Re: Auto Offset

On Wed, 03 Dec 2003 12:26:42 -0800, Luhan Monat
<[EMAIL PROTECTED]> wrote:

>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.

---
What don't you understand about "I prefer not to have a processor 
involved"... ?

-- 
John Fields
Professional circuit designer
http://austininstruments.com