Re: Current source design (tricky?)

From: John Larkin (jjSNIPlarkin_at_highTHISlandPLEASEtechnology.XXX)
Date: 03/12/05 

Date: Sat, 12 Mar 2005 13:16:51 -0800

On Sat, 12 Mar 2005 12:37:53 -0800, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

>The "capacitive load
>accomadation" feature described for the LM8261
>is strictly limited to large signal conditions.

But it's not. If you reflect the "Miller" capacitance to an equivalent
capacitance at the output node, adding external C simply makes this
net C look bigger. They have merely moved the dominant pole to the
output pin. It's not much more profound than putting two capacitors in
parallel. If the c-load fix only worked large-signal, it would
oscillate at low level when c-loaded, and it doesn't.

These *are* great parts for driving ADC references and mosfet gates.

Too bad the input offset shifts with common-mode voltage, as most r-r
front-ends still do. I think there's a subtlety there as regards not
making a schmitt trigger out of the whole thing.

>how one might confuse it for Miller effect is a
>puzzle. Care to explain that, Fred?

The National data*** calls it Miller Effect, but that's sort of an
ancient blanket term for any sort of capacitive feedback.

Fred's not at all nice, but he's usually right.

John

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