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ADS1248: External Reference voltage

Saribel Daza
Prodigy 50 points
Part Number: ADS1248
Other Parts Discussed in Thread: ADS124S08

I would like to change our design from using the internal 2.048V reference to using an external 5V reference.  Please see attached redline.  My question is: How is the reference selected?  Or how does the ADS1248 select the reference voltage (select voltage across pins 9 and 10 or the voltage across pins 5 and 6)?

  • 0
    TI__Mastermind 47625 points

    Hi Saribel,

    Thanks for your post and welcome to the forum! 

    The ADS1248 has multiple options for references. The ADS1248 can select the internal reference (output on Pin 9 & 10), or from two external references. The first external reference would connect to pin 5 & 6, while the second external reference would connect to pin 7 & 8. This can be seen in the Pin Functions section of the datasheet. There is a reference MUX to select which reference the ADC uses to convert, as seen in section 9.3.6 Voltage Reference Input. This MUX is controlled using the 9.6.4.3 MUX1—Multiplexer Control Register 1 (offset = 02h) [reset = x0h]. 

    One issue is that the ADS1248 will not be able to use a 5V reference. If you take a look at table 7.3, you will see that the max allowable AVDD voltage is 5.25V and the max allowable reference voltage is (AVDD - AVSS) -1. With AVDD = 5V and AVSS = GND, the max allowable reference voltage is 4V. 

    If this is for a refresh, you may want to consider the ADS124S08 family as it is the latest and greatest device for type of design.