Re: A6302 current probe excessive offset

Albert Otten
 

On Wed, Sep 18, 2019 at 10:18 PM, cmjones01 wrote:


I played with reducing the value of the resistor which feeds the offset
correction in to one pin of the amplifier, but even that wasn't enough. I
have now added a 3k3 resistor from the offset adjust pot's wiper to one end
of the hall sensor's output, and that allows me to bring the offset down
but at the expense of a very fussy offset adjustment and slight reduction
in gain. I could probably correct the latter by increasing the hall current
a bit, or devoting one AM503 to it with increased gain.
Some thoughts without studying the schematics too long.
I choose a similar remedy for a P6302. Soldered R18 to "A" and decreased the value. Wired side "E" of the Hall sensor to "K" (gnd) via 18k. Both action pulled the + of the opamp more negative.
A slight reduction in gain is only possible (I think) when there is not enough amplification by the DC amplifier (the feedback to zero out the DC flux component is not satisfying). Maybe your action put a heavier load on the Hall output. In my opinion the effect would be that the low frequency gain is now slightly smaller than the high frequency gain. AM503 Gain adjust R344 is common to both, so can't help. I'm not sure whether I ever carefully checked the frequency response of the P6302.
With that probe the scope trace is also very "jumpy". The probe is nearly useless in the most sensitive settings of the AM503. From all old messages at the time (especially by Steve Ditter) I concluded that the Hall sensor was desintegrating.

Albert

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