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'[EE] Accurate low-current sensor'
2011\08\10@000102 by PICdude

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Back on my current sensor search.  Looking for something that will  accurately (1% error or less) measure up to 3mA (both directions).   Looking at some zetex parts, but the 1% is questionable unless I  compensate for temperature, etc.

Not finding much else at Digikey/Mouser/etc.  Any of you here have any  other recommendations?  I'm flexible on output type, supply voltage,  etc.

Cheers,
-Neil.

2011\08\10@002536 by Marcel Duchamp

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On 8/9/2011 9:00 PM, PICdude wrote:
> Back on my current sensor search.  Looking for something that will
> accurately (1% error or less) measure up to 3mA (both directions).
> Looking at some zetex parts, but the 1% is questionable unless I
> compensate for temperature, etc.
>
> Not finding much else at Digikey/Mouser/etc.  Any of you here have any
> other recommendations?  I'm flexible on output type, supply voltage,
> etc.
>
> Cheers,
> -Neil.
>
>

Have you looked at the LTC6101 from Linear Technology?

One thing about sensors like this is that they exhibit errors due to offset voltage which means that as your measured current drops toward zero, the error goes to infinity.  You can possibly avoid this by always loading your circuit with a known amount of current and then subtracting that out of the readings.  The other thing is to use as large of a value of sense resistor as is possible since a large measured voltage vs any fixed offsets improves the accuracy as well.


You probably will have to figure out a way around the various sources of errors as attempting to do analog measurements better than 1% over temperature, line voltage, etc. quickly gets challenging

2011\08\10@010438 by PICdude

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Quoting Marcel Duchamp <spam_OUTmarcel.duchampTakeThisOuTspamsbcglobal.net>:

> Have you looked at the LTC6101 from Linear Technology?

Had not, but now that I check why it did not come up on my digikey  search, I am finding that "current sense amp" yields more results.

This looks nice, though I have to figure out what it does for  bi-directional current sensing.  Guessing for now that I'll need to  use two simultaneously around a single sense resistor.  Will read the  datasheet tomorrow.


> One thing about sensors like this is that they exhibit errors due to
> offset voltage which means that as your measured current drops toward
> zero, the error goes to infinity.  You can possibly avoid this by always
> loading your circuit with a known amount of current and then subtracting
> that out of the readings.  The other thing is to use as large of a value
> of sense resistor as is possible since a large measured voltage vs any
> fixed offsets improves the accuracy as well.

I'll be sending the output to a PIC, so compensating for offset can be  done there.


> You probably will have to figure out a way around the various sources of
> errors as attempting to do analog measurements better than 1% over
> temperature, line voltage, etc. quickly gets challenging.

I can temperature compensate (same PIC measuring a temp sensor), but  was looking for simplicity.  Not a big deal though.

Cheers,
-Neil.

2011\08\10@015240 by RussellMc

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> Back on my current sensor search.  Looking for something that will
> accurately (1% error or less) measure up to 3mA (both directions).
> Looking at some zetex parts, but the 1% is questionable unless I
> compensate for temperature, etc.

Can you please explain what you are actually trying to do. That MAY be
in the prior thread youallude to but even if that tells us something
it may not be what you want.

You are really telling us HOW you think you can solve what you arre
trying to achieve. That may or may not be the best solution.

So, what's your problem?


     Russell

2011\08\10@023139 by Forrest Christian

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There are bidirectional versions of very similar parts....

On 8/9/2011 11:04 PM, PICdude wrote:
{Quote hidden}

>

2011\08\10@040253 by alan.b.pearce

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> > Have you looked at the LTC6101 from Linear Technology?

Another way to do it may be the Hall Effect sensor range from Allegro Microsystems. They do come in SO-8 packages to measure a current in a trace running under the chip. You could wrap the chip in a number of turns of wire to increase the sensitivity to the 3mA you want.

I am not sure if they have internal temperature compensation, it is a while since I looked at them.

http://www.allegromicro.com/en/Products/Categories/Sensors/index.asp is the home page for all their sensors, all sorts of interesting stuff.



-- Scanned by iCritical.

2011\08\10@083754 by Sean Breheny

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Hi Marcel,

I don't think that adding in an additional "real" offset current and
then subtracting that off later does anything to improve the problem
of percentage error near 0 Amps. This is simply the nature of the
concept of percentage - there will always be some false offset (which
is unaffected by whether you add in some other fixed current) and to
whatever extent you do not know the value of this fixed offset, it
will contribute to a percentage error which climbs to infinity near 0
current. The only way around this that I know of would be to modulate
the current you are trying to measure so that you can then AC-couple
the output of the sensor and get rid of any DC-offset.

Sean


On Wed, Aug 10, 2011 at 12:25 AM, Marcel Duchamp
<marcel.duchampspamKILLspamsbcglobal.net> wrote:
> One thing about sensors like this is that they exhibit errors due to
> offset voltage which means that as your measured current drops toward
> zero, the error goes to infinity.  You can possibly avoid this by always
> loading your circuit with a known amount of current and then subtracting
> that out of the readings.  The other thing is to use as large of a value
> of sense resistor as is possible since a large measured voltage vs any
> fixed offsets improves the accuracy as well.
>

2011\08\10@163139 by Richard Prosser

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On 11 August 2011 00:37, Sean Breheny <.....shb7KILLspamspam.....cornell.edu> wrote:
{Quote hidden}

Hi Marcel

I used a chopper stabilised opamp in a somewhat similar application -
Used for measuring 1000s of amps, rather tha mA. The input voltage
drop was only about 3mV max however so a low offset amplifier was a
must. (Even a few 10s of mV drop at 1kA represents a "useful" power
loss). It also included temperature compensation by using a 120ohm
copper "resistor" (a 12mH choke) as the input resistor to the opamp.
If the temperature of the choke and the copper busbar were the same
then the temperature dependence cancelled nicely. In practice there
was a small error, but generally less than 0.5% over the range -20 to
+80C.

>From memory I used a texas TLC2652 opamp and found it worked very well
once the circuit was setup to minimise track current effects and
localised temperature increases.  Other chopper stabilised opamps
worked nearly as well and the non-stabilised OP177 was nearly good
enough.

RP

2011\08\10@183351 by PICdude

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As part of a larger project for someone else (which controls some  valves etc on a test rig), we're interfacing into an existing system  to "see" what a sensor is doing.  I've not actually tested this yet,  but was told the sensor output is in the range of -3 to +3 mA, and  they measured ~15V.  It is supposed to be a closed-loop or feedback  system, so sticking a small resistor in the line should not be a  problem.

I know of a couple ways to measure current, and since the resistor  method seems to be much less costly, am leaning that way for now (vs.  a hall-effect sensor).

Cheers,
-Neil.



Quoting RussellMc <apptechnzspamspam_OUTgmail.com>:

{Quote hidden}

>

2011\08\10@212546 by IVP

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part 1 653 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

> I used a chopper stabilised opamp in a somewhat similar application -
> Used for measuring 1000s of amps, rather tha mA. The input voltage
> drop was only about 3mV max however so a low offset amplifier was
> a must. (Even a few 10s of mV drop at 1kA

Just read an article about a DMM adapter using the MAX4239ASA+
chopper amp. 0.1uV offset typical, 2.5uV over the temperature range.

Article claims resolution of 10pA on a 4.5 digit meter, 0.2% accuracy

Unfortunately the circuit is not bidirectional

http://parts.digikey.com/1/parts/1270799-ic-amp-low-noise-8-soic-max4239asa..html

Joe

part 2 15658 bytes content-type:image/gif; name="dmm_current_adapter_sm.gif" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\08\10@231242 by Marcel Duchamp

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On 8/10/2011 6:24 PM, IVP wrote:

> Just read an article about a DMM adapter using the MAX4239ASA+
> chopper amp. 0.1uV offset typical, 2.5uV over the temperature range.
>
> Article claims resolution of 10pA on a 4.5 digit meter, 0.2%
> accuracy
>
> Unfortunately the circuit is not bidirectional
>

I think it is... that's the reason for the "synthetic" ground.  The ground opamp splits the 3V battery into +/- 1.5V rails and since the input is referred to ground it can go both ways, no

2011\08\10@233307 by PICdude

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Hmmm... isn't this the Dave Jones micro-current project?


Quoting IVP <@spam@joecolquittKILLspamspamclear.net.nz>:
{Quote hidden}

2011\08\10@235348 by IVP

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> input is referred to ground it can go both ways, no?

Ah, yes

> Hmmm... isn't this the Dave Jones micro-current project?

It is

2011\08\11@035321 by RussellMc

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part 1 2561 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

> Unfortunately the circuit is not bidirectional

Note other comments - the designer has tried to make it so - but has
almost fail by apparently failing to note that the input CMR is
asymmetrical.

Note that (according to the data sheet) input common mode range is
from 0.1V below negative rail (OK so far) to -1.3V below positive
rail. With the circuit shown with a 3V lithium battery supply being
split to +/- 1.5V supplies, that gives it a positive input range of
(1.5-1.3) = +0.2V !!!!.  ie CMR (common mode range) with a 3V supply
is +/- 0.9 Volt centred around +0.8V. As in this circuit the local
ground is at 1.5V (0.7V above the opamps's CMR centre point, input +ve
headroom is almost zero.

If the specs are correct (as they probably are) it would make sense to
skew the derived ground around 0.8V  for maximum AC swing or even
lower if maximum positive DC is required.

Worse, with the circuit as is, when/if the supply droops to 2.6V  the
amplifier will have ZERO positive input capability. A typical lithium
CR2032 at low loading (about 0.3 mA) reaches 2.6V about half way
through its capacity - and as used here at several mA load it may stop
working at a small fraction of full battery capacity. Increasing R6 to
150k looks like a good idea.
_______

The MAX4239 is an excitingly impressive amplifier
2 uV max offset voltage at 25 C and $US2.47/1 in stock at Digikey.
1 MHz gain-bandwidth product or 6.5 MHz GBP at above 10 V/V gain for
the max4239 and 600 uA supply current.
Makes me want to rush out and design something :-).

Datasheet

   http://datasheets.maxim-ic.com/en/ds/MAX4238-MAX4239.pdf

Pricing (Digikey)

  http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=max4239&stock=1


   Russell McMahon


On 11 August 2011 13:24, IVP <KILLspamjoecolquittKILLspamspamclear.net.nz> wrote:
{Quote hidden}

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