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Original question:

ADS124S06: ADS124S06 AVSS-SW

ADS124S06: ADS124S06 Unused REFN0

Masa Katayama
Intellectual 970 points
Part Number: ADS124S06

Dear Sir,

A customer would like to tie AVSS-SW to ground and switch REFN0 for external application to connect ground or float. I read section 10.1.5 on page 89 in d/s as follows;    

"REFN0 is an exception; this pin can be accidently shorted to AVSS through the internal low-side switch. Leave the REFN0 pin floating when not in use or tie the pin to AVSS."

It looks keeping REFN0 floated is okay, but I wonder if it is better to have a capacitor between REFN0 and ground to avoid (AVSS-SW switch) noise?

 

Best regards,

Masa 

  • 0
    TI__Guru** 112725 points

    Hi Masa-san,

    There may be some confusion with respect to how the AVSS switch (AVSS-SW) works.  It is only useful if REFN0 is connected.  Note the functional block diagram.

    The AVSS-SW will only break the connection to whatever is connected to REFN0.  Normally this would be EXC- on a bridge circuit to break the excitation current.  The action of the AVSS-SW is controlled through a register setting in the IDACMAG (0x06) register.  The switch is default open.  If the switch is set to closed, it will open when the device is set to power-down mode.  See section 9.3.12 on page 55 of the ADS124S06 datasheet.  There will not be a lot of switching that would affect REFN0 and this would also be in power-down as well.

    If REFN0 is not being used and is floating, then you could also float the AVSS-SW pin.  The key point here is you don't want to connect AVSS-SW to AVSS and REFN0 to AVDD as the switch could inadvertently be closed in the register setting causing a supply short.  Instead of floating AVSS-SW and REFN0 you could connect both pins to AVSS.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 970 points

    Hi Bob,

    It sounds this internal switch can not used for just as a stand-alone analog switch for outside application like below.

    Correct?

    Best regards,

    Masa

  • 0 in reply to Masa Katayama
    Intellectual 970 points

    Hi Rob,

    I am putting more detail of the block diagram after amplifier output to ADS124S06.

    The ADC uses only internal reference voltage and will not use power down mode.  

    We are very keen to the analog switch reliability. Please advise if this will work or not.

     

    Best regards,

    Masa

  • 0 in reply to Masa Katayama
    TI__Guru** 112725 points

    Hi Masa-san,

    I'm sorry as I misunderstood your original question.  In your block diagram you have something connected to REFN0 and I initially thought the REFN0 pin was floating.  Using the PSW bit in the IDACMAG register (0x06), it is possible to open and close the switch manually.

    There is no requirement for placing a cap on the REFN0 input to ground.  There are also alternatives to the external INA.  You could use the PGA of the ADS124S06 along with VBIAS to set the common-mode to (AVDD-AVSS)/2.  I would suggest taking a look at A Basic Guide to Thermocouple Measurements for additional configurations.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 970 points
    Hi Bob,
    Thank you very much for your clarification. Very helpful.
    Masa