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.

 

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.

 

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?

 

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.

 

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.

 

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!

 

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!

 

Originally posted by Papabravo@Mar 5 2006, 06:36 PM
An elegant solution nonetheless. Three cheers for you!

[post=14691]Quoted post[/post]​

Thanks!

 

I haven't had a chance to try this cct yet but will let you know how I go. Thanks for all your efforts.

 

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.

 

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.

 

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.

 

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.

 

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.