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ADS7828-Q1: Input range in differential and single ended mode

Thomas F
Prodigy 190 points
Part Number: ADS7828-Q1
Other Parts Discussed in Thread: ADS7828

Hello,

I plan to use the ADS7828-Q1 ADC in my design. However, I am a bit confused about the input ranges in differential / single-ended modes and about the typical application "8.2.1 ADS7828-Q1 With Current Shunt Monitor" which is detailed in in the datasheet.

1/ From the datasheet page 4 (6.3 Recommended Operation Conditions) and the video "SAR ADC Input types" from TI (https://training.ti.com/ti-precision-labs-adcs-sar-adc-input-types, I understand that the ADS7828-Q1 has pseudo-differential inputs : negative input (VIN-) must remain between -0.2V...0.2V. However :

  • In Figure 25, VIN- is connected to a 3.3V reference voltage.
  • In datasheet page 22, it is written "The fully differential voltage input of the ADS7828-Q1 device..."

Which kind of input has this ADC : pseudo-differential or fully-differential ? Can VIN- go a few volts above GND ?

2/ In Section "8.1.1.3 Single-Ended Inputs", it is explained that in single-ended mode, one bit of resolution is lost because only half of the input scale is covered. This would be true for a fully differential input ADC, not a pseudo-differential input. The datasheet of the ADS7828 (not ADS7828-Q1) doesn't mention this at all. On top of that, in Figure 24, we can read "Output Codes 0-32767" although this ADC has only 12 bits.. This makes me think that this whole section of the datasheet is wrong.

In single-ended mode, is one bit lost ? 

3/ In this post https://e2e.ti.com/support/data_converters/precision_data_converters/f/73/t/695297, Abhijeet wrote that "For single ended configuration (Input referred to COM pin), analog input range is up to VDD. ". This means that VREF isn't used in single-ended configuration ?

In summary, my questions are :

  • In single-ended mode
    • Is VREF or VDD used to define the LSB ?
    • Is one bit lost ?
  • In differential mode :
    • Is the input pseudo-differential or fully differential ?

Thank you,
Thomas

  • 0
    TI__Mastermind 25020 points
    Hello Thomas,
    Thank you for your question, we are looking into and will get back to you soon. Regards Cynthia
  • 0
    TI__Expert 5575 points
    Hi Thomas

    Please see my comments below

    This device can be used in single ended or pseudo- differential configuration (VIN- swing is restricted to -0.2V to 0.2V). This device is not fully differential. I think there are some mistakes in datasheet description section and that need to be corrected.

    1. VREF is used for LSB calculation. I guess the confusion of whether to consider VDD or VREF for LSB calculation may have come from device mode of operation. This device can be used with internal reference or with external reference driving REFIO pin.

    When used with internal reference the device input range is 0 to VREF (0 to 2.5V) typically.

    If you plan to use this device with external reference, then reference can go up to VDD. That's where device input range can be 0V to VDD (as VREF = VDD).

    2. I do not think one bit is lost if you use this device in single ended configuration. We need to correct that part of the datasheet.

    I hope this answers your questions. We will work on getting these errors corrected.

    Thanks & Regards
    Abhijeet
  • 0 in reply to Abhijeet Godbole
    Prodigy 190 points
    Hi Abhijeet,

    thank you for the quick and good reply. It answers my questions. In summary :

    - LSB is defined by VREF. VREF can be internal (2.5V) or external (up to VDD).
    - Input is pseudo-differential.
    - No bit is lost in single ended configuration.

    I have another question : 2 versions of this device exist (ADS7828E and ADS7828EB). ADS7828EB offers better performances (better linearity, lower offset etc.).
    However, there is only one version for the automotive part (ADS7828EIPWRQ1). Which specifications do apply to this part ? The ones of the *E or *EB version ?

    Thank you and regards,
    Thomas
  • 0 in reply to Thomas F
    TI__Expert 5575 points
    Hi Thomas

    For ADS7828-Q1 (ADS7828EIPWRQ1 orderable), please refer to ADS7828E electrical specifications.

    Thanks & Regards
    Abhijeet
  • 0 in reply to Abhijeet Godbole
    Prodigy 190 points
    Hi Abhijeet,

    Thank you for the answer.

    Regards,
    Thomas