Re: trimpots at speed

From: Frank Miles (fpm_at_u.washington.edu)
Date: 03/04/05 

Date: Fri, 4 Mar 2005 18:16:09 +0000 (UTC)

In article <9ipe21dkn14p8kn3vetkevqv1t70jaq5fe@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:
>I'm designing an optical-electrical converter (fiber-coupled pin
>diode, opamps, stuff like that) and figured I might want to offer a
>calibrated version, 1 v/mw or something. So I figure I'll just use a
>small trimpot (4mm surfmount) in one of the gain stages. I'll use 800
>MHz opamps and maybe get a couple hundred MHz overall bw.
>
>So, is there anything wrong with using trimpots at this speed? What's
>the fastest anybody *has* used trimpots? Any advice?

Trimpots have been used for a long time in oscilloscope vertical amplifiers.

What follows will probably be obvious to you: if you use them in a
low-impedance path, the inductance associated with the path length will
get you eventually; on a high-impedance node, it will be the stray capacitance
that will be problematic.

Be careful with design, part selection, and layout and you will almost
certainly be able to achieve >200MHz bandwidth. It's been done, and with
larger parts than the tiny parts now available.

        -frank 

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