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ADS1174: Hardware, reliability of the clock chain for conversions

Simon Kissling
Prodigy 60 points
Part Number: ADS1174
Other Parts Discussed in Thread: ADS1178

Hello.

I am looking to develop a device in which the analog measurements are synchronized on UTC. The goal is to have several devices with analog measurements that are synchronized at +/- 100ns. Imagine that we have a 10MHz clock perfectly synchronized on UTC (using a GPS chip) and that the /SYNC pin of the ADS1174 in each device are synchronously pass from the low state to the high state using a pulse also synchronized with UTC. Will the samples of the ADS1174 be synchronized? Will the samples stay synchronized for a long period of time (several days, that is the question for the reliability of the clock chain)?

Thank for your help,

Best Regards

  • 0
    TI__Guru 66516 points

    Hello Simon,

    Thanks for your post!

    Yes, each channel in the ADS1174 will be synchronized to within one modulator clock cycle by design, regardless of the configured data rate. The digital filters are also synchronized so that they start the convolution phase during the same modulator clock cycle. The only internal delay that you would expect between channels of the same device is the aperture delay, typically less than 500ps.

    The /SYNC pin is used to synchronize the channels of multiple ADS1174 devices to each other or to an external event. You do not need the /SYNC pin just to synchronize the channels of one ADS1174. When synchronizing multiple devices, the master CLK input and /SYNC input must be shared. Pulsing the /SYNC pin will synchronize the channels of all connected devices to within one master CLK cycle.

    Over time, I do not see any reason why the channels of a single device, nor the channels across multiple devices, would become out of sync as long as the master CLK and /SYNC pins are not interrupted in any way. With a 10 MHz clock, your channels should easily be synchronized to within +/- 100 ns.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Hi Ryan,

    Thank you a lot for your answer :) You answered to my queries.

    However, it brought me to another question. I search to synchronize the different ADS1174 every second with the /SYNC pin. But, when this pin is used, the conversions stops and the digital filter are reset. So, the ADC miss one clock at minimum and the measures are corrupted. Is it a way to avoid such a corruption? Maybe with a change of the ADC family?

    Best Regards

    Simon

  • 0 in reply to Simon Kissling
    TI__Guru 66516 points
    H Simon,

    There is no need to issue a /SYNC pulse every second. You only need to pulse the /SYNC pins one time at start-up. Once all devices are synchronized, leave the /SYNC pin high and the ADCs will convert continuously and remain synchronized.

    Does this approach make sense for your application? What is the reason you wish to synchronize the ADS1174's every second?

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Hi Ryan,

    Thank you for your response.

    Yes, first I thought to do like you explain. But if the GPS signal is lost (GPS gives the synchronized clock signal), I have to detect the loss and "manually" re-synchronize the ADS. If the ADS is re-synchronized each second, the synchronization will automatically be done at the recovery of the GPS signal. It isn't mandatory but I think it's a good thing to have this possibility.

    Best Regards

    Simon

  • 0 in reply to Simon Kissling
    TI__Guru 66516 points
    Understood. You can certainly do it that way, but also keep in mind that each time you synchronize the ADCs, the digital filter will require at least 127 conversion periods (tNDR) before new data is considered valid. So depending on the output data rate, this settling time may take more than 1 second.

    This settling time requirement for synchronization comes from the wideband FIR filter architecture used in the ADS1178. If this seems like too much, perhaps we can consider a device which uses a sinc filter instead. These filters typically settle much faster (within 1-4 conversions), but their passband is significantly smaller due to the early roll-off.

    Best Regards,
  • 0 in reply to Ryan Andrews
    Prodigy 60 points

    Thank you for your response. The application can't lose any sample, so I will detect the loss of the GPS signal and re-synchronize the ADS when the signal is back.

    Thanks for your help!

    Best Regards

    Simon