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ADS1158: Problem with starting of oscillator

Ivo Senn
Prodigy 20 points
Part Number: ADS1158

Hello E2E Users

In our application a TI 'ADS1158IRTCT' is used, and we are facing now some startup problems of the oscillator circuit for the ADC.

That means the ADC oscillator circuit is sometimes (often) not starting up after a power up or after a power-cycle. It can e.g., startup for 3 times with a power-cycle but the 4. times it won't.

 The oscillator circuit consists of a crystal 'FC-135 32.7680KA-A0' with two load capacitors of 15pf 'C0G, NPO 1%'. The PLL capacitor is a 22nF 'X7R 10%'.

 All those capacitors are connected on the common side to AVSS which is in our case -2.7VDC.

 Now we are having some questions:

  1. In the datasheet/application note of the ADS1158 two 4.7pf capacitors are used (page 15) for the crystal, but the datasheet of the crystal says 12.5pf load capacity which would mean two 15pf to be used. Why is that and what is correct?
  2. Could the two 15pf capacitors for the crystal we are using in our circuit cause this startup problem?
  3. Is it that or what else could cause this issue?
  4. We didn't face this problem witha lot ofPCBA's but now it came up.

    Could this be a question of tolerances? If yes, which tolerances?

  1. What is the risk for runningPCBA's (with no startup problem) stop running from your point of view?

---

Regards Ivo

  • 0
    TI__Mastermind 45731 points

    Hi Ivo Senn,

    We have seen this issue before, and unfortunately there is no simple fix. The crystal clocking circuit on the ADS1158 can be very difficult to get oscillating consistently. This is due to a combination of the PCB layout/routing, the selected crystal, and the load capacitors. A good first option is to try to change the values of load capacitors to see if that improves the reliability of the circuit. I would always start with the load capacitors recommended by the crystal datasheet, then move up or down from there.

    It is hard to quantify the risk of issues with the PCBs that have no startup issues. If the crystal circuit is marginally stable, tolerances on the load caps or the drive circuitry inside the ADC could cause the circuit to go unstable. Temperature changes could also cause issues if the circuit is marginally stable

    The best suggestion would be to use a discrete clock oscillator as opposed to a crystal. I have not heard of any startup issues with a discrete clock oscillator. Unfortunately this would require a PCB change.

    -Bryan