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Offset circuit needed

Hi,

Has anyone experienced the same problem as I am right now: I use 4-20 mA
transducers in combination with a 249 Ohm resistor. This gives me analog
signals of 1 - 5 volts. What I like to do is build some electronic hardware
that can offset the voltage with 1 volt. This gives me a signal between 0 -
4 volts.

Has anyone of you build this before or can anyone give me some example on
how to make this?

Thanx.

Ron@ld
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Ronald,

Do you have any NI hardware? You could certainly program a DAC to 1 volt, then wire this in series with your signal and offset it. What is the reason that you can't have the 1 volt offset? Are you saturating an input?

Mark
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Yes, I do have NI hardware. PCI-MIO-16X E4 or something. Most of my
transducers are 4..20mA transducers. e.g. a pressure transducer from 0 - 16
[bar]. When I want to use them at low pressure (e.g. 0 - 400 mbar) I can't
get the desired resolution. My board is capable of 12bits resolution but in
this case the 249 Ohm resistor gives me at least 1 volt. Best resolution can
be found at 2 volts.......
If I can subtract in any way this 1 volt, I can produce more accurate
measurements.

That's why


markwysong schreef in berichtnieuws
506500000005000000241C0000-984280909000@quiq.com...
> Ronald,
>
> Do you have any NI hardware? You could certainly program a DAC to 1
> volt, then wire this in series with your signal and offset it. What
> is the reason that
you can't have the 1 volt offset? Are you
> saturating an input?
>
> Mark
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Ronald,

Several companies make signal conditioners that will
do what you want. From NI, you can buy a 5B32 module
(be sure to get the 4-20 mA, part # 776232-01, not
the 0-20 mA version). This will take a 4 to 20 mA signal
and produce a 0 to 5 volt signal. You can then adjust the
gain on you MIO card to improve your resolution.

Be aware that these modules will typically introduce a
lot of filtering, and may not be as accurate as the rest of
your system. Look carefully at the specifications.

You can also replace the 249 Ohm resistor with a different
value, and adjust the gain to improve your resolution.

For example, assuming 0 to 400 mbar desired range
on 0 to 16 bar sensor:

With your 249 Ohm resistor, gain = 1, you are using
1% of your range, for a resolution of ~ 41 counts.

If you change the gain to 5, you will use 4.98%
of your range, for a resolution of ~ 204 counts.

If you use a 200 Ohm resistor, gain = 10,
you will use 8% of your range, for a resolution
of ~ 327 counts.

You can improve this to about 9% if you use
non-standard resistor values.

Regards,
Reed

//-----------------------------------------------
> Reed Blake
> Beta Technology
> Industrial & Scientific Computing.



Ron@ld wrote in message <3ab10c95@newsgroups.ni.com>...
>Yes, I do have NI hardware. PCI-MIO-16X E4 or something. Most of my
>transducers are 4..20mA transducers. e.g. a pressure transducer from 0 - 16
>[bar]. When I want to use them at low pressure (e.g. 0 - 400 mbar) I can't
>get the desired resolution. My board is capable of 12bits resolution but in
>this case the 249 Ohm resistor gives me at least 1 volt. Best resolution
can
>be found at 2 volts.......
>If I can subtract in any way this 1 volt, I can produce more accurate
>measurements.
>
>That's why
>
>
>markwysong schreef in berichtnieuws
>506500000005000000241C0000-984280909000@quiq.com...
>> Ronald,
>>
>> Do you have any NI hardware? You could certainly program a DAC to 1
>> volt, then wire this in series with your signal and offset it. What
>> is the reason that you can't have the 1 volt offset? Are you
>> saturating an input?
>>
>> Mark
>
>
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