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ADS4242 Analog Input driving stage

Dwijen Pandya1
Prodigy 90 points
Other Parts Discussed in Thread: THS4509, ADS4242

I am using THS4509 to drive ADS4242.  I have following doubts about this combination.

  • On page 9 of the datasheet output VCM of ADC is mentioned as 0.95V    

on the page 80 of the datasheet it is mentioned that termination resistors are connected to 1.5V VCM of the device  

Why these values of VCM is different? Which is correct value?

  • In the eval board circuit, VCM is applied to analog input through 24.9 ohm resistors.      

If I am using THS4509 cant I connect ADC VCM directly to THS4509? I am operating THS4509 at -Vss=0V to +Vss=5V, Would there be any issue if I connect common mode of THS4509 to 0.9V VCM of ADC as this VCM is less then mid supply voltage of THS4509? 

  • Which scheme is better, connecting VCM of ADC to THS4509 common mode or connecting VCM of ADC through 24.9 ohm resistor and keeping commond mode of THS4509 at 2.5V mid supply range?
  • 0
    TI__Expert 8895 points

    Hi,

    VCM is 0.95V for ADS42xx family. I'll be asking to update datasheet for this. And also I'll get back to you with other two answers soon.

    Thanks,

    KW

  • 0 in reply to KW Nam
    Prodigy 90 points

    Also let me know what input driving circuit was used for ploting characteristic on page 52 of the datasheet. Input signal was applied through transformer or through THS4509 for ploting this data?

    Dwijen

  • 0 in reply to Dwijen Pandya1
    TI__Expert 8895 points

    Hi,

    Please refer to my comments for your questions below.

    If I am using THS4509 cant I connect ADC VCM directly to THS4509? I am operating THS4509 at -Vss=0V to +Vss=5V, Would there be any issue if I connect common mode of THS4509 to 0.9V VCM of ADC as this VCM is less then mid supply voltage of THS4509?

    KW] As long as the signal does not hit the boundry (0 ~ 5V), direct connection should be fine between THS5409 and ADS4242.  Our EVM also has the direct connection for these devices.

    Which scheme is better, connecting VCM of ADC to THS4509 common mode or connecting VCM of ADC through 24.9 ohm resistor and keeping commond mode of THS4509 at 2.5V mid supply range?

    KW] The signal swing of ADS4242 is VCM +/- 0.5V, resulting 2Vpp, and therefore we suggest connecting VCM of ADC to THS5409 common mode. If 2.5V of mode is chosen, you might have matching problem between differential channels from voltage dividing block.

    Also let me know what input driving circuit was used for ploting characteristic on page 52 of the datasheet. Input signal was applied through transformer or through THS4509 for ploting this data?

    KW] Those results are based on transformer IF input to ADS4242 device.


    Thanks,

    KW