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ADS1158 Unipolar Operation?

Yunho Jeong
Prodigy 20 points
Other Parts Discussed in Thread: ADS1158

Hi,

According to the manual, it is possible to configure ADS1158  to be unipolar by supplying 0 to REFN and +5 to REFP.

The problem is, I have configured everything to be unipolar as in the manual, but it still detects negative voltage.

at about 5V, I am getting ADC reading of 32767, but -1 at -5V, and any value in-between.

If it is operating in unipolar mode, shouldn't it be reading any value below reference?

  • 0
    TI__Mastermind 44680 points

    Hi Yunho,

    Welcome to the TI E2E Forums!

    "Unipolar mode" refers to your supply configuration. To configure the ADS1158 in unipolar mode you would connect AVSS to GND and supply +5V to AVDD. (You can then apply "0 to REFN and +5 to REFP." as you mentioned).

    The ADC can measure differential voltages of about ±VREF, or ±5V in your application. However, since you are using a unipolar supply, your input signals may need be level shifted to be within the ADC power supply range (0-5V). Therefore:

    • a +5V differential signal can only be measured when ADCINP = +5V and ADCINN = 0V
    • a -5V differential signal can only be measured when ADCINP = 0V and ADCINN = +5V


       

    Yunho Jeong said:
    at about 5V, I am getting ADC reading of 32767, but -1 at -5V, and any value in-between.

    A reading of 32767 or 0x7FFF would indicate that your differential voltage is out-of-range, or greater than +1.06*VREF.

    A reading of -1 or 0xFFFF also indicates that your differential voltage is out-of-range, but in this case less than -1.06*VREF.


    When you say you're applying 5V or -5V, is that the input voltage to ADCINP or is that the differential input voltage (ADCINP - ADCINN)?
    ...If it is the differential voltage, than what is your common-mode voltage?


    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Prodigy 20 points
    Thank you for your help, Chris

    I did connect AVSS to GND and +5 to AVDD as well. I meant to say, while VREFP = +5 and VREFN = 0, ADS1158 was using only half of full resolution. I am using single ended input configuration, and I thought ADC should only measure voltages between 0 and 5, correct?

    I read this thread http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/42990/150562#150562 and it deals with a differential voltage issue, while mine has an issue with a single-ended application.

    When you say you're applying 5V or -5V, is that the input voltage to ADCINP or is that the differential input voltage (ADCINP - ADCINN)?
    ...If it is the differential voltage, than what is your common-mode voltage? --> It is input voltage to ADCINP
  • 0 in reply to Yunho Jeong
    TI__Mastermind 44680 points

    Hi Yunho,

    I'm glad to help! To answer your questions...

     

    Yunho Jeong said:
    I did connect AVSS to GND and +5 to AVDD as well. I meant to say, while VREFP = +5 and VREFN = 0, ADS1158 was using only half of full resolution. I am using single ended input configuration, and I thought ADC should only measure voltages between 0 and 5, correct?

    Correct, when using a unipolar supply, the ADC inputs (ADCINP & ADCINN) can accept input signals ranging from 0V to 5V!

    When your input signal is single-ended, ADCINN will be tied to constant voltage, such as 0V or 2.5V. ADCINP can still accept inputs of 0V to 5V; thus, the differential voltage (ADCINP - ADCINN) will either range from 0-5V or +/-2.5V, depending on the voltage of ADCINN. What voltage do you connect to ADCINN?

    Yunho Jeong said:
    I read this thread http://e2e.ti.com/support/data_converters/precision_data_converters/f/73/p/42990/150562#150562 and it deals with a differential voltage issue, while mine has an issue with a single-ended application.

    Even though your input signal is single-ended, the ADC still sees a differential input (between ADCINP and ADCINN). Therefore, the same conditions apply to your case. The only difference being that if you set ADCINN to 0V and ADCINP swings from 0V to 5V, then you will only ever get POSITIVE output codes (from 0x00 to 0x7FFF) from the ADC.

     

    I would recommend connecting ADCINN to 2.5V and VREFP to 2.5V, then you would be able to take advantage of the ADC's full input range (which, in this case, would be +/-2.5V). You would then see both POSITIVE and NEGATIVE output codes, because the difference: ADCINP - ADCINN would swing between -2.5V and +2.5V.

    Best Regards,
    Chris