Re: Auto Offset

On Wed, 03 Dec 2003 13:43:57 -0500, Mark <[EMAIL PROTECTED]>
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


   in--------c-------+----------out
                     |
                    switch
                     |
                    gnd


Just keep the switch closed when you know there's no pulse. A fet or
an CMOS analog switch thingie will work.

John