> "Tim Wescott" <t...
@seemywebsite.com> wrote in message
>> On Wed, 04 Apr 2012 19:58:10 -0700, Adrian Nievergelt wrote:
>>> While designing a analog end user circuit which is noise critical, i
>>> came across the problem that i need variable resistors in the signal
>>> path of my op amps. Now wiring signal currents out onto a user interface
>>> pot is somehow just not feeling right. I was thinking of ofsetting the
>>> pot, by designing in a jfet based variable resistor (
>>> http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00335682 for those
>>> with access to IEEE ). Which is basically linearizing a jfet with an
>>> opamp and use that as resistor. This part of the circuit could then be
>>> addressed with a quasi DC voltage from aforementioned pot. I have two
>>> 1. That the jfet in combination with the op amp will in the end
>>> introduce more noise into the system than wiring out the pot would
>>> bring. 2. That the jfet would have the signal thermally drifting, even
>>> with the linearizing op-amp in place.
>>> Does anyone have experiences or thoughts to this, or maybe just simply a
>>> much better solution (no digital electronics please)?
>> Noise how? Are you worried about picking up EMI? Are you worried that
>> the pots themselves will generate thermal noise? What?
>> If it's thermal or excess noise in the pots, choose a pot material that's
>> not subject to that problem. If it's EMI, consider shielding the leads
>> going to and from the pot -- if you're really concerned, build a little
>> housing for the back of each pot, that surrounds it and makes a good
>> solid connection to the shielding on the wire. Better yet, just use a
>> good enclosure.
> ......or mount the pots on the board and mount the board to the rear of the
> user panel, eliminating wiring to/from the pots.