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ADS1291: Startup process power up and wait time

Brian Eng
Prodigy 10 points
Part Number: ADS1291
Other Parts Discussed in Thread: ADS1298

Hi,

I have some questions about the startup process for the ADS1291.

In figure 44 of the datasheet, one of the boxes reads “Set PWDN/RESET = 1 Wait for 1 s for Power-On Reset” while Figure 72 and Table 29 indicate a wait requirement of 2^12 * tMOD. We use the internal oscillator, so I believe 2^12 * tMOD = 2^12 / (128kHz +/- 1.5%) = 1/32 s +/- 1.5% = 31.7ms, much less than 1s. What is the correct minimum amount of time to wait before pulsing RESET low?

Also, Section 10.1 states “At the time of power-up, all of these signals [all digital and analog inputs] should remain low until the power supplies have stabilized, as shown in Figure 72”, but RESET in Figure 72 appears to come up with the power supplies. Are input signals required to remain low, or can they come up with the power supplies provided the signals remain below the absolute max limits of AVDD+0.3V and DVDD+0.3V?

  • 0
    TI__Guru 66346 points

    Hello Brian,

    Thank you for your post and please excuse our delay.

    The power-up procedure should be corrected to match the updates we made to our ADS1298 data sheet. There, we explain that the tPOR is actually dependent on the larger of two delays: 

    1. A minimum delay of at least 2^n clock periods
    2. R-C time constant for internal bias voltage on VCAP1 to ramp to ≥ 1.1 V

    Whichever condition takes longer will determine the amount of time to wait before issuing a nRESET pulse/command and resuming normal operation. I've included a screenshot of the equivalent flow chart from the ADS1298 data sheet below.

    Regarding your second question, as long as the absolute voltage on any analog or digital input does not exceed the Abs Max rating relative to the current supply voltage level, it is ok to allow their respective voltages to ramp with the supply. For example, a pull-up resistor on nRESET pin is fine.

    Regards,

    Ryan