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[on:]

I've been working on a -2kV power supply that requires a high side current monitor. I need to read four -2.5V~0V DC channels. I just realized that the arduino's adc's can't read negative voltages.  I've already begun populating the board and need to make this work. The only voltage inverters I've seen have a floor of about 1.5V. What's the best way to get my voltages in the positive range to be read by the adc. I'm already using all of my digital pins so this would have to be some kind of break-out board.

Thanks for any advice in advance

[Reply #1 on:]

Use an op amp to add a 2.5v dc shift to the signal.

[Reply #2 on:]

Specifically a unity gain inverting amp configuration should do the job - the op-amp needs to work at the ground rail both inputs and output.

[Reply #3 on:]

Here's a circuit for -7V to 0V sensing....can be modified for -2.5V to 0V sensing by making R1 and R2 equal (or R1=2*R2 if you want a gain of 2). R3 can be removed, it adds a little positive shift to the signal to keep it away from exactly 0V where op-amps are not as accurate:

http://ruggedcircuits.com/html/circuit__14.html

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[Reply #4 on:]

thanks for the replies, they've helped greatly!

[Reply #5 on:]

I basically have one shot to get this right so I was hoping that someone could take a look at my schematic and just confirm I'm doing this correctly. I used the rugged circuits design and left in the small positive offset.

I selected the same op amp used in rugged circuits design(I think) http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-13057-1-ND
schematic here http://i.imgur.com/MTtJB.png

Thanks

[Reply #6 on:]

It looks OK, but...

* If you're going to use 4 op-amps you can save yourself some time/money by using a quad op-amp like the TLV2374, which is really just 4 TLV2371's in one package

* I suggest a decoupling capacitor (1uF ceramic) across the op-amp power pins

* The voltage created by the 976 ohm and 82k resistor divider can be shared among all op-amps. Thus you could eliminate R7, R11, R15, R16, R12, R8. You can also improve noise immunity by putting a small capacitor across the 976 ohm resistor (1nF to 1uF, ceramic).

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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons