So isolate the grounds.  We do that all the time in low level analog stuff, where you tie back at a single point, typically thru a ferrite, and next to the digital interface.

--- On Mon, 6/2/08, Xiaofan Chen <xiaofanc@gmail.com> wrote:

From: Xiaofan Chen <xiaofanc@gmail.com>
Subject: Re: [EE] How to design a 2 channel 4-20mA transmitter?
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Date: Monday, June 2, 2008, 5:35 AM

On Mon, Jun 2, 2008 at 8:06 PM, Pang <mailtopang@yahoo.com> wrote:
> Having some problem designing multiple channel of
> 4-20mA transmitter. We would like to design a card
> that will provide 3 channels of 4-20mA transmitter
> that are independent of each other using XTR117. But
> based on the equivalent diagram provided by the
> datasheet, we notice that the current of different
> channels within the same card will be affecting each
> other, since all the XTR117 shares a common ground.
>
> Are there any two or three channels 4-20mA transmitter
> within the same chip?

I am not familiar with XTR117.

You can always use discrete solutions. From what I
see in the I/O side, most of the vendors are using
discrete based solutions.

Regards,
Xiaofan
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