• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 2214 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

ADS1282EVM-PDK: Problems using the evaluation board in Bipolar Mode and installing the drivers to use it with ADCPro

ADS1282EVM-PDK: Difference between Unipolar and Bipolar mode

Hiago Santos da Gama
Prodigy 230 points

Part Number: ADS1282EVM-PDK

Hi,

I am currently working with the ADS1282EVM-PDK and I read in its datasheet that it can operate in the unipolar or bipolar mode. However, I don't understand the difference between these two modes, since I was able to read a negative voltage on the ADC using the evaluation board in the unipolar mode. 

So, what's the difference between the unipolar and bipolar mode in the evaluation board? Is there any difference in the way the ADC operates? 

Thank you in advance,

Hiago Gama.

  • 0
    TI__Mastermind 44620 points

    Hi Hiago,

    The ADS1282 can read positive and negative differential input voltages no matter which supply mode you operate the device (both signals may have an absolute positive value, but the difference can still be negative).

    The main difference between using a 0-5V analog supply versus a -2.5 to 2.5V analog supply depends on the common-mode range of your input signal. If you want to measure a differential signal with a common-mode voltage around 0V, then using the bipolar supply configuration allows the internal PGA of the ADS1282 to be biased such that the input signal's common-mode voltage is right in the middle of the PGA's input range, allowing you to take advantage of the PGA's full input range.

    However, if you tried to connect this same signal (0V common-mode voltage) to the ADS1282 with a unipolar supply (AVDD = 5V, AVSS =0V), then you would violate the PGA's input common-mode range specifications and get a distorted conversion result. The reason for this is that the amplifier's output voltage is not fully linear within the supply rail range, but only if the amplifier output voltages remain at least 400 mV away from the supply voltage (any closer and the outputs saturate).

    See equation 4 (on pg 14) in the ADS1282 datasheet. I hope that helps!

  • 0 in reply to Christopher Hall
    Prodigy 230 points
    Hello Chris,

    Thank you for your assistance! I just need to understand a little bit better:

    1) If I use a signal generator to send a 2Vpp (-1V to +1V) sinusoidal signal to the ADS1282 in the unipolar mode, should I be able to read this signal correctly (I did that and the values were correct in the ADCPro, I just don't understand why)?

    2) Will the ADS1282 always saturate with values above 2.5V or below -2.5V, independently of using unipolar or bipolar mode?

    Best regards,

    Hiago
  • 0 in reply to Hiago Santos da Gama
    TI__Mastermind 44620 points

    Hi Hiago,

    1. Are you using the ADS1282EVM or are you using a custom PCB that connect the signal directly to the ADS1282?

    On the ADS1282EVM, there is an amplifier stage which level-shifts the signal to "AVSS + 2.5 V":

    2. There are actually three different requirements on the input signal range that must be met in order to prevent non-linear effects or output code saturation:

    • PGA input operating range (given by equation 2). The input signal must not be lower than AVSS+0.7V, or higher than AVDD-1.25V.
       
    • PGA output operating range (given by equation 4). The output voltage of the PGA (after gain) must not be lower than AVSS+0.4V, or higher than AVDD-0.4V.
       
    • Full-scale input voltage (given in electrical characteristics table). The maximum differential input voltage must not exceed +/- Vref / (2x PGA). So, in case of a 5V reference and PGA gain of 1 V/V, the maximum differential signal must not exceed +/- 2.5V.
       

    The first two requirements are to ensure that the PGA is operating in a linear region, while the last requirement is to ensure that the digital output code does not clip. So to answer your question, you can measure UP TO +/- 2.5V differential signals in either unipolar or bipolar supply mode, but the common-mode voltage requirement for this signal will change based on which supply mode you select.

  • 0 in reply to Christopher Hall
    Prodigy 230 points
    Thank you, Chris!

    I think now I understand the difference between these two modes. I am using the ADS1282EVM-PDK so I can learn how the ADS1282 works. We are going to use this ADC in a custom PCB and I needed to know these details for its designing.

    Best Regards,

    Hiago