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Original question:

ADS8555: Unipolar operation

ADS8555: Two questions about ADS8555

Lin_Charles
Expert 4260 points
Part Number: ADS8555
Other Parts Discussed in Thread: ADS8860, ADS9120

Hi team,

Recently I study the datasheet of ADS8555, I have two questions about this device,

1. Is it possible to operate the ADS8555 inputs as unipolar while keeping the HVSS at AGND and HVDD at 5V? For this device do not have internal LPF, maybe we can use ii in some application required higher BW.

2. If this application is good, what is the relationship between input voltage range and BINARY CODE?

Thanks. 

  • 0
    TI__Mastermind 37330 points

    Hi Charles,

    1. This ADC requires a headroom on the HVDD/HVSS, for example, ≥6.5V required on HVDD for 5V input. If the application requires higher BW, you can use ADS8555 with high voltage power supplies, however, you will lose the dynamic range and will not be able to achieve specified performance (SNR etc.) if your input signal is too small, for example, your input signal is only 0~5V but the ADS8555's input range is +/-2Vref (+/-5V) or +/-4Vref (+/-10V) if the Vref is 2.5V. Another option is, you can use multiple low voltage precision ADCs, for example, ADS8860 or ADS9120 which does not have internal LPF. 

    2. Please refer to the "BTC – Binary Two's Complement" section in the app note below:

      Coding Schemes Used With Data Converters

    Regards,

    Dale

  • 0 in reply to Dale Li
    TI__Expert 4260 points

    Hi Dale,

    Thank you for your response here. 

    For Q1, I saw in our datasheet table 6.3 that we recommend HVDD at 2*VREF (min), which means if we use VREF=2.5V, we can use 5V power supply for HVDD. I cannot understand why we need headroom for this device. Could you please share more details? 

    For Q2, let us ignore the SNR performance and consider the feasibility of this application. If we connect HVDD to 5V, HVSS to GND, RANGE set to high,  the input voltage range will be 0V-5V when VREF=2.5V. From the app note you share to me, (Midscale + 0.5 LSB to full scale) corresponds to  (0000 to 7FFF), and we need to ignore the code between 8000-FFFF. Is it right from your side? Thank you for your help here.

  • 0 in reply to Lin_Charles
    TI__Mastermind 37330 points

    Hi Charles,

    1. You can use 2*VREF=5V for HVDD, but the input range will have to be limited to keep the resistance on internal switch small and linear.

    2. Your understanding about the code is correct, however SNR is only one example, the performance specified in the datasheet can not be guaranteed without the headroom. Also, the specifications in the datasheet were characterized with the conditions: HVDD=10V to 15V, HVSS=-15V to -10V.

    Regards,

    Dale