Re: variable high resistance
- From: kensmith@xxxxxxxxxxxxxxx (Ken Smith)
- Date: Mon, 16 Jan 2006 15:04:28 +0000 (UTC)
In article <b82dnblalNi_lVfeRVn-jA@xxxxxxxxxxxxx>,
Walter Harley <walterh@xxxxxxxxxxxxxxxxxxxx> wrote:
>Something I was working on recently got me wondering:
>
>Suppose I want to have an adjustable resistance, that will be linear (that
>is, ohmic) for AC voltages less than 1Vp and audio frequencies. The
>resistance needs to vary from about 50k to 5MEG, controlled by some other
>(analog) voltage or current. The relationship between control signal and
>resistance does not need to be particularly linear or repeatable from part
>to part.
How about a semi-digital method?
50K
------+--------/\/\/---------
! !
! A1 ----- A2 !
X --!>----! Ref !---!>-- Y
! !
N ==============! !
-----
DAC
A1 and A2 are good quality buffering amplifiers. The circuits between X
and Y are arranged so that it is non-inverting. When N = 0, the input
looks like a 50K. When N is full scale, the input impedance is very high.
If you want a knob the user turns, replace the DAC with a pot the user
adjusts.
--
--
kensmith@xxxxxxxxx forging knowledge
.
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