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Author Topic: Making a -0.25V ref  (Read 2227 times)

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Offline richcj10Topic starter

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Making a -0.25V ref
« on: July 17, 2013, 02:15:14 am »
I am using a LMP7721 op-amp in a design. I want to give V-    -0.25v to allow a true 0-5v output.
I have a design for a LM2787 to output that kind of voltage. But, they are out of stock and in the wrong package for me to use.
Any other suggestions?

Thanks as always,

Rick
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Offline codeboy2k

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Re: Making a -0.25V ref
« Reply #1 on: July 17, 2013, 04:24:52 am »
You don't need anything fancy... That opamp has a 6V supply range. You can simply use a diode drop... use a 5V voltage regulator with it's ground node raised above the (now -650mV rail) by a diode drop. 

The point where your diode meets the ground pin of the regulator is the "system ground reference", or "0" volts so the speak, or the point of the GND symbol. The other rail is now -650mv below ground.

see attached spice.

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Online Andreas

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Re: Making a -0.25V ref
« Reply #2 on: July 17, 2013, 07:45:08 pm »
A Schottky diode (BAT48 or LL103) will give around 0.25-0.35V at 1-10mA.

with best regards

Andreas
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Offline richcj10Topic starter

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Re: Making a -0.25V ref
« Reply #3 on: July 20, 2013, 03:07:16 am »
What about the noise created from the diode?
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Offline SeanB

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Re: Making a -0.25V ref
« Reply #4 on: July 20, 2013, 05:14:12 am »
100n across the diode reduces it a lot.
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