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Original question:

ADS7950: help getting started with ADS7950

ADS7950: ADS7950 - accuracy of 2Vref configuration

tom greene
Prodigy 170 points
Part Number: ADS7950

we are planning to use an external 2.5V reference with 0.2% tolerance and will configure the device for 2Vref to sample 5V range input signals. however, i cannot find any details in the datasheet as to how accurate 2Vref is compared to Vref. i am not sure if your device internally has a precision gain of 2.0 or attenuation of 0.5 for sampling 5V signals. either way, there must be some tolerance but i cannot find it. if there is a wide tolerance, use of a precision external reference would have no benefit. please advise.

thnks

Tom Greene

  • 0
    TI__Mastermind 24940 points

    Hello,

    The specifications given in the datasheet apply to both ranges, Vref and 2Vref, unless otherwise noted. The device scales down the input to half, thus Vref specifications apply to both ranges. 

    Regards

    Cynthia 

  • 0 in reply to Cynthia
    Prodigy 170 points

    hi Cynthia

    thanks for the reply. we tried using a 2.5V external reference with 0.2% tolerance but the bit count error grows as the signal input increases. attached is a xls file with data collected inputting the same dc voltage to all 4 channels. results show consistency regardless of mux channel selected and we have only 4 counts spread out of 5000 samples. by the time we reach 4.8V for signal input the bit count error is ~45mv, ~1% full scale error. if you are scaling the input to half, there must be a tolerance on the scaling. could it be close to 1%?adc_reading_TI.xlsx

  • 0 in reply to tom greene
    TI__Mastermind 24940 points

    Tom,

    What is the input source? the fact that the device measures well at low input voltages but not at high would lead me to look at the system around the device. 

    Also, I would like to share some collateral that you may find interesting. there is a blog series, that cover the Total Unadjusted Error of an ADC. There is also good video as part of TI Precision labs titled, Understanding and Calibrating the Offset and Gain for ADC Systems that I also think might be of interest

    Regards

    Cynthia 

  • 0 in reply to Cynthia
    Prodigy 170 points

    hi Cynthia

    thanks for the reply and material. i have recently discovered most of the error is from a 100k termination at the mxo pin. there doesnt appear to be a spec listed in electrical characteristics table for on resistance but i did find one figure showing 200ohm. after removing the load, we're getting ~3.5mv calc error. i still would to know what the scaling accuracy is of your internal 2:1 divider when setting 2Vref

    thanks

    Tom

  • 0 in reply to tom greene
    TI__Mastermind 24940 points

    Tom,

    I am glad you were able to figure that one out.

    When using the 2Vref, the device scales down the input to half, but to get more information on the accuracy of it would take some deeper digging. Overall the datasheet spec would  govern.

    Regards

    Cynthia