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I have low power (USB 5V, up to 500mA) available on a microcontroller board I am developing. Because the voltages I am going to measure (1.2-2.3V) using the MCU are not filling the ADC's range (0-3.3V), an opAmp seems suitable to scale it appropriately.

Case is, an opAmp needs not just a positive, but also a negative supply voltage to work properly. The opAmp I am looking at currently is the AD8027. So to provide its supply voltage this IC seems quite nice (judging by all its features, such as low-dropout-regulators): LM27762. But running TIs simulation web-software (Webench Power Designer), I am not getting the expected value of +3.3 and -3.3 Volt, when supplying it with the 3.3V of the microcontroller. It's more at a stable +3.1 and -2.9 Volt with attached load.

So my question is, how much will this degrade performance and quality? Will this still work to some degree? Are there better solutions to this kind of problem? Can I even trust the simulation?

Thanks in advance!

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  • \$\begingroup\$ You do not need a negative supply voltage, if you design the circuit correctly: ti.com/lit/an/sloa030a/sloa030a.pdf \$\endgroup\$ – τεκ Oct 29 '18 at 19:49
  • \$\begingroup\$ How comes that 1.2 - 2.3V range doesn't fit into 0 - 3.3 V range? The specified range is right in the middle of ADC range. Something is missing here... \$\endgroup\$ – Ale..chenski Oct 30 '18 at 4:48
  • \$\begingroup\$ I want to increase resolution, should have made that clear, sorry \$\endgroup\$ – Luftbaum Oct 30 '18 at 15:45
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For that particular application you can use a single-supply op amp that can reach ground on its output (given that you have 5V available you might not even need a rail-to-rail). In some cases you would probably have to add some protection (e.g., a resistor) to avoid damaging the input of your 3.3V-powered ADC, however. This means that you don't need to generate a negative voltage, which means you don't need the charge pump.

But to answer your question, as long as you satisfy the supply requirements of the op-amp it will be ok (6V > 2.7V min of the AD8027). These have enough power-supply rejection ratio (PSRR) to be relatively immune to supply variations. But in your particular application, and given that you are using a rail-to-rail op amp, the 3.1V positive supply will be the maximum voltage that you will be able to produce.

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