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'[EE:] nA meter'
2004\09\10@052616 by Jinx

face picon face
Any circuit suggestions for a multimeter plug-in that could measure
1nA (hopeful) to a few uA ? I was thinking perhaps A -> V and
use the low DC volts range

I've got a couple of multimeters - one goes down to 20uA and the
other to 2000uA. Measuring a sleeping 12F675 gets you 00.0uA
or 0.000mA (yes, it is alive, I can wake it up to do stuff). Helpful
in the sense that it shows the battery will last a flipping long time,
but there's nothing there to hang your hat on

Googled and Googling around a bit but not too fruitful yet. Maybe
I'm not searching for the right thing, and there's such a bewildering
array of parts and technologies I might not even recognise what I
need when I see it anyway

The old NatSemi Linear Apps book has a 200pA-100uA circuit
(LB37) for the LM216A and a 100uA  moving coil meter, but the
LM216A seems to have dropped off the radar. I'm certain there
will be an equivalent or better around, but short of downloading
squillions of op amp datasheets...........

TIA

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2004\09\10@055809 by cdb

flavicon
How about one of those Zetex or Philips MMR's or a Sentron Hall effect
chip?

With either of these (they have internal magnets) placing the sensor
chip in a ferrite ring with a number of turns to amplify leading to an
op-amp or insamp thence to multimeter.

I have some ZMZ20M sensors.

Polykom are the agent in Australia - but they never seem to reply to
emails. I had to get mine from W2K.co.uk.

There are the Philips equivalent, but have yet to find who will supply
them in small quantities.

I'm about to start on a project using these, but i don't want to go
into that  in public.

Colin

--
cdb, spam_OUTcdbTakeThisOuTspambarnard.name on Friday,10 September,2004

I have always been a few Dendrites short of an Axon and believe me it
shows.

Light travels faster than sound. That's why some people appear bright
until they speak!



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2004\09\10@061943 by andrej.nemec

flavicon
face
Take a look at:

http://www.elv.de

and search for part. nr. 68-157-50

Andrej

On 10 Sep 2004 at 21:26, Jinx wrote:

{Quote hidden}

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2004\09\10@064002 by Jinx

face picon face
> http://www.elv.de  and search for part. nr. 68-157-50
>
> Andrej

That's pretty good - 100pA and 9.45 Euros. Wouldn't mind
one of those. Babel had to help out with the lingo

During the measurement of particularly small rivers starting from
few well (10^-9 A) this nano-ammeter carries good services out. For
the determination of the power input of CMOS elements or also for
the measurement of remainder and leakage currents the sensitivity
of conventional circuit analyzers is not sufficient frequently. But
the additional circuit presented here offers itself with the special
advantage that between the messklemmen nearly no voltage drop arises.
The messbereichsumfang reaches when connecting a 3,5-stelligen of
circuit analyzer of 100 Pa with a measuring range final value of
0,2 µA up to 2 mA. The signal current into to it proportional an
output voltage within the range between 0 to 200 mVs converted for
the direct connection of a digital circuit analyzer. By connection
of a measuring instrument with a final value of 1 V even rivers up
to 10 mA can be measured.

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2004\09\10@065303 by Jinx

face picon face
> Polykom are the agent in Australia - but they never seem to reply to
> emails. I had to get mine from W2K.co.uk.
>
> There are the Philips equivalent, but have yet to find who will supply
> them in small quantities.

I'll ask at Polykom and Active Components on Monday.

I just wonder about trying to isolate magnetic fields caused by
nA currents

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2004\09\10@070835 by Spehro Pefhany

picon face
At 09:26 PM 9/10/2004 +1200, you wrote:
{Quote hidden}

You may be able to use a cheap DVM on a voltage range with
an external resistor (check the specifications). My HP 5-1/2 digit
meter can be switched to >10G ohm on low volts, so it's easy to measure
nA level leakages.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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2004\09\10@072340 by Russell McMahon

face
flavicon
face
> Any circuit suggestions for a multimeter plug-in that could measure
> 1nA (hopeful) to a few uA ? I was thinking perhaps A -> V and
> use the low DC volts range
>
> I've got a couple of multimeters - one goes down to 20uA and the
> other to 2000uA. Measuring a sleeping 12F675 gets you 00.0uA
> or 0.000mA (yes, it is alive, I can wake it up to do stuff). Helpful
> in the sense that it shows the battery will last a flipping long time,
> but there's nothing there to hang your hat on


If you want to specifically measure a uPs sleep current or similar you can
cheat and power it from a small cap at a known voltage.
First use a voltmeter to measure the cap voltage and watch what it does with
time with the meter only as load.
Now add the uP load and note the difference. Mr Coulomb will allow you to
decide what current is drawn.

   RM

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2004\09\10@074953 by David Bearrow

picon face
www.national.com/an/AN/AN-71.pdf

is an application note for the LM4250 Programmable Operational Amplifier.
Go down to page 7. They have a schematic for a nanoammeter.

David Bearrow

At 04:26 AM 9/10/04, you wrote:
{Quote hidden}

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2004\09\10@080820 by Jinx

face picon face

> http://www.national.com/an/AN/AN-71.pdf

Thanks, I did find that one on the latest search (actually
nsc04471.pdf from another site). LM4250 I can get from
RS, and it's not too pricey

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2004\09\10@093821 by Engineering Info

picon face
Just to add to everyone elses suggestions.  Take a look at the MAX472.  
Should be able to design it for whatever range you want.

Jinx wrote:

{Quote hidden}

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2004\09\10@182913 by Jinx

face picon face
> Just to add to everyone elses suggestions.  Take a look at the
> MAX472. Should be able to design it for whatever range you want.

Thanks, it looks useful for many current-related applications. The
only fly in the ointment at nA ranges might be this, (but I haven't
gone through all the ANs in detail yet) -

http://pdfserv.maxim-ic.com/en/ds/MAX471-MAX472.pdf

Output Offset Error at Low Load Currents.

Large RG values reduce IOUT for a given load current. As IOUT
gets smaller, the 2.5µA max output offset-error current becomes
a larger part of the overall output current. Keeping the gain
high by choosing a low value for RG minimizes this offset error


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2004\09\10@190704 by Bob Blick

face picon face
Jumped into this a little late so forgive my ignorance of the thread.

If you want to build a nanoammeter, why not use a decent FET opamp to
buffer the input to your multimeter, measuring a largish-value shunt
resistor?

Accuracy +-5% shouldn't be too hard to get.

-Bob



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2004\09\10@193229 by Spehro Pefhany
picon face
At 04:07 PM 9/10/2004 -0700, you wrote:
>Jumped into this a little late so forgive my ignorance of the thread.
>
>If you want to build a nanoammeter, why not use a decent FET opamp to
>buffer the input to your multimeter, measuring a largish-value shunt
>resistor?
>
>Accuracy +-5% shouldn't be too hard to get.
>
>-Bob

Or just use the multimeter on low volts range with a resistor. Most of
them use a variant of the 7106 chip which has a typical input current
of 1pA (10pA maximum). On the 200mV range there are usually no resistors
shunting the input. So, using a 1M 1% resistor will give you 100pA,
resolution, 200nA full scale, and typically better than 1% accuracy (with
a low-end 3.5 digit DVM).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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2004\09\10@213112 by Jinx

face picon face
part 1 731 bytes content-type:text/plain; (decoded 7bit)

> So, using a 1M 1% resistor will give you 100pA, resolution,
>200nA full scale, and typically better than 1% accuracy (with
> a low-end 3.5 digit DVM)

I've tried that, and it seems to work, thanks. You need to short
the 1M first so that the PIC powers up properly and executes the
SLEEP. After a couple of minutes the reading has dropped from
an initial 23mV to a low low low 1.5mV. Impressive. Attached
is the actual circuit under test. Intention is for it to be a sensor that
you can download parameters to and receive data from. So far
it's meeting all expectations admirably, but there's a little tinkering
to do yet (on Vref, internal s/w scaling, multi-functioning pins etc)





part 2 3400 bytes content-type:image/gif; (decode)


part 3 194 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

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part 4 1 bytes

2004\09\11@041357 by Peter L. Peres

picon face

On Fri, 10 Sep 2004, Jinx wrote:

> Any circuit suggestions for a multimeter plug-in that could measure
> 1nA (hopeful) to a few uA ? I was thinking perhaps A -> V and
> use the low DC volts range

Hook up any cmos low input current opamp like:

  +9V-100k--*-100k-9Vgnd
            |
+supply-->--*--+
               |    out-----+
pic vdd <---*---            |
            |               |
            *----- 1uF PMP -*
            |               |
            +------r1-------+

with modern parts you can 'measure' to 10pA or so. The voltage on r1 is
(r1 * Imeas) and this is your output. F.ex. r1=1Meg 1% I=1nA -> Ur1=1mV,
r1=10Meg I=1nA ->Ur1=1V (not recommended). If you qualify your dvm input r
you can use it as r1. To find out offset and such you have to play a
little. The opamp can be powered by a 9V battery. Watch out for
cleanliness. At these currents fingerprints are resistors and the circuit
will pick up hum and cell phones and noise from the air.

Peter
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2004\09\13@131002 by Peter L. Peres

picon face


On Fri, 10 Sep 2004, Spehro Pefhany wrote:

> Or just use the multimeter on low volts range with a resistor. Most of
> them use a variant of the 7106 chip which has a typical input current
> of 1pA (10pA maximum). On the 200mV range there are usually no resistors
> shunting the input. So, using a 1M 1% resistor will give you 100pA,
> resolution, 200nA full scale, and typically better than 1% accuracy (with
> a low-end 3.5 digit DVM).

But the input impedance is large and the sampling/zeroing switches can be
'felt'.

Peter
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