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ADS1258-EP: Expected Converter Codes

Scott Metzger
Prodigy 60 points
Part Number: ADS1258-EP
Other Parts Discussed in Thread: ADS1258

Hello, 

I have a PCB that I want to use a ADS1258EP on. The sensors I will attach to the inputs will span 0V - 4.096V. I plan on running the device in autoscan mode with the following pin configuration: Vref=4.096, AINCOM=2.048V, AVDD=5V, AVSS=0V, DVDD=3.3V. If I want to maximize the converter span is the correct approach? Or should I use Vref=AINCOM=2.048V. I realize that there will be some unused head space (.06 on each side of span). I did search through the converter forums and there seems to be some confusion about this topic. Thanks for any input

Scott Metzger

  • 0
    TI__Mastermind 30036 points

    Hi Scott Metzger,

    The ADS1258 full-scale range (FSR) is +/-1.06666*VREF (let's ignore the 1.06666 for now). If VREF = 4.096V, then the FSR extends from -4.096V to +4.096V (note that this voltage is measured differentially between two analog inputs, and is not with respect to ground i.e. you cannot apply a -4.096V signal, referenced to ground, into this ADC). In your case, your sensor can only use half of this range (0 to 4.096V), so you would want to reduce the VREF to 2.048V. Now your FSR extends from -2.048V to +2.048V. If AINCOM is also 2.048V, and you are always measuring your input signal with respect to AINCOM, then when your input is 0V, you will measure 0V - 2.048V = -2.048V. When the input is 4.096V, you will measure 4.096V - 2.048V = +2.048V. This allows you to use all of your ADC's range, except for that extra 6.6% I ignored at the beginning 

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 60 points

    Thanks for replying to my question. It might be worth adding a table in the datasheet showing how Vref and AINCOM interact in a typical application. Also a warning that using a Vref with a value greater then 2.5V will limit the ADC's ability to use it's full scale. 

  • 0 in reply to Scott Metzger
    TI__Mastermind 30036 points

    Hi Scott Metzger,

    Thanks for the note. This is mentioned in section 3.3.1 of the ADS1258EVM user's guide

    The datasheet provides an application diagram for a differential input, as well as a single-ended to differential input stage. The single-ended application diagram (Figure 67 in the ADS1258 datasheet) actually shows the configuration I noted in my post, although it uses bipolar supplies so AINCOM is connected to ground in that case.

    -Bryan