On Sat, 3 Oct 2009 14:50:15 +0100, Nemo
>
>
>
> wrote:
> >I'd appreciate some witty put-downs and scathing feedback on the
> >following.
>
> >* I want to take the opportunity of a circuit redesign on a piece of
> >scientific instrumentation to use a differential output to reduce EMC
[....]
The ADA4922-1 pretty much does this. Nice part, lots of swing and lots
> of power dissipation. Drive two coaxes with precision 50-ohm source
> terminator resistors.
Make or buy some of that stuff with 2 coax cables glued side by side. You want to keep the enclosed area between the shields small. Even the best coax is not 100% shielded. This will tend to make a current flow in the shields. It isn't a problem unless the area enclosed is very large.
Ground the shields on both ends. Source
> termination reduces the dynamic load on the opamps and helps keep
> distortion and power dissipation down. Don't terminate the receive
> end.
Keep the capacitance small at the receive end. You want to go from Z =3D 50 ohms to infinite. Short PCB traces will do this.
If you need suppress EMI at the transmit end, run the whole mess through a single ring core
What's your dynamic range? What's the physics? I'm doing a very
> similar thing, moving a wide-dynamic-range signal across a room on two
> coaxes, KHz to 20 MHz, and need to reject ambient RF. We're going for
> maximum swing (20 v p-p differential) to climb above the noise.
> Source-only termination gives 1.00000 gain through each cable leg,
> maintaining the CMRR.
>
> John