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ADS8339I gives out a doubled value

Nikolai Ehrhardt
Prodigy 20 points
Other Parts Discussed in Thread: OPA333

Hello, i have following problem.

I controlling a ADS8339I via SPI, but it gives out the doubled value of the input. It's said, that this device provides an unipolar full range to Vref Input.

I can observe on the oscilloscope, that the halfword is shifted one bit left, therefore i get the doubled value, but why does the device do that. I#m loosing one bit and i can read Vref/2 as maximum.

Thank you.

  • 0
    TI__Genius 9165 points
    Hello Nikolai,

    The most probable cause is the data captured is left shifted by 2.

    Please share oscilloscope capture of SPI transaction for review. Please help with following information.
    1. Schematic of the ADC interface section.
    2. Let us know which mode you are using.
    3. Provide a known DC input & share the input V, reference voltage, scope plot & the ADC code you read from this transaction.

    Thanks & Regards,
    Shridhar.
  • 0 in reply to Shridhar%20More
    Prodigy 20 points

    Well you are correct, i should make a picture, i maybe can give it to you this evening.

    I told you that i can see the halfword one bit shifted to the left there, the reference is created as the datasheet explains with ref5040 and the backlooped ths4821 and the opa333. it is very stable.

    I'm providing a dc constant input.

    I'm using spi-Mode 1.

    the pictures won't tell you anything more.

    The used MCU is AT32UC3B

    Thank you.

    I maybe can make some pictures this evening.