ericvic

I know this is a very simple question but for some reason my brain is not working very well and I haven't been able to find any answers by searching.

I have a sensor that will output a range of say 0 - 2v. What I want to do is to offset that voltage so that it starts at 2.4v. So that it would be 2.4v - 4.4v. How can I do that?

The exact voltages are not important, I can tweak those but I need an explaination of how to get this done and then hopefully my brain will start working again

Thanks,
Eric

Nigel Goodwin

You really need to give more details, at first glance it looks like you need an opamp - but it might be even easier than that?, without details it's hard to know?.

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ericvic

Let me see if I can provide more information. I don't have any exact numbers because it is work in progress.

The sensor is going to be a photodiode running through an opamp and the output will be something like 0-2v dc. The ADC I'm using, this is the only part I can't change, has an input threshold of 2.4v so I need to get the sensor (opamp) output moved up to above the 2.4v threshold of the ADC. The opamp and the ADC will be powered by 5vdc. Hope that provides more info.

Thanks,
Eric

Nigel Goodwin

Can you post the circuit as you presently have it?, it may be possible to bias the opamp to do what you want?.

But what sort of A2D has a 2.4V threshold?.

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ericvic

I don't have any circuit drawn up yet.

The ADC is a Dallas 1-Wire chip, the DS2438.

Thanks for your help.

Eric

dknguyen

Uhhh...isn't this IC a digital output?

ericvic

Yes it is a digital output but an analog input.

dknguyen

This IC is a sensor in itself though...

ericvic

It has a temperature sensor built in but that is not what I'm using. I'm using a photodiode through an opamp that needs to go to the VAD input of the DS2438.

Nigel Goodwin

Why are you using a battery monitor chip instead of a real A2D, it seems a highly bizarre choice?. For that matter, what processor are you using?, can't you just use a processor with internal A2D?.

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Russlk

This circuit is approximatly what you want. A gain of one is not possible with this circuit, so I made the output 2.4 to 4.8 volts. You may be able to incorporate it into your input opamp.

Attached Images

scan0006.jpg (210.1 KB, 14 views)

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ericvic

This is going to be part of a 1-Wire network so I have to use a 1-Wire chip and since I already use the DS2438 in other devices I wanted to keep using that chip.

ericvic

Thanks for the circuit. I have a question though. It shows a BIAS of -12v. Do I have to have a negative supply or can I get away with just a +5v or +12v supply?

Thanks,
Eric

Russlk

The -12v bias is essential for this circuit to work. You can make -12v with a "simple switcher" IC.

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Roff

You can do it with a single +5V supply, a dual rail-to rail I/O op amp, and 4 resistors. See below.
I used LMC6482. You can substitute any 5 volt capable, dual rail-to rail I/O op amp. The resistors are in the ratio 0.48:1.00.

Attached Images

level shifter.PNG (10.8 KB, 18 views)

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