• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 234 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

ADS8166: Question 2 - Custom Channel Sequencing vs Break before make mux

ADS8166: Question 3 - Limitations of not buffering Mux-outputs to ADC-inputs

rolek
Prodigy 40 points
Part Number: ADS8166

There are several points in the datasheet's diagrams where there is shown an option between using a buffer or to provide a direct connection between MUXOUT-P and ADC-AINP, and  again between MUXOUT-M and ADC-AINM. 

Are they to be done in the same fashion when selecting between these options?

What are the performance limitations for going with a direct connection?

How does the Thevenin equivalent resistance looking out the mux inputs affect the performance (say 5kΩ, or 1kΩ)?

Some sort of kickback from the ADC's sampling caps?

Are the ADC's caps arranged also in a T-switch structure or otherwise modified between samplings?

  • 0
    TI__Mastermind 36170 points

    Hi Rolek,

    When changing channels, there are two main capacitors that must be charged during the acquisition phase.  Please refer to Figure 33 in the datasheet.  The first capacitor is the Cmux=13pF, which is the total effective capacitance of the MUX itself.  The second capacitor is the actual input sampling capacitor of the ADC converter, Cs=60pF.  The advantage of adding a buffer between MUXOUT and ADC-AIN is that one amplifier can drive the ADC, and if the source resistance driving the inputs is low enough, you do not need additional amplifiers driving each of the multiplexor inputs.  This is not always the case depending on the sampling rate and resolution of the ADC.

    Take a look at this application note that discusses this subject in much more detail.

    http://www.ti.com/lit/an/sbaa308a/sbaa308a.pdf

    There may be some applications where the impedance of a sensor is very high and a buffer is required.  In this case, it may be easier to short the MUXOUT and AINP together, and just use a buffer per each input channel.  Again, it depends on your system.  The ADS8166 gives you the flexibility to optimize where and how many buffers to use.

    Regards,
    Keith N.
    Precision ADC Applications