tolerance, +/- 5% if they're really picky, which is +/- 0.25v. Before you design a way to drop your 5.17v, you need to make sure you can provide the accuracy your load demands. From your question, it appears you're not too picky about the difference between 170mV and 200mV.

Another direction is to look at your supply. Is it adjustable? Can you use a supply with the correct output voltage. Can you use a supply with slightly higher output voltage that will allow you to regulate down with more headroom?

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The only reason I insist on it is that if the voltage has less telorance, The ADC would be more accurate.

If you use the power supply as a voltage reference for your ADC, you can't expect any kind of precision. Since you mention 1A current draw, the 5V supply will vary a little bit as the current varies. Every time the load current changes, the voltage will jump a bit as the regulator settles.

If you want accurate ADC readings, you must use a dedicated voltage reference for your ADC. Your microcontroller or ADC most likely has a pin for this. Since it draws very little current from the reference, the voltage will be much more stable.

So, you need a reference that can be supplied from 5V. You can use a 4.096V reference, for example.

Here's a link.

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You could use a schottky diode like 20l15t. it has a forward voltage of 0.25V maybe you will find something even lower. You could also try a Buck-converter like the LT1375

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    \$\begingroup\$ Current is measured in different units \$\endgroup\$ – Gregory Kornblum Jul 28 '17 at 6:47
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    \$\begingroup\$ Note that if/when current drops in a diode, the voltage drop will drop as well. \$\endgroup\$ – Bort Jul 28 '17 at 6:48
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The best way to get accurate 5Vdc is to use 9V or 12V adapter and step down to 5V.

Buy a DC-DC module that have a potentiometer on it and you can set accurately 5V.

If you're going to use it with ADC that requires low noise, choose LM317 option instead. LM317 have better noise performance than 7805. Not to mention you can adjust 5V accurately using pot.

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