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Value tends to fluctuate in differential mode

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chicken_feet

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Hi all,

May have posted on the wrong list.

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 it 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 is common to Gnd.

Are there any explanation to this?

Thanks.

Best rgds,

KaKiaYam
 

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