Current measurement question

[panfilero]

hello,

I have to measure DC current through a range of about 100mA to 50uA, I
need to make bi-directional current measurements.... so I'm planning
on getting an instrumentation op-amp and a shunt resistor... but I've
never done this before, I was wondering if anyone had any experience
with this could give me some tips on how to approach it... I think the
range may give me difficulty, I'm going to feed the output of the
instrumentation amplifier to an analog to digital converter that can
take -10V to 10V inputs...

thanks

 

[Jon Kirwan]

On Mon, 4 May 2009 17:06:54 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

I have to measure DC current through a range of about 100mA to 50uA, I
need to make bi-directional current measurements.... so I'm planning
on getting an instrumentation op-amp and a shunt resistor... but I've
never done this before, I was wondering if anyone had any experience
with this could give me some tips on how to approach it... I think the
range may give me difficulty, I'm going to feed the output of the
instrumentation amplifier to an analog to digital converter that can
take -10V to 10V inputs...

2000:1 is the range. If linear, you will need enough bits to resolve
at least your 50uA -- 12 bits, say.

How many volts is your measuring circuit allowed to drop, in-line? At
what potential relative to ground is it?

And I may not be able to help that much. I know a few things, but I'm
just a hobbyist and probably can only offer a few thoughts.

Jon

 

[whit3rd]

On May 4, 5:06 pm, panfilero <panfil...@gmail.com> wrote:

hello,

I have to measure DC current through a range of about 100mA to 50uA, I
need to make bi-directional current measurements.... so I'm planning
on getting an instrumentation op-amp and a shunt resistor... but I've
never done this before, I was wondering if anyone had any experience
with this could give me some tips on how to approach it... I think the
range may give me difficulty, I'm going to feed the output of the
instrumentation amplifier to an analog to digital converter that can
take -10V to 10V inputs...

thanks

Why bother with amplifiers and resistors? Just connect a
swinging-needle meter.

Or, do you need multiple ranges, high precision, machine-readable
outputs? It'll be difficult to handle 100 mA and 50 uA cases
with a single shunt for current sensing, if they both need to be +/-
1%
accuracy.

 

[David L. Jones]

"panfilero" <panfilero@gmail.com> wrote in message
news:f920101c-c1e1-4125-9ae2-7e8e562303bc@21g2000vbk.googlegroups.com...

hello,

I have to measure DC current through a range of about 100mA to 50uA, I
need to make bi-directional current measurements.... so I'm planning
on getting an instrumentation op-amp and a shunt resistor... but I've
never done this before, I was wondering if anyone had any experience
with this could give me some tips on how to approach it... I think the
range may give me difficulty, I'm going to feed the output of the
instrumentation amplifier to an analog to digital converter that can
take -10V to 10V inputs...

You'll need a 12 bit converter for 50uA resolution on a 100mA range.
Make sure you take the effect of shunt resistor voltage drop into account
with whatever it is you are measuring.
You don't say if you need much accuracy or resolution down at 50uA?, that
will determine if you need multiple ranges or not.
A high side current shunt resistor with say a MAX4372 amplifier would be a
standard solution.
Or if your amp is battery isolated you can get away with the shunt anywhere,
similar to my uCurrent adapter:
http://www.alternatezone.com/electronics/ucurrent/

Dave.

 

[Ross Herbert]

On Tue, 5 May 2009 13:11:06 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:

:On May 4, 5:06 pm, panfilero <panfil...@gmail.com> wrote:
:> hello,
:>
:> I have to measure DC current through a range of about 100mA to 50uA, I
:> need to make bi-directional current measurements.... so I'm planning
:> on getting an instrumentation op-amp and a shunt resistor... but I've
:> never done this before, I was wondering if anyone had any experience
:> with this could give me some tips on how to approach it... I think the
:> range may give me difficulty, I'm going to feed the output of the
:> instrumentation amplifier to an analog to digital converter that can
:> take -10V to 10V inputs...
:>
:> thanks
:
:Why bother with amplifiers and resistors? Just connect a
:swinging-needle meter.

I assume you mean a "centre-zero meter".

:
:Or, do you need multiple ranges, high precision, machine-readable
utputs? It'll be difficult to handle 100 mA and 50 uA cases
:with a single shunt for current sensing, if they both need to be +/-
:1%
:accuracy.

 

[whit3rd]

On May 6, 8:03 pm, Ross Herbert <rherb...@bigpond.net.au> wrote:

On Tue, 5 May 2009 13:11:06 -0700 (PDT), whit3rd <whit...@gmail.com> wrote:

:On May 4, 5:06 pm, panfilero <panfil...@gmail.com> wrote:

:> I have to measure DC current through a range of about 100mA to 50uA, I
:> need to make bi-directional current measurements....

:Why bother with amplifiers and resistors? Just connect a
:swinging-needle meter.

I assume you mean a "centre-zero meter".

That's one possibility, but polarity and shunt-value
switches (or single switch, if it has enough complexity)
can do the same job. It's not totally clear from the post what
is needed, but the idea that this task required multiple regulated
power supplies, A/D converters, high-performance instrumentation
amplifiers... I just had to throw cold water on that.

Some of the best big systems I've used were cobbled together
with CCTV cameras aimed at a panel of clearly labeled meters.
That kind of system (Hey, it's Post-It-Note compatible!) just
can't be beat.