For analog isolation a very effective method is V/F (voltage to frequency) =

and F/V (frequency to voltage) translation.  There are some very good =

components with less 1% nonlinearity.  Sometime you can use an optocoupler =

that directly interfaces with a D/A converter hat you can scale for the =

range and span you desire.  These, I believe are the most effective and =

linear.  I have achieved a Pearson Product Moment of 0.99999 with these =

techniques.
    If you would like assistance implementing either, contact me off list =

and I will be happy to assist you.



----- Original Message ----- =

From: "Crist=F3v=E3o Dalla Costa" <cdallacosta@gmail.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, March 26, 2007 9:51 PM
Subject: Re: "isolated" analog input


> On 3/26/07, Forrest W. Christian <forrestc@imach.com> wrote:
>>
>> I'm currently working on a remote telemetry project.   One of the
>> requirements is the ability to measure various DC (and possibly AC)
>> voltages.   These may be above or below ground, ground referenced, or
>> not.   I do have some flexibility with specifying what inputs are
>> permitted (voltage), but don't have enough flexibility to say that "all
>> signals must be ground referenced and must not be negative in reference
>> to ground".  (Actually I might be able to do this, but I'm not going to
>> be happy about it long term).
>
>
>
> You should probably use instrumentation amplifiers with weakly grounded
> inputs. Basically you'll use a resistor network to bring down the voltage =

> to
> levels readable by the amplifiers then, for each input, both the input
> ground and the input signal will be connected to your circuit's ground vi=
a =

> a
> 1 MB resistor or similar. The input ground goes to the opamp's inverting
> input and the signal to the non-inverting. The opamp will subtract the
> common mode and leave you the signal.
>
> Ideally the opamps ground should convert to 50% of the ADC's full scale so
> that it can swing both over and below ground. Make sure the full scale
> readable swing is at least 4 X the highest input voltage you want to
> measure. You should probably use a 16 bit ADC if you want good resolution.
>
> Use diodes to clamp inputs, high resistors in series to limit current, =

> etc.
> HTH.
> -- =

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