Searching \ for '[EE] Virtual shunts and measuring small currents' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=current
Search entire site for: 'Virtual shunts and measuring small currents'.

Exact match. Not showing close matches.
PICList Thread
'[EE] Virtual shunts and measuring small currents'
2005\09\04@151120 by Matthew Miller

flavicon
face
Hi everyone,

In my thread last week Peter posted a schematic for a virtual shunt current
measurement circuit. I wanted to know more about this type of circuit, but a
search on "virtual shunt" combined with some other terms turned up no useful
links. Could this type of circuit have a different common name?

Also, if anyone has suggestions or tips for measuring small (10nA-100uA)
currents feel free to share them! :)

Thanks.

Matthew

--
...a GUI is only so we can display more xterms at the same time.
   - Rodney L Caston

2005\09\04@162931 by Spehro Pefhany

picon face
At 03:07 PM 9/4/2005 -0400, you wrote:
>Hi everyone,
>
>In my thread last week Peter posted a schematic for a virtual shunt current
>measurement circuit. I wanted to know more about this type of circuit, but a
>search on "virtual shunt" combined with some other terms turned up no useful
>links. Could this type of circuit have a different common name?

It's just an op-amp with a "virtual ground". The op-amp forces the input
to ground (give or take Vos of the op-amp) through a feedback resistor.
The output voltage is -(Iin * Rf), ignoring the effect of input bias
current of the op-amp and offset voltage. Those two details (and others)
affect the accuracy, of course.

>Also, if anyone has suggestions or tips for measuring small (10nA-100uA)
>currents feel free to share them! :)

A regular DVM will measure small currents fairly accurately. Often there is
no resistor on the input on lower ranges (eg. 199.9mV F.S.). So a simple 10M
1% shunt  resistor will allow you to measure current with a resolution of
10pA.
Putting a small *film* capacitor eg. 0.1uF polypropylene (or polyester if you
don't have PP) across the 10M will help smooth the reading. The voltage
drop will be < 200mV, if you can read it, obviously. Go with a lower
resistor for higher currents.

My HP33401A can be set from the front panel to have no input resistor, so with
1uV resolution, I can measure current with a resolution of 100fA, which is
sufficient for many applications.

If you want to go lower still, check into some of the 'electrometer' op-amps,
and the special construction techniques required (I think Bob Pease had an
article on it), which will allow you to delve well into fA territory at
room temperature.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\09\04@192959 by Matthew Miller

flavicon
face
Hi Spehro,

On Sun, Sep 04, 2005 at 04:36:43PM -0400, Spehro Pefhany wrote:
{Quote hidden}

Thanks for the information. This is very helpful!

Hopefully, I'll be able to take some better measurements soon.

Matthew

--
If at first you don't succeed, destroy all evidence that you tried.
   -- Steven Wright

2005\09\05@174200 by Peter

picon face


On Sun, 4 Sep 2005, Matthew Miller wrote:

> Hopefully, I'll be able to take some better measurements soon.

Matthew, the 'virtual shunt' measurement has one big advantage over
other methods: the voltage drop on the shunt is small and constant. This
is very important at low voltage. For example a PIC will 'wake up' and
draw 2-3mA then go to sleep and draw uA. The 2-3 mA have to come from
somewhere. The simple DVM and capacitor methods cannot cope with this.
The 'virtual shunt' can cope fine (up to about 20mA load). Of course it
will read wrong during this time, but the pic will not brown out or
reset.

Peter

2005\09\05@201317 by Matthew Miller

flavicon
face
Hi Peter,

On Tue, Sep 06, 2005 at 12:41:57AM +0300, Peter wrote:
>
> Matthew, the 'virtual shunt' measurement has one big advantage over
> other methods: the voltage drop on the shunt is small and constant. This
> is very important at low voltage. For example a PIC will 'wake up' and
> draw 2-3mA then go to sleep and draw uA. The 2-3 mA have to come from
> somewhere. The simple DVM and capacitor methods cannot cope with this.
> The 'virtual shunt' can cope fine (up to about 20mA load). Of course it
> will read wrong during this time, but the pic will not brown out or
> reset.

Thanks for the information. I think the virtual shunt method is a good way
to go. I just need to hunt around and see if I have any suitable op-amps.

I know I have some LMC6062 on hand which have a 100uV offset voltage and
10fA input bias current. Do you think this op-amp would work O.K.?

Thanks.

Matthew

--
"Get off my way or I'll pee right through ya!" -- Richard Feynnman

2005\09\06@033636 by Peter

picon face
Matthew Miller <namiller2 <at> naxs.net> writes:

>
> Hi Peter,
>
> On Tue, Sep 06, 2005 at 12:41:57AM +0300, Peter wrote:
> >
> > Matthew, the 'virtual shunt' measurement has one big advantage over
> > other methods: the voltage drop on the shunt is small and constant. This
> > is very important at low voltage. For example a PIC will 'wake up' and
> > draw 2-3mA then go to sleep and draw uA. The 2-3 mA have to come from
> > somewhere. The simple DVM and capacitor methods cannot cope with this.
> > The 'virtual shunt' can cope fine (up to about 20mA load). Of course it
> > will read wrong during this time, but the pic will not brown out or
> > reset.
>
> Thanks for the information. I think the virtual shunt method is a good way
> to go. I just need to hunt around and see if I have any suitable op-amps.
>
> I know I have some LMC6062 on hand which have a 100uV offset voltage and
> 10fA input bias current. Do you think this op-amp would work O.K.?

The LMC6062 would work except it has too low Vcc limit for use with 2 9 Volt
batteries (15V vs. 18V). Something old and cheap will work better here. TL
072/074 or 082/084 etc.

In general the newer opamps have better characteristics but lowe voltage
tolerance. You could use the LMC6062 with just one 9V battery and a 22k/22k
voltage divider to produce the floating ground reference for the + input of the
opamp.

But you have to fit a power switch as it may drain the battery faster than
before. The 2 x 9V battery solution with a TL072 would last in excess of two
weeks continuous on two cheap (60 cent) 9V batteries. I know this because I once
forgot to disconnect the batteries.

Peter

2005\09\06@113024 by Matthew Miller

flavicon
face
On Tue, Sep 06, 2005 at 07:34:41AM +0000, Peter wrote:
>
> The LMC6062 would work except it has too low Vcc limit for use with 2 9 Volt
> batteries (15V vs. 18V). Something old and cheap will work better here. TL
> 072/074 or 082/084 etc.
>
> In general the newer opamps have better characteristics but lowe voltage
> tolerance. You could use the LMC6062 with just one 9V battery and a 22k/22k
> voltage divider to produce the floating ground reference for the + input of the
> opamp.
>
> But you have to fit a power switch as it may drain the battery faster than
> before. The 2 x 9V battery solution with a TL072 would last in excess of two
> weeks continuous on two cheap (60 cent) 9V batteries. I know this because I once
> forgot to disconnect the batteries.

Thanks Peter. I do have some TL082's laying around.

Matthew

--
"I ran it on my DeathStation 9000 and demons flew out of my nose." --Kaz

More... (looser matching)
- Last day of these posts
- In 2005 , 2006 only
- Today
- New search...