On Tue, Aug 4, 2009 at 1:04 PM, Pang wrote: > 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. Do you have buffers? This can be an issue. > 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. This can happen and is one of the required tests for our device. > 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 different > channel affects the other? There are various reasons this can happen. Cross-talk, wrong input range, wrong configuration of the ADC, etc. Take note you are dealing with 24bit Sigma-Delta ADC and each LSB is really very small. It would help that you post part of the schematics. -- Xiaofan http://mcuee.blogspot.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist