Powering mcu question

[DarkZero]

i wanted to build a small current meter panel and i stole Dave's design, now i want to use same power supply that powers the op amps for the mcu without messing the things about the GND i simply cannot wrap my mind around it plz some suggestions

[danadak]

Several issues/solutions -

1) I assume the MCU CM range is + only with respect to its ground. Therefore
the OpAmp feeding MCU input has to be offset appropriately to force this to
happen. Easy with a R network to gen the offset.

2) MCU tied to ground as shown in schematic.

3) Assuming MCU is 5V, then use a 2.5V zener from the positive supply source
to MCU, one approach. 7.5 - 5V (assuming MCU is 5V) = 2.5V

4) Or use a 3 term adj reg, gnd leg to - side 15V supply, input to + side of 15V supply,
set it to 7.5 + 5 (assuming MCU is 5V) = 12.5V, and reg output goes to MCU Vdd pin.

Regards, Dana.

[danadak]

One consideration is U2 has to carry the MCU Vss ground leg current. Since it is
7.5V above supply - leg for every 1 mA of MCU Vss current the OpAmp has to dissipate
7.5 mW. So do a current budget on MCU and all its I/O load that flows thru its ground
pin and calc Pdiss. If too much for OpAmp consider a power OpAmp, or another 3 term
reg to establish the system ground potential. Note that reg would have to have a source/
sink output capability, not in every 3 term reg.

Also, for stability, if you use large C bypass from + or - supply to ground, examine OpAmp
datasheets for C loading limitations. If necessary add a zero into OpAmp feedback net to
lift the phase to get adequate phase margin.


Regards, Dana.

[rstofer]

Section 4.3 of Op Amps For Everyone:
http://www.cypress.com/file/65366/download

Basically you use a 5V single supply rail to rail op amp to offset and scale the input to match the range of the MCU.

[DarkZero]

my whole circuit draws 75mA max but ill take it that's it's 100mA so i guess i can get away with 150mA supply. so either i find some op-amp that can source or sink 100mA or i get some chip that can do -V because i don't want to use transformer with 2x secondary.

but let me ask maybe a stupid question is it possible to boost current capability with a transistor in this situation ?  or should i just wack in 7660

[danadak]

Then generate a - supply, some techniques -

https://www.dropbox.com/s/k1yn4m7heaixvmg/ADinput%20negative%20inputs.zip?dl=0


Regards, Dana.

[Jeroen3]

Why do you need a 15v supply?