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Ottawa,Canada
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i need this for a Due input so 0-3.3

my signal is ~1-4v so i need to subtract 1v from the input to get it in range
  it has to be fast (signal changes every 2us)  & not AC coupled ..

IF i had a negitive supply i could sum the inputs  ... but i don't have negitive supplys at hand (ment to be in a portable device )....

 does anyone have a solution ? 
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IF i had a negitive supply i could sum the inputs 
Most op-amps are going to require a positive & negative supply if the output needs to go all the way to zero (or negative).    Otherwise a differential amplifier (difference amplifier) with a gain of one, and +1V on the negative input would do the trick.

« Last Edit: September 23, 2013, 05:11:11 pm by DVDdoug » Logged

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.75-3 & doing in software doesn't cut it - you loose 1/4 the range ... no point in having 12 bits (4096 ) if you toss out 1/4 of them
 
plenty of rail to rail single supply opamps /
 
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Most (all?) "rail to rail" op-amps, aren't.  Close maybe, but still asymptotic.  Only way to get 100% of a voltage range is for the supplies to exceed it.
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. but i don't have negitive supplys at hand
How about using a voltage mirror chip for the negitave rail?
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Here's a circuit that should work with any 5V rail-to-rail output op amp.


* Scan 208.JPG (66.04 KB, 1653x1165 - viewed 48 times.)
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Most (all?) "rail to rail" op-amps, aren't.  Close maybe, but still asymptotic.  Only way to get 100% of a voltage range is for the supplies to exceed it.
understood / but i don't want to lose 25% of my width (5-10% if i really have to ..)
 and at some point don't mind if 0V is hard to get to ...
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Here's a circuit that should work with any 5V rail-to-rail output op amp.
i'll give that one a try
 i had tried today with a similar circuit / but to find out that i have to wait for smd holders for the proper opamp   - 8dip ones i was trying with didn't have the bandwidth ...

 
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.75-3 & doing in software doesn't cut it - you loose 1/4 the range ... no point in having 12 bits (4096 ) if you toss out 1/4 of them
 
plenty of rail to rail single supply opamps /
 
Losing 1/4 of the range still leaves 11.59 bits, and that might be more than the effective bits anyway.
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i had tried today with a similar circuit / but to find out that i have to wait for smd holders for the proper opamp   - 8dip ones i was trying with didn't have the bandwidth ...

This could be my own ignorance, but what about that circuit requires higher bandwidth than you can get in a DIP-8 package?  Was it oscillating?
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