# Forum: ARM programming with GCC/GNU tools a/d (differential) converter question

 Author: Kxx S. (kskolik) Posted on: 2008-05-29 16:51

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Hi,

I'm new to ARM processors. And well, I found out that there's not that
much information around as it is for AVRs. :-(

Anyway, I need to do some voltage measurements using my NXP LPC2294.
I want to measure the voltage between to motor phases (acutally 2 times
between 3 phases, Vab and Vbc, if the phases are named a, b and c).

The thing is, the A/D conversion needs to be a differential one. Some
AVRs do have the capability of using two A/D channels to do a
differential conversion.

How can I do that with the ARM7 processor? Is there a way to do it?

I hope it's the correct forum.

Cheers,
Kai

 Author: Martin Thomas (mthomas) (Moderator) Posted on: 2008-05-29 19:00

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Kai Skolik wrote:

> The thing is, the A/D conversion needs to be a differential one. Some
> AVRs do have the capability of using two A/D channels to do a
> differential conversion.

So how would you solve this if you had just an AVR that does not offer
differential conversion?

Suggestions:
(1) calculate the differential result from the conversion software
(check if this is accurate enough for you since the conversions are not
done at the same time).
(2) use external OP-AMPs and connect the outputs to the ARM-ADC-INputs
http://en.wikipedia.org/wiki/Operational_amplifier...
(3) use an external AD-converter which offers the needed feature

Personally I would choose a setup with external OP-AMPs. Should be a
rather cheap and easy solution. Maybe you need some external
signal-conditioning anyway.

 Author: Kxx S. (kskolik) Posted on: 2008-05-30 08:46

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Martin Thomas wrote:
> Kai Skolik wrote:
>
>> The thing is, the A/D conversion needs to be a differential one. Some
>> AVRs do have the capability of using two A/D channels to do a
>> differential conversion.
>
> So how would you solve this if you had just an AVR that does not offer
> differential conversion?

I didn't do that personally. I guess the AVR do measure the voltage
between two a/d converter channels. So, of course, two pins are needed.

>
> Suggestions:
> (1) calculate the differential result from the conversion software
> (check if this is accurate enough for you since the conversions are not
> done at the same time).

That's what I was thinking of as "plan B". Simply do two (in total four)
conversions between each channel and Aref (if it's called Aref).

> (2) use external OP-AMPs and connect the outputs to the ARM-ADC-INputs
>
http://en.wikipedia.org/wiki/Operational_amplifier...

In that case, the amplifier has to be connected to the same GND as the
ARM (or at least to the reference voltage of the a/d converter). Right?

> (3) use an external AD-converter which offers the needed feature
>
> Personally I would choose a setup with external OP-AMPs. Should be a
> rather cheap and easy solution. Maybe you need some external
> signal-conditioning anyway.

-Kai

 Author: Kxx S. (kskolik) Posted on: 2008-06-02 10:07

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Ok, good morning!

I have two external voltage measurement devices. they do have a
differential, isolation amplifier. The output is a single pin. And the
output signal is one voltage against GND. And GND on the output side is
"floating". That means, it can be connected to (any) potential and the
output signal is the voltage against that potential.

That means: I do not need to do any differential a/d-conversions. The
only thing that needs to be done is to connect the GND pin of the
measurement boards to the reference-Pin of the A/D-converter.

But thanks a lot for your help! :-)

-Kai

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