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ADS1252 Power up timing

Takumi Suzuki1
Mastermind 8610 points
Other Parts Discussed in Thread: ADS1252

Hi,

I would like to ask you some questions about "Power-up" timing specifications of ADS1252.

Data sheet describes about "power-up sequence" as below, data sheet page9.

***** 

Prior to power-up, all digital and analog-input pins must be
LOW. At the time of power-up, these signal inputs can be
biased to a voltage other than 0V, however, they must never
exceed +VDD.
Once the ADS1252 powers up, the DOUT/DRDY line pulses
LOW on the first conversion; this data is not valid. The sixth
pulse of DOUT/DRDY is valid data from the analog input
signal.

*****

I understand that,

 During power-up(+Vdd, from 0V to 4.5V), all inputs (+Vin, -Vin, CLK, Vref, SCLK) must be lower than +Vdd (+Vdd>all inputs)

Is my understanding correct?

Or all inputs must be lower than 0.8V(digital), 0.5V(analog, Vref) during +Vdd reaches to 4.5V ??

I would like to confirm the meaning of "LOW".

And do you have the power-up timing specifications between +Vdd and all inputs signals?

Because there is no "Reset" pin with ADS1252.

I think to reset ADS1252 is only "power-up" sequence. It is important to know the timing to reset correctly.

(Please let me know if you know how to reset after power on.)

As I ask you this question, my customer has a problem about abnormal behavior of ADS1252 when powered-up.

After power-up, ADS1252 sometimes does not work (output data is always 0xFFFFFF).

In case of happenning this situation, ADS1252 begins to work by "power-down and power-up" again.

I suppose this is due to there is something input voltage at some inputs of ADS1252 during power-up.

Could you tell me any advice about this?

Thank you and Best Regards,

  • 0
    TI__Mastermind 44680 points

    Hi Takumi-san,

    Takumi Suzuki1 said:
     During power-up(+Vdd, from 0V to 4.5V), all inputs (+Vin, -Vin, CLK, Vref, SCLK) must be lower than +Vdd (+Vdd>all inputs)

    This is correct, no input should exceed the power supply voltage to ensure that the ADC powers up correctly!

     

    The main intent is to prevent a violation of the absolute maximum rating of the device during power-up. Since, every pin has ESD diodes to the supplies, exceeding the supply voltage (at any time) would forward bias these diode and could cause latch-up or even damage the device.

    As a side note, keep in mind that the the ADS1252 is specified to operate with a minimum 4.75V Vdd supply. With only 4.5V, the ADC will probably work fine, but the performance may not be as good.

     

    Takumi Suzuki1 said:

    I think to reset ADS1252 is only "power-up" sequence. It is important to know the timing to reset correctly.

    (Please let me know if you know how to reset after power on.)

    Correct, there is no "reset" per se; however, performing a SYNC would restart the ADC conversions, which is a kind of "reset".

     

    Takumi Suzuki1 said:

    After power-up, ADS1252 sometimes does not work (output data is always 0xFFFFFF).

    In case of happenning this situation, ADS1252 begins to work by "power-down and power-up" again.

    I suppose this is due to there is something input voltage at some inputs of ADS1252 during power-up.

    Could you tell me any advice about this?

    I would suggested performing a SYNC command after power up (by holding SCLK high for at least 4 conversion periods, but less than 20). This might resolve the issue, but if the device is powering up into a odd mode (for example a latch-up state), then the only way to resolve would be to prevent the problem during power-up, as you mentioned.

    Also, increasing the supply voltage from 4.5V to 5V could help.

    Chris

  • 0 in reply to Christopher Hall
    Mastermind 8610 points

    Hi Chris-san,

    Thank you very much for your kind and detail reply !

    I understood well.

    But let me ask you one more question.

    My customer plans to add a "Power-Off-ON" circuit to resolve the issue.

    Would you tell me the low voltage value of Vdd to make a reset correctly?

    Around under 0.5V ??

    I appreciate your support.

    Best Regards,

  • 0 in reply to Takumi Suzuki1
    TI__Mastermind 44680 points
    Hi Takumi-san,

    That is a great question! However, I don't know a specific value that will guarantee proper operation...the closer you can get to 0V before applying power again the better.

    We had looked at a similar issue on another device and found that, in addition to the initial supply voltage (before re-applying power), the supply ramp rates also had an effect. Under certain conditions, power-on-reset (POR) issues occurred for supply voltages as low as 0.3 V. When powering up from a partially charged state, the device may not reset, or it may even power-up into an unknown state. Therefore, I would recommend trying to fully-discharge the supply capacitors before re-applying power.

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Mastermind 8610 points
    Hi Chris-san,

    Thank you very much for your kind answer!
    I understood well and let me tell this to my customer.

    Best Regards,