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'[PIC] 4-20mA conversion for A2D'
2007\04\21@042137 by Jinx

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Until recently I've been using

http://www.maxim-ic.com/appnotes.cfm/appnote_number/823

with 4-20mA sensors, and it works very very well, on a DMM as
well as down to the resolution of a PIC A2D. No fiddling or twiddling
needed, unlike the op-amp designs I tried

Unfortunately a new design means a revamp, and the MAX472 is
not recommended for new designs. Sigh

The client found the RCV420

http://focus.ti.com/docs/prod/folders/print/rcv420.html

but it's relatively expensive (from where I can get them)

I found this, the INA138 - (231kB)

http://focus.ti.com/lit/ds/symlink/ina138.pdf

It looks similar, a little simpler even, but nowhere in the ANs does
it say suitable for 4-20mA. It seems to me it could do it

Googling around finds several devices, just wondering whether
anyone has a proven cheap accurate method for 4-20mA before
I go ordering parts

TIA

2007\04\21@080239 by Nigel Duckworth

picon face
I've used the MAX4173 for high-side current measurement;

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1971

Nigel



Jinx wrote:
{Quote hidden}

2007\04\21@131244 by Gerhard Fiedler

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Jinx wrote:

> Until recently I've been using
>
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/823
>
> with 4-20mA sensors, and it works very very well, on a DMM as well as
> down to the resolution of a PIC A2D. No fiddling or twiddling needed,
> unlike the op-amp designs I tried

They say there:

"The circuit performs well with 1% resistors. Monte Carlo analysis
indicates a worst-case output error of 5% due to resistance mismatch alone,
but the performance with unmatched components was better than 1% at
full-scale."

This is about what I'd expect for a typical opamp design -- the result is
mainly determined by the precision (or matching) of the resistors.

I've used the LT1637 as high-side current sensor amp with good success. One
nice feature of it is that it can have its inputs above its supply. Does it
have to be high-side? Low-side measurement is easier...

What is the fiddling and twiddling you had to do with opamp designs? After
all, in your application, the MAX472 is little more than an opamp. (It has
that absolute value output feature, but in a 4-20 mA current loop the
current direction is known.)

Gerhard

2007\04\21@183642 by Jinx

face picon face
> Does it have to be high-side? Low-side measurement is easier...

I guess it doesn't have to be high-side. Resistor-across-the-ADC has
been mentioned on the list before, detailed here

http://celesco.com/faq/420.html

A basic 250R is going to produce 1V -  5V with 4-20mA, which is
less than A2D FSD, OK for some measurements, eg ambient
temperature but maybe not others. I have a 5m depth gauge for example
that is pretty good to the mm with Maxim circuit (1.000m = 1.000V),
although of course a 10-bit PIC degrades this to 5mV steps

Maybe 300R to make 1V - 6V and then level-shift (doesn't account
for broken connection though). Does imply 0.1% or better resistors.
And what's that going to cost, plus the attendant temperature
management

> What is the fiddling and twiddling you had to do with opamp
> designs? After all, in your application, the MAX472 is little more
> than an opamp

Well, integral designs like the RCV420/INA168/MAX4137 are
factory-trimmed for accuracy. Internally they're still basically op-
amps. I could get reasonable results with discrete circuits, based
on their principles, but the tolerances needed are a little hard to
find OTS

A 5-pin SOT23 is such an easy answer, especially if it's cheap

> (It has> that absolute value output feature, but in a 4-20 mA
> current loop the current direction is known.)

That's true. There are quite a few subtle variations on the theme
(Rachmaninov would've been proud), some devices are unipolar,
some have polarity detection, voltage ranges, etc etc

2007\04\21@184150 by Jinx

face picon face
> I've used the MAX4173 for high-side current measurement;
>
> www.maxim-ic.com/quick_view2.cfm/qv_pk/1971
>
> Nigel

Thanks. Looks functionally and internally very similar to the INA168
I was thinking about

2007\04\21@194152 by Gerhard Fiedler

picon face
Jinx wrote:

>> Does it have to be high-side? Low-side measurement is easier...
>
> I guess it doesn't have to be high-side.

FWIW, I think low side sensing is inherently more precise (with equal
precision of the resistors).


>> What is the fiddling and twiddling you had to do with opamp designs?
>> After all, in your application, the MAX472 is little more than an opamp
>
> Well, integral designs like the RCV420/INA168/MAX4137 are
> factory-trimmed for accuracy.

I did miss this feature. So I guess what you want is an opamp with a fixed
(and calibrated) amplification?

I saw that Maxim offers replacements for the MAX472 -- if you're a lucky
guy you might just try these :)  Linear also has current sense amps of that
type (like the LT1787 or LT6100; I tend to prefer Linear for this sort of
thing) for both high side and low side sensing. They also have trimmed-gain
resistor amplifiers like the LT1991.

Gerhard

2007\04\21@195814 by Jinx

face picon face
> > Well, integral designs like the RCV420/INA168/MAX4137 are
> > factory-trimmed for accuracy.
>
> I did miss this feature

That would be the self-contained current monitors. Some require you to
add your own external resistors

> So I guess what you want is an opamp with a fixed (and calibrated)
> amplification?
>
> I saw that Maxim offers replacements for the MAX472 -- if you're a
> lucky guy you might just try these :)

I think it was the 4072. After hearing so many stories of Maxim being
troublesome with supply I might agree with you about using other parts,
not just from a technical POV

>  Linear also has current sense amps of that type (like the LT1787 or
> LT6100 .... LT1991.

Cheers, will look into them too

2007\04\21@200748 by Jinx

face picon face
> FWIW, I think low side sensing is inherently more precise (with
> equal precision of the resistors)

I might actually re-think this. If I bump up the range to say 0.2V -
5.32V (for an offset 0 - 5.12V effective range) with a higher PIC
Vcc, then that leaves < 0.2V for no-current detection

Hmmmm

2007\04\22@061231 by Pearce, AB (Alan)

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part 1 390 bytes content-type:text/plain; (decoded quoted-printable)

<>  Linear also has current sense amps of that type (like the LT1787 or
>> LT6100 .... LT1991.
>
>Cheers, will look into them too

Another nice thing about Linear is they are as esay to get samples from as Microchip. Limit is 2 of a part, but you can get 3 parts in an order. Cannot remember how often you can order though.


part 2 3285 bytes content-type:application/ms-tnef; (decode)

part 3 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2007\04\23@035530 by Alan B. Pearce

face picon face
>I've used the LT1637 as high-side current sensor amp with good success. One
>nice feature of it is that it can have its inputs above its supply. Does it
>have to be high-side? Low-side measurement is easier...
>
>What is the fiddling and twiddling you had to do with opamp designs? After
>all, in your application, the MAX472 is little more than an opamp. (It has
>that absolute value output feature, but in a 4-20 mA current loop the
>current direction is known.)

For high side current sense you may like to look at the Zetex ZXCT10xx rage
of devices. Some are voltage output, some are current output. On the front
page at http://www.zetex.com/ select the "current monitors" in the third
menu on the list. Some of the devices are available through Farnell and RS.

2007\04\23@041753 by Jinx

face picon face
> Zetex ZXCT10xx

Thanks. Downloaded datasheets and ANs


'[PIC] 4-20mA conversion for A2D'
2007\05\02@220133 by Jinx
face picon face
www.globalw.com/downloads/WQ/WQSensormanual.pdf

500kB

"All Global Water water quality sensors produce a 4-19 mA output
signal. 4-19 mA is an industrial standard signal for process control
monitoring"

It's not - is it ?

Google hits

4-20mA 1,040,000
4-19mA 138

2007\05\02@230713 by Marcel Duchamp

picon face
Jinx wrote:
> www.globalw.com/downloads/WQ/WQSensormanual.pdf
>
> 500kB
>
> "All Global Water water quality sensors produce a 4-19 mA output
> signal. 4-19 mA is an industrial standard signal for process control
> monitoring"
>
> It's not - is it ?
>

Nope. 4-20 mA is the standard.

Funny thing here... I used to work with the guy that started that
company.  A salesman guy, not an engineer.  One would think that they
would get lots of "WTF? 19mA?!"




2007\05\03@222121 by Jinx

face picon face

> Nope. 4-20 mA is the standard.
>
> Funny thing here... I used to work with the guy that started that
> company.  A salesman guy, not an engineer.  One would think
> that they would get lots of "WTF? 19mA?!"

I got a sample one to try. It's actually 4.017 - 18.868mA for a
15ft range, or (their figure) 0.9900666mA/ft (ambitious use of 7
decimal places)

Which works out to, after giving the old grey matter a workout,
a 315.2R load to make 1.024V/m. Which it does. Bit of a pain
that it was (a) imperial (b) not standard 4-20mA

But it works well enough with the resistor ft -> m conversion. It's
expensive, so a little effort to get it around to my way of thinking
is justified

2007\05\03@230023 by Marcel Duchamp

picon face
Jinx wrote:
>> Nope. 4-20 mA is the standard.
>>
>> Funny thing here... I used to work with the guy that started that
>> company.  A salesman guy, not an engineer.  One would think
>> that they would get lots of "WTF? 19mA?!"
>
> I got a sample one to try. It's actually 4.017 - 18.868mA for a
> 15ft range, or (their figure) 0.9900666mA/ft (ambitious use of 7
> decimal places)

Oh, sure, no doubt it is rock solid out to the last digit...;>


2007\05\04@001846 by Vasile Surducan

face picon face
On 5/2/07, Marcel Duchamp <spam_OUTmarcel.duchampTakeThisOuTspamsbcglobal.net> wrote:
> Jinx wrote:
> > www.globalw.com/downloads/WQ/WQSensormanual.pdf
> >
> > 500kB
> >
> > "All Global Water water quality sensors produce a 4-19 mA output
> > signal. 4-19 mA is an industrial standard signal for process control
> > monitoring"
> >
> > It's not - is it ?
> >
>
> Nope. 4-20 mA is the standard.

2-10mA is good as well. Some older standards figure 0-10mA or 0-20mA
too, as long there is another choice for checking the cable
continuity.

2007\05\04@131618 by Howard Winter

face
flavicon
picon face
Marcel,

On Thu, 03 May 2007 19:58:55 -0700, Marcel Duchamp wrote:

> Jinx wrote:
>...
> > I got a sample one to try. It's actually 4.017 - 18.868mA for a
> > 15ft range, or (their figure) 0.9900666mA/ft (ambitious use of 7
> > decimal places)
>
> Oh, sure, no doubt it is rock solid out to the last digit...;>

Or course, as long as the water level is stable to within 30 nanometres!   :-)

Cheers,


Howard Winter
St.Albans, England


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