You'll get better results if you attach the low side of the current 
sense resistor to VREF-
with VREF+ attached to VDD or to a lower reference voltage. The GND 
losses between
the PIC and the current sense resistor will introduce errors.

I think the hall effect current sensors are non-linear at low currents.

--Bob

Chetan Bhargava wrote:

>>Sense resistor, hall effect sensor, or something like that.
>>    
>>
>
>I saw the allegro micro hall effect device. That is for way more
>current (50A) and might not measure small current values accurately.
>
>  
>
>>You won't be able to measure 20A directly.
>>    
>>
>
>Actually I have to measure current in two different apps. So I was
>thinking if only one front end would work. Anyway I can go lower to
>0-5A.
>
> 
>  
>
>>1 ohm gives you 1V at 1A, and dissipates 1W.
>>0.1 ohm gives you 0.1V at 1A, and dissipates 0.1W
>>If you can tolerate the 2V forward drop, then this would scale pretty well to a 2.5V input range ADC.
>>If you have to take the current sense resistor smaller, then you'll need an op-amp to bring it back up.
>>    
>>
>
>So if I use a 1 ohm, ground line sense resistor, I can get a 0-5v (for
>0-5A) across the resistor and PIC16D876 ADC can measure that range!
>Right?
>
>
> 
>  
>
>>Alternately, Maxim makes current sense chips that integrate the amplifier.
>>They can solve any $0.50 problem for only a few dollars. :)
>>    
>>
>
>That is true :-) One of my friends at Linear suggested me to use
>LTC2433-1 as he wrote a design note for linear "DN341". I don't like
>that idea as that is an added external component.
>
>
>Regards,
>
>Chetan Bhargava
>www.bhargavaz.net
>_______________________________________________
>http://www.piclist.com
>View/change your membership options at
>http://mailman.mit.edu/mailman/listinfo/piclist
>
>
>  
>


-- 
Note: Attachments must be sent to
 attach@engineer.cotse.net, and 
 MAY delay replies to this message.
        520-219-2363

_______________________________________________
http://www.piclist.com
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist