> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Pang
> Sent: 04 August 2009 06:04
> To: pic
> Subject: [EE]: Value tends to fluctuate in differential mode
> 
> Hi all,
> 
> 
> I am designing a 4 channel differential 4->20mA analog input using the
> ADS1243I from Texas Instruments. This chip can also be used for 8
channel
> single ended analog input. The front end basically consist of
resistors
> network, since we are catering for both voltage input and 4-20mA input
> using jumpers.
> 
> The problem comes when the value of some differential channels
affecting
> the values at another channel. Usually, it's some positive offset to
the
> value of other channels.
> 
> For example -
> - all the channels are configured to use current differential input.
There
> will be a resistor across the channel to convert the current to
voltage.
> - Channel 1 is input with a 4mA with a corresponding steady value of
0x00
> - When Channel 2 is input with a 10mA current, the value of Channel 1
will
> be read as 0x05 and tends to fluctuate.
> - Now, measuring the voltage at Channel 1 shows that the voltage
across
> the resistor remains the same, both prior to and after the injection
of
> current at channel 2.
> 
> - All these fluctuation and offset will cease when all channels are
> convert to single ended, meaning, all the -ve of the channels is
common to
> Gnd.
> 
> Are there any explanation to this?
> 
> We could not understand the issue here as all the channels are
independent
> from each other. Why would the input of a diferent channel affects the
> other?
> 
> Thanks.
> 
> Best rgds,
> 
> pang

Are all the voltages on the A/D inputs within the common mode range and
below the supply voltage?

Regards

Mike

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