--- On Sun, 8/3/08, Forrest W Christian <forrestc@imach.com> wrote:

> From: Forrest W Christian <forrestc@imach.com>
> Subject: Re: [EE] ADC Input Buffer Circuit
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Date: Sunday, August 3, 2008, 7:02 PM
> Richard Prosser wrote:
> > Could you run separate sensors & connect them by
> a) optoisolators  at
> > the digital level, or b) radio (zigbee  etc) ? Passing
> analogue
> > readings through an opto can be done but it needs
> care.
> >  Alternatively you could do a voltage-frequency
> conversion and use
> > simple transformers.
> 
> I've actually thought about providing some basic inputs
> which are all 
> ground/Vss referenced and then building a separate device
> where this 
> won't work which includes some sort of isolation
> circuitry.
> 
> > You don't state the update rate so I'm
> guessing it will be slow.
> 
> Slow... once every few seconds...  Mainly these need to be
> able to 
> monitor the battery conditions at the site, plus things
> like water level 
> and the like.   They're more "condition
> sensing" than something which 
> needs quick updates.
> 
> 
> > You may be able to group sensors to perform multiple
> tasks - e.g one
> > group measuring negative referenced signals (e.g low
> voltage side
> > battteries, temperature), one group measuring positive
> referenced
> > signals (High voltage side batteries, current shunts).
> 
> As I continue to work though this, I'm almost convinced
> that I'm going 
> to have to shed the ability to measure things not
> referenced to 
> ground/vss.  It would make things a lot easier and turn
> this project 
> into something which requires a quad opamp and a few
> precision resistors.
> 
> The shunts are actually fairly easy...  There are several 
> high-common-mode-voltage amplifiers set up for this. 
> Unfortunately, 
> none will allow you to do more than a few volts of
> "signal"...  That is, 
> you can measure a +-5V voltage difference riding on up to
> +-100V of 
> common mode voltage.   Getting above the +-5V or so is
> fairly difficult.
> 
> It's when you start to say "It's a 0-24V
> signal, and I don't know what 
> it's going to be referenced to" which gets sticky.
> 
I suggest if you have a high side shunt resistor for measuring current you look at linear tech's line up.  Unfortunately NONE of there stuff is cheap. You stated cheap so you may need to shed the idea of the huge voltage range or measuring everything.  What I suggest is you first make something that works, then find out what you don't need and make it a lot cheaper.  How many units are intended in the final design? 1000's? 

Stephen


      
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