AD7858: Channel Switching and Software Conversion

Question

Accepted Answer

Question I am programming bean for AD7856 and AD7858. I would like to known why two 
 separate writes are required to set a new channel address and initiate 
 conversion on that new channel in software for AD7858 and not for AD7856. (Data 
 sheets page number 27) Answer AD7858
 When a change of channel and convert start command are both contained in single 
 write operation to the control register, 
 1. the input multiplexer is changed first 
 2. and then the conversion is started. 
 In other words the new conversion is carried out on the channel you have just 
 selected. This does not leave sufficient settling time for the newly selected 
 channel, and so the conversion result will be garbage. Therefore we recommend 
 two separate write operations, to allow sufficient settling time and 
 acquisition time after the MUX has switched. 
 
 AD7856
 When a change of channel and convert start command are both contained in single 
 write operation to the control register, 
 1. the conversion is started first, on the previously selected channel, 
 2. then the input MUX is switched to the new channel,
 To convert on the channel you have just selected, you must sent another start 
 convert signal.

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AD7858: FAQ