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ADS8665: Waiting time after setting the ADS8665 RANGE_SEL register

Conor
Mastermind 8765 points
Part Number: ADS8665
Other Parts Discussed in Thread: ADS8686S

Tool/software:

Hi,

I am using an ADS8665 with the RANGE_SEL register value changed from the initial value. After setting this register, I need to wait a certain amount of time before I can get the AD conversion data I expect. Specifically, if I get data 82us after setting RANGE_SEL, the correct value is output, but if I get data 32us later, an unexpected value is output. I checked the ADS8665 datasheet, but I can't find any information about the waiting time after setting the RANGE_SEL register. Is a waiting time necessary after setting RANGE_SEL? If so, please tell me the waiting time until RENGE_SEL switches.

■ADS8665 usage conditions:
- AIN_P-AIN_GND voltage: approx. 10mV (AIN_P driven directly from op amp)
- CPHA=0, CPOL=0, SCLK: 3.28MHz
- ADS8665 register settings: Only RANGE_SEL changed, others default

■Waveform information

Thanks,

Conor

  • 0
    TI__Guru**** 170945 points

    Hi Conor,

    I am not aware of any specific need to add wait time for settling with a range change.  Is it only the first sample after the change that you have issue with?

  • 0 in reply to Tom Hendrick
    Mastermind 8765 points

    Hi Tom,

    This does not only occur in the first sample after the register is set.

    If you read the sample value several times before the wait time in the previous document has elapsed, you will get a value different from the expected value.
    The sample value read at this time will gradually approach the expected value over time.
    (↓Example of sample after setting)
    RENGE_SEL setting ⇒ 6CBh ⇒ 30Bh ⇒ 0C7h ⇒ 02Eh ⇒ 00Ch ⇒ 006h ⇒ 005h ⇒ 004h (expected value)

    In addition, the wait time due to this event is required not only the first time after powering on, but every time the RENGE_SEL setting is changed.

    Thanks,

    Conor

  • 0 in reply to Tom Hendrick
    Mastermind 8765 points

    Hi Tom,

    Generally, SAR data is determined one bit at a time from MSB to LSB in one clock, so 12 clocks may be required for a 12-bit SAR.
    Is it possible that the above is the cause of the data differing depending on the read timing?
    If so, is there any mention of the above in the data sheet?

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Guru**** 170945 points

    Hi Connor - 

    Take a look in the datasheet, specifically page 32:

    In hardware mode, the range selected by the HW_RNGSEL[1:0] pins is applicable for all channels. A logic change on the HW_RNGSEL[1:0] pins has an immediate effect on the analog input range; however, there is typically a settling time of approximately 120 µs in addition to the normal acquisition time requirement for the low-pass filter option 2.

    and then on page 45:

    The HW_RNGSELx signals program the analog input range. The selected input range is applied to all 16 analog input channels. A logic change on these pins has an immediate effect on the analog input range. Allow for a typical settling time of 120 µs, in addition to the normal acquisition time requirement after the range change. The recommended practice is to hardwire the range select pins according to the desired input range for the system signals.

  • 0 in reply to Tom Hendrick
    Mastermind 8765 points

    Hi Tom,

    Are you referring to the ADS8665? I checked page 45 of the ADS8665 datasheet, but the above content was not mentioned.

    Also, is the 120 µs mentioned anywhere? I would like to know the basis for this number. If it is not mentioned in the datasheet, is there any reference information that can serve as the source of the number?

    Thanks,

    Conor

  • 0 in reply to Conor
    TI__Guru**** 170945 points

    Hi Conor,

    Sorry for the confusion - the LPF filter in the ADC needs time to settle after the input range is changed, the BW of LPF on ADS8665 is 15kHz which is the same BW as the option 2 filter on ADS8686S, so the settling time is also approximately 120us. Please see the description from the ADS8686S datasheet, we'll update the ADS8665 datasheet with the next revision.