Hi all,
I am designing a 4 channel differential
4->20mA analog input using the ADS1243I from Texas Instruments. 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. Each channel will be 'independent' from each other as they are differential.
- Channel 1 is input with a 4mA with a corresponding steady reading value of 0x00
- When Channel 2 is input with a 10mA current, the value of Channel 1 will change to 0x05 and have a tendency to fluctuate.
- Now, measuring the voltage across the resister at Channel 1 shows that the voltage
across 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?
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
