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With the ADS1243, is it ok to use a 3.0V reference with Vdd=3.4V?

Ron Evans
Prodigy 165 points
Other Parts Discussed in Thread: ADS1243

With the ADS1243, is it ok to use a 3.0V reference with Vdd=3.4V?  I am getting unusual readings, rather random.  I can affect the reading by connecting the analog input to the ref thru a 100-ohm resistor so I appear to be reading the correct channel.  I have verified the correct SPI clock phase.  The input is about 0.5V.  Here are some of the readings, MSByte first:

236 97 101
251 97 93
255 97 97
180 97 93

I know you may need more info, just asking the reference question first.

Thanks
Ron 

  • 0
    Prodigy 165 points

    Please tell me if these signals look ok.  XIN clock is from the PIC microcontroller, 1.25 MHz.
    Ron


     

  • 0 in reply to Ron Evans
    TI__Guru** 113765 points

    Hi Ron,

    You can use 3.0V reference using RANGE=1, but this reduces the full-scale input range to +/-Vref/(2*PGA) or +/-1.5V measurement range with a gain of one. If your sensor is single ended (referenced to ground) then you need to make sure the Buffer is OFF.  If your sensor output is 0 to 3V, then you need to offset the sensor output by 1.5V and correct the reading in software.  You do this by placing the AINN input to 1.5V and the AINP input is connected to the sensor.  Any data below 1.5V will show as a negative voltage and any data above 1.5V will show as a positive result.  The total range is 0 to 3V.  In software you correct the data by adding 1.5V (or the equivalent number of codes) to the result.

    The codes you first sent is showing as a negative value.  You need to change the SCLK polarity so that it dwells low.  Data should be read on the falling edge, and change on the rising edge.  With SCLK dwelling high, you are reading and writing the data incorrectly.

    Best regards,

    Bob B

     

  • 0 in reply to Bob Benjamin
    Prodigy 165 points

    Bob Benjamin,
    Many thanks.  I changed SCLK to be idle low and that allowed me to read the device correctly.  I still need to sort out the input range etc. 
    Ron