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### Topic: simple opamp circuit ? (Read 1 time)previous topic - next topic

#### ralphnev

##### Sep 23, 2013, 11:23 pm
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 ?

#### DVDdoug

#1
##### Sep 24, 2013, 12:06 amLast Edit: Sep 24, 2013, 12:11 am by DVDdoug Reason: 1
Quote
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.

#### ralphnev

#2
##### Sep 24, 2013, 12:13 am

.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 /

#### SirNickity

#3
##### Sep 24, 2013, 10:14 pm
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.

#### Grumpy_Mike

#4
##### Sep 24, 2013, 10:19 pm
Quote
. but i don't have negitive supplys at hand

How about using a voltage mirror chip for the negitave rail?

#### dc42

#5
##### Sep 24, 2013, 10:45 pm
Here's a circuit that should work with any 5V rail-to-rail output op amp.
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#### ralphnev

#6
##### Sep 24, 2013, 11:43 pm

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 ...

#### ralphnev

#7
##### Sep 24, 2013, 11:46 pm

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 ...

#### MarkT

#8
##### Sep 25, 2013, 03:14 am

.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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#### SirNickity

#9
##### Sep 25, 2013, 09:14 pm
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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