Wow, how did I miss the TI parts?

Looked everywhere else, and I was sort of stuck between a $1.20 national=20
part w/shunt, and the Allegro part...  The INA21X's have too low of a=20
CMRR voltage, but the INA270 looks like it might be perfect for the job.

Thanks!

-forrest

On 6/23/2011 12:39 PM, Mark E. Skeels wrote:
> Forest,
>
> It might be worth investigating using a very small series resistor with
> a TI INA21X. current shunt monitor.
>
> http://focus.ti.com/docs/prod/folders/print/ina214.html
>
> The chip output reference can be attached to something other than ground
> and the small value resistor needed allows very low insertion loss and
> low wattage rating.
>
> Different parts in the family have different gains, so you can choose
> one that fits your needs.
>
> Data sheet: http://focus.ti.com/lit/ds/symlink/ina214.pdf
>
> I can't spend a lot of time to see if this really fits your app, but I
> thought I'd mention it here because I bet if you take a look you'll find
> a nice way to use it and I suspect it will save you some $ over the
> Allegro part.
>
> Just make sure you have dedicated sense lead connections to the inputs
> to avoid voltage drop in the main current path which leads to errors. If
> you do this right you can get good results.
>
>> Mark Skeels
>> Engineer
>> Competition Electronics, Inc.
>> TEL: 815-874-8001
>> FAX: 815-874-8181
>> www.competitionelectronics.com
>> On 6/23/2011 11:38 AM, Michael Watterson wrote:
>>> what is resolution of ADC compared to accuracy of the sensor? Does it
>>> actually matter if full adc range isn't used? If so save on HW and just
>>> scale in SW. Unless you use precision resistors etc adding an op-amp ma=
y
>>> reduce accuracy?
>> I'm going to be using a 10 bit ADC in a PIC.  Vdd/Vss references, 3.3V
>> So 3.3/1024 =3D ~3mV/step.
>>
>> For various reasons (primarily increasing the number of places this part
>> can be used), i'd really like to use the +-25A version of the Allegro
>> product.   In this case, I'm most interested in 0-+5A.
>>
>> 0A corresponds to 1.67V.   5A corresponds to (5*0.055+1.67)=3D 1.945V.
>> So, a range of 275ma over the range of interest...   or just under 100
>> steps.
>>
>> Or using the other math... 55mV per AMP, or about 1/20th of an amp per
>> step.  Not sure that is enough resolution for me.
>>
>> I was considering moving the ADC references but there just isn't enough
>> to gain - you need at least 2V from Vref+ to Vref-, which means I'd be
>> lucky to gain another 100 steps.
>>
>> Although I have to admit, the most usable option I've come up with so
>> far is to give up and use the 12.5A version, and move the Vref- to
>> around 1.2V, which should put us at 2.1V/1024 =3D 2mV/step, and the abou=
t
>> 55 steps/amp... or 0.02A per 'step', which seems quite a bit better, and
>> then do math in software to get the right values.
>>
>> -forrest
>>

--=20
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