Level shifting

Thread Starter

windoze killa

Joined Feb 23, 2006
605
Hi guys. This post is to hopefully replace and rekindle one that vanished with the rest of the projects topic.

I am try to work out how to take an I/P voltage that is between 2.5V and 5V and produce an O/P of 0V to 5V. I only have a +5V supply to work with which is where I am having the problem. I have tried everything but just can't come up with an answer.

There has to be someone that can trip over an answer.

Please

someone

anyone

Ok. I will stop crawling now.
 

kubeek

Joined Sep 20, 2005
5,732
Originally posted by windoze killa@Mar 4 2006, 09:25 AM
Hi guys. This post is to hopefully replace and rekindle one that vanished with the rest of the projects topic.

I am try to work out how to take an I/P voltage that is between 2.5V and 5V and produce an O/P of 0V to 5V. I only have a +5V supply to work with which is where I am having the problem. I have tried everything but just can't come up with an answer.

There has to be someone that can trip over an answer.

Please

someone

anyone

Ok. I will stop crawling now.
[post=14635]Quoted post[/post]​
maybe you could use a reversed version of open collector - high side transistor and pull-down resistor.
 

Thread Starter

windoze killa

Joined Feb 23, 2006
605
Originally posted by kubeek@Mar 4 2006, 09:21 PM
maybe you could use a reversed version of open collector - high side transistor and pull-down resistor.
[post=14638]Quoted post[/post]​
Not quite sure what you mean?
 

pebe

Joined Oct 11, 2004
626
Originally posted by windoze killa@Mar 4 2006, 07:25 AM
Hi guys. This post is to hopefully replace and rekindle one that vanished with the rest of the projects topic.

I am try to work out how to take an I/P voltage that is between 2.5V and 5V and produce an O/P of 0V to 5V. I only have a +5V supply to work with which is where I am having the problem. I have tried everything but just can't come up with an answer.

There has to be someone that can trip over an answer.

Please

someone

anyone

Ok. I will stop crawling now.
[post=14635]Quoted post[/post]​
A fairly simple way would be to use 2 opamps. The first would be in inverting mode with a gain of 2 and with the +ve input set at 2.5V. A 2.5V to 5V signal would then give 5V to 0V. A second amp would be used as a unity gain invertor.

Your limitation is the 5V supply. There's no headroom outside the input voltages. But you could use a rail to rail op amp like the LT1782/83 if you were prepared to accept that the output would only swing to within a few millivolts of the rails.

If the signal you are measuring is isolated from the 5V supply, then you could use a 555 set up as a voltage doubler to give a 10V supply, then you you would have a wider choice of op-amps. A lot depends on your circuit constraints.
 

Papabravo

Joined Feb 24, 2006
14,407
Originally posted by pebe@Mar 4 2006, 12:30 PM
A fairly simple way would be to use 2 opamps. The first would be in inverting mode with a gain of 2 and with the +ve input set at 2.5V. A 2.5V to 5V signal would then give 5V to 0V. A second amp would be used as a unity gain invertor.

Your limitation is the 5V supply. There's no headroom outside the input voltages. But you could use a rail to rail op amp like the LT1782/83 if you were prepared to accept that the output would only swing to within a few millivolts of the rails.

If the signal you are measuring is isolated from the 5V supply, then you could use a 555 set up as a voltage doubler to give a 10V supply, then you you would have a wider choice of op-amps. A lot depends on your circuit constraints.
[post=14652]Quoted post[/post]​
I'm not sure that the two opamp circuit works like you think it does. I think the first inverter with a gain of 2 and an offset of 2.5 Volts suffers clipping.
Attached is my three op-amp circuit which inverts with gain -1, then doubles with gain of 2, then inverts with gain of -1. The 2.5 volt offset is applied to the first and third amplifiers.

[attachmentid=1228]

The comment about not being able to reach the supply rail with a typical opamp is spot on.
 

pebe

Joined Oct 11, 2004
626
Originally posted by Papabravo@Mar 5 2006, 12:04 AM
I'm not sure that the two opamp circuit works like you think it does.  I think the first inverter with a gain of 2 and an offset of 2.5 Volts suffers clipping.
Attached is my three op-amp circuit which inverts with gain -1, then doubles with gain of 2, then inverts with gain of -1.  The 2.5 volt offset is applied to the first and third amplifiers.

[attachmentid=1228]

The comment about not being able to reach the supply rail with a typical opamp is spot on.
[post=14670]Quoted post[/post]​
OOOPS! You are quite right. This is the circuit I described, but the reference voltage to the +ve input of the first amp should be 2/3 rail volts - not the 1/2 rail that I said.

I should have drawn it out first - not tried to work it out in my head!
 

Papabravo

Joined Feb 24, 2006
14,407
Originally posted by pebe@Mar 5 2006, 07:24 AM
OOOPS! You are quite right. This is the circuit I described, but the reference voltage to the +ve input of the first amp should be 2/3 rail volts - not the 1/2 rail that I said.

I should have drawn it out first - not tried to work it out in my head!
[post=14682]Quoted post[/post]​
An elegant solution nonetheless. Three cheers for you!
 

aac

Joined Jun 13, 2005
35
Originally posted by windoze killa@Mar 4 2006, 02:25 AM
Hi guys. This post is to hopefully replace and rekindle one that vanished with the rest of the projects topic.

I am try to work out how to take an I/P voltage that is between 2.5V and 5V and produce an O/P of 0V to 5V. I only have a +5V supply to work with which is where I am having the problem. I have tried everything but just can't come up with an answer.

There has to be someone that can trip over an answer.

Please

someone

anyone

Ok. I will stop crawling now.
[post=14635]Quoted post[/post]​
I don't know how percise you are trying to be or temperature requirements and such but you can shift your signal down with a 2.5V zener and a resistor. The gain of 2 could be a non-inverting opamp with equal feedback resistors.
 

pebe

Joined Oct 11, 2004
626
Originally posted by windoze killa@Mar 6 2006, 09:22 AM
I haven't had a chance to try this cct yet but will let you know how I go. Thanks for all your efforts.
[post=14707]Quoted post[/post]​
A simpler circuit.
 

Thread Starter

windoze killa

Joined Feb 23, 2006
605
Originally posted by aac@Mar 8 2006, 12:25 AM
I don't know how percise you are trying to be or temperature requirements and such but you can shift your signal down with a 2.5V zener and a resistor. The gain of 2 could be a non-inverting opamp with equal feedback resistors.
[post=14749]Quoted post[/post]​
I don't require it to be too precise. If the output went fron .5 to 4.5 that would be close enough.
 

Thread Starter

windoze killa

Joined Feb 23, 2006
605
Originally posted by pebe@Mar 8 2006, 03:59 AM
A simpler circuit.
[post=14754]Quoted post[/post]​
This circuit did the job but it has all changed now. I have also posted in another forum that I have to use an LM324 which isn't rail to rail But I now have a 10V supply.
 

pebe

Joined Oct 11, 2004
626
Originally posted by windoze killa@Mar 23 2006, 11:39 PM
This circuit did the job but it has all changed now. I have also posted in another forum that I have to use an LM324 which isn't rail to rail But I now have a 10V supply.
[post=15364]Quoted post[/post]​
I've replied in the other forum.
 
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