Re: Auto Offset

Mark wrote:

> I'm using a LEM current transducer to measure pulsed currents.  The 
> current sensor produces a current output which I feed to a resistor 
> producing a voltage that represents the current flow. The voltage feeds 
> an opamp for offset correction and buffering.  The sensor has good 
> accuracy (+/- 0.5%), but suffers from an initial offset  
> (+/- 0.15mA) and worse, a thermal induced offset (+/- 0.35 mA over temp).
>
> Since I know when the current pulses are coming, I want to to force the 
> offset to zero before taking a measurement.  I've seen circuits to do 
> this using the following techniques:
>
> 1. An A/D followed by a D/A.  With zero current input, the offset is 
> driven to zero, and the offset correction is latched in the D/A until 
> the next "calibration" cycle.
>
> 2.  A/D and a digital pot controlled by a processor, works similiar as 
> above.
>
> What are the merits of either method.  I prefer not to have a processor 
> involved, but do have access to FPGA resources.
>
> Is ther a simple way to do this?
>
> Mark

Mark,

Since you are incorporating a processor here, make 2 a/d readings: one 
just before and one during the pulse.  Then subtract the first from the 
second.

--
Luhan Monat, "LuhanKnows" At 'Yahoo' dot 'Com'
http://members.cox.net/berniekm
"The future is not what it used to be."