Quoting Mike Harrison <mike@whitewing.co.uk>:

> On Thu, 3 Jan 2008 09:55:38 -0800 (PST), you wrote:
>
>> I've used the 2 wire RTDs  before...essentially measure the voltage  
>>  across the device for the temperature, but I've been asked to look  
>>  at 3 wire or full bridge RTD's.  Now, they talked like they are  
>> one  and the same but I think a full bridge is a 4 wire?  4 wire  
>> seems  simple enough in that it has a constant current source to  
>> get  better accuracy but what about 3 wire devices?  A current  
>> source  that dumps into a common ground?  Just need some  
>> clarification on  that.
>>
>
> With 4 wire, you apply current on one pair and measure voltage   
> differentially on the other. Easiest
> way is to apply a constant-ish current, and take the ADC reference   
> as the voltage across a precision
> series resistor in the excitation leg - this gives a direct reading   
> of resistance independent (
> within reason) of the excitation current, avoiding the need for   
> expensive voltage references.
>
> 3-wire makes the reasonable assumption that the voltage drop on both  
>  excitation wires is the same.
>
> Assume the wires to one end are A,B and the other is C
>
> Apply current to A, with C grounded
> Measure voltage across B & C - this is the RTD voltage plus the   
> voltage drop on the C wire
> Measure voltage between A and B - this is the voltage drop on the A   
> wire. Subtract this value from
> the BC reading to get the 'real' RTD voltage value.

Another approach is to use a second current sink that's twice the
value of the main one, and just apply that to the compensation
lead. That causes the leadwire resistance to cancel out without
any calculation, and with a single-ended input, provided, of course,
that the leadwires are of equal resistance. Otherwise there's an
error proportional to the difference in the leadwire resistances.
With 100R (or less) sensors, narrow sensing ranges, and long runs
of relatively small-gauge wire, the leadwire resistance can be
substantial compared to the sensing range, so differences from
different lots of wire can have a significant effect. Nonetheless,
4-wire sensing is rarely used outside of lab situations.

Best regards,
Spehro Pefhany
-- 
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