I didnt read too much of the datasheet but am wondering about something.

In the section about the input multiplexer it mentions there is a 
single-cycle settling digital filter that provides valid data on the first conversion
Do you change channels at the falling edge of DRDY as it states?


Peter van Hoof





----- Original Message ----
> From: Pang <mailtopang@yahoo.com>
> To: pic <piclist@mit.edu>
> Sent: Tuesday, August 4, 2009 1:04:11 AM
> 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
> 
> 
>       Get your preferred Email name!
> Now you can @ymail.com and @rocketmail..com. 
> http://mail.promotions.yahoo.com/newdomains/aa/
> 
> 
> -- 
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist



      
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist