LTC298X ADC Internal Clocking and LTC2984/LTC2986 Universal Typical Application Example

Hi 


I just want to confirm the following: 

1.The internal ADC's on the LTC298X parts are clocked by an internal oscillator and not through a derivative of the SPI clock? 
I figure it would be illogical for a register based part like this to draw its conversion clock from a possibly variable speed SPI clock that might drive other SPI slaves during conversions. I am sure this is the case but as far as I can see the datasheets do not make mention of an internal oscillator. I thought this worth some clarification for our intended use.

2.The LTC2986 datasheet includes a "Typical Application" example for a Universal Protected Multi-Sensor Input. Would it be possible to include 2 of these Universal Protected Multi-Sensor Inputs on the 20 channel LTC2984, which would make use of a either a shared CJ or Rsense where required. 

A table at the end of an article (below) makes mention of the LTC2984 not having functionality for "Universal protected inputs (2- 3- and 4-wire)" where the LTC2986 does include such protection. I deduce from this that the LTC2984 is unable to handle one Universal Protected Multi-Sensor Input (let alone 2). 

https://www.analog.com/media/en/technical-documentation/lt-journal-article/LTJournal-V27N1-00-df-LTC298x-LoganCummings.pdf

If you could provide some further information or confirmation on the above that would be much appreciated. 

Garrick

Top Replies

    •  Analog Employees 
    Jun 17, 2020 +1 verified

    - Hi,

    1) Correct, the internal oscillator is factory trimmed for power-line frequency rejection. In the global settings configuration register there is the option to select how the oscillator…

  • +1
    •  Analog Employees 
    on Jun 17, 2020 11:31 PM 5 months ago

    - Hi,

    1) Correct, the internal oscillator is factory trimmed for power-line frequency rejection. In the global settings configuration register there is the option to select how the oscillator is divided down to best reject 50 or 60Hz power line frequencies. By default the part centers the rejection null at 55Hz as a compromise for worldwide usage. The same register is where you would set output to be in Fahrenheit or Celsius - see pg 59 of the LTC2983 datasheet (or the same global config register section of the other DSs). 

    2) Correct, the LTC2986 and LTC2986-1 added the universal *protected* multi-sensor input mode - the trick is the Kelvin sense of the 3-wire RTD, everything else should be possible with the LTC2983 or LTC2984. (e.g. Note that a 2-wire RTD in protected universal mode looks an awful lot like a 4-wire RTD to the part...)

    Hope it helps,

    -Logan