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'Can you suggest an OP AMP?'
1997\06\03@181046 by cinetron

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Hi,
I'm trying to read current by using a low value shunt resistor. The
resistor value is .01 ohms. I want to be able to read .05 amp steps
from  0 to 10 Amps using a PIC 16c73 with a VREF of 2.5 volts. I'm
planning to use an OP AMP with a gain of 20 so that 1 Amp will yield .2
volts into the PIC. Can anyone suggest a good single supply OP AMP that
I can run off of 5 volts that would have a fairly low input offset? Do I
need an instrumentation OP AMP? Are these available in a single package
or do I have to make them out of 3 OP AMPS? Any suggestions would be
greatly appreciated. While I'm asking for info. does anyone know where I
can buy .01 ohm 1 watt resistors. Thanks!
                                                       Jim

1997\06\03@193302 by Steve Hardy

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> From: Jim Ruxton <spam_OUTcinetronTakeThisOuTspampassport.ca>
>
> Hi,
>  I'm trying to read current by using a low value shunt resistor. The
> resistor value is .01 ohms. I want to be able to read .05 amp steps
> from  0 to 10 Amps using a PIC 16c73 with a VREF of 2.5 volts. I'm
> planning to use an OP AMP with a gain of 20 so that 1 Amp will yield .2
> volts into the PIC. Can anyone suggest a good single supply OP AMP that
> I can run off of 5 volts that would have a fairly low input offset? Do I
> need an instrumentation OP AMP? Are these available in a single package
> or do I have to make them out of 3 OP AMPS? Any suggestions would be
> greatly appreciated. While I'm asking for info. does anyone know where I
> can buy .01 ohm 1 watt resistors. Thanks!
>                                                         Jim
>

One of the CMOS type opamps would be the go, since the output swings
rail-to-rail, and handles ground referenced inputs.  I haven't kept
up with the latest instrumentation amps, but I don't think one is
justified.  If needed it would be better to construct your own using
a quad opamp pack, since I think IOA's are fairly pricey for a one-off.
Regarding resistors: at this low level it is probably better to make
your own using constantan wire.  This is typically about 6 ohms per
metre for 28SWG, and is solderable.  Use a number of strands in parallel.

Look at Analog Devices, Linear Technology, Burr Brown, Nat Semi etc.
for info on op amps.  Farnell and RS Components are UK-based companies
which probably have a presence in Canada.  You can get small qty of
an extensive range of components from them (Farnell probably better
range of opamps) for a premium price, however this is better than
shopping around unless you get lucky.  They also supply pretty
cool current shunts if you don't want to make your own.

Having said this, it may be better for you to get a proper linear
hall effect current sensor.  These are only $20 or so, and give the
advantage of negligible voltage drop in the measured circuit, 150KHz
bandwidth, and total isolation of the electronics.  They take a +-
15V supply.  You pass the conductor whose current is to be measured
through the hole in the middle of the sensor.  You can wrap several
turns through it to increase sensitivity (and reduce range).

Isn't it annoying when you ask a simple question then some smart-arse
tells you that you're doing things the hard way in the first place?

Regards,
SJH
Canberra, Australia

1997\06\03@200026 by Site Y

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>On Tue, 3 Jun 1997 18:09:04 -0400 Jim Ruxton <.....cinetronKILLspamspam@spam@passport.ca>
wrote:
>Hi, I'm trying to read current by using a low value shunt resistor.
> .............. Can anyone suggest a good single supply OP AMP
>that I can run off of 5 volts that would have a fairly low input offset?


Texas Instruments TLC4501

>Do I need an instrumentation OP AMP?

 Not in these elightened times

> Are these available in a single package or do I have to make them out
of 3 OP AMPS?

Yes, but, you don't need one.

>can buy .01 ohm 1 watt resistors. Thanks!

 Vishay (610) 644 1300 or Caddock (909) 788 1700

Enjoy!
  Marv

1997\06\04@112656 by Tim Crist

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    >Date:    Tue, 3 Jun 1997 18:09:04 -0400
    >From:    Jim Ruxton <cinetronspamKILLspamPASSPORT.CA>
    >Subject: Can you suggest an OP AMP?

    Jim,

       Try the Maxim MAX433_ESA/ESD the Vos <= 600 uV max @ 25C.]
    Beware of generic CMOS Op-amps some of them have high Vos.
    Don't forget that the Quantization and Noise effects of your
    ADC will add at least two counts of error = 39.2 mV (worst case).
    I'm sure that Analog Devices, Mot, NSC, and others have comparable
    parts, But I have had good luck with Maxim.  Ask for some free
    samples at 1-800-998-8800.  (Note:  I have no affiliation, just
    a satisfied customer).  Also, Ibias is as high as 65 nA, so size
    you input resistors accordingly.  Make sure you use Kelvin style
    lead attachments to you sense resistor so that you don't pick
    up ground shifts (small signal wires coming off of the leads
    of the resistor back to you Op-Amp in addition to the fat high current
    traces that go Load to ground).  Check out Dale (Vishay) and IRC for
    .01 ohm resistors.

    Good luck,

    Tim Crist

1997\06\04@124001 by STEENKAMP [M.ING E&E]

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> Hi,
>  I'm trying to read current by using a low value shunt resistor. The
> resistor value is .01 ohms. I want to be able to read .05 amp steps
> from  0 to 10 Amps using a PIC 16c73 with a VREF of 2.5 volts. I'm
> planning to use an OP AMP with a gain of 20 so that 1 Amp will yield .2
> volts into the PIC. Can anyone suggest a good single supply OP AMP that
> I can run off of 5 volts that would have a fairly low input offset? Do I
> need an instrumentation OP AMP? Are these available in a single package
> or do I have to make them out of 3 OP AMPS? Any suggestions would be
> greatly appreciated. While I'm asking for info. does anyone know where I
> can buy .01 ohm 1 watt resistors. Thanks!
>                                                         Jim
You can always take a length of enameled copper wire.  The
resistance/length is readily available (or you can compute it with R=pl/A
where p=resistivity, l=length and A=area).  I used 0.45mm diameter wire
with a resistance of 0.155ohm per meter.  I needed a voltage drop of
10mV for a full scale current of 3A.  I used two lengths of 50mm wire in
parallel and it works like charm.  The current sensor consists of two op-
amps, but I am not sure what the resolution or stability of the circuit
is (it was not designed by me).  I only need a 'rough' reading.

Niki
.....steenkmpKILLspamspam.....firga.sun.ac.za

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