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Original question:

ADS1278: Excessive failures where the 1278 shorts AVDD to AGND

ADS1278: Power up

Piero Cambriani1
Prodigy 40 points
Part Number: ADS1278
Other Parts Discussed in Thread: THS4521

Hi,

about power up for ads1278 ADC component, in the datasheet i read that i need to check power-supply sequence for proper order, including the ramp rate of each supply.

But I could not find the delay times between the power supplies to meet the start up sequence.

Could you tell me what are the delay times between DVDD, IOVDD and AVDD  in start up sequence?

Thank you in advance.

Piero

  • 0
    TI__Mastermind 36240 points

    Hello Piero,

    Welcome to the TI E2E community.

    The power-up requirements for the ADS1278 are only required to ensure a proper power-up reset.  If you bring the supplies up in a different order, the ADS1278  will not be damaged and can still be properly reset by asserting the /SYNC pin after the supplies have stabilized to the recommended operating range and /DRDY first toggles.

    If you want to ensure a proper power-up reset, then DVDD/IOVDD should be brought up first (with AVDD held at gnd) and once stabilized to the recommended voltage range, then ramp AVDD.  The exact timing is dependent on the ramp rate of DVDD and IOVDD.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 40 points

    Hello Nicholas,

    about ADS1278 driver, I found on the datasheet a basic single ended input signal interface with THS4521 component. For my design the ADS1278 needs to run at 128 Ksps. The driver input is single ended Vin =2.5Vp-p the gain G=1, so I chose the driver resistors all at the same 1K value and Vref=2.5V. Could you tell me if 5.6nF value for feedback capacitors, as shown in the ADS1278 datasheet, is the right value for high speed mode?

    Thank you in advance.

    Best ragards,

    Piero Cambriani

  • 0 in reply to Piero Cambriani1
    TI__Mastermind 36240 points

    Hello Piero,

    The circuit in Figure 90 of the datasheet implements a low pass filter with a 3dB corner of 106kHz, which supports the input bandwidth of the ADS1278  when running at 128ksps.

    If you change all resistors to 1k and keep the 5.6nF, the 3db corner frequency drops to 27.9kHz, which may be too low for the customer's system.  Since you are using 1k resistors, you can increase the filter response to 115.5kHz by changing the capacitors to 1.5nF.

    Also, if you are using a 2.5V reference, the full scale input will be 5Vpp.  You can reduce the two input resistors to 499ohm (and increase the feedback capacitors to 3nF) for a differential gain=2 in this case and use the full scale input of the ADS1278.

    Regards,
    Keith